Calculator

Concrete Calculator

Calculate concrete volume, dry mix allowance, ready-mix quantity, bags, and estimated cost.

How it works

Method

The calculator applies the formula shown in the result cards and updates instantly as values change.

Assumptions

Editable rates, odds, values, and percentages should match your current source, supplier, or platform data.

Privacy

Calculations run in your browser. No extra API request is needed for these estimates.

Calculate the Concrete Required for Your Project

A concrete calculator converts project measurements into a practical estimate for slabs, foundations, footings, walls, columns, post holes, steps and other concrete elements. Enter the relevant dimensions to find the net concrete volume, adjusted purchase quantity, approximate number of bags and estimated material cost.

The volume formula is usually simple. Accurate measurement is the more important part. Excavations can vary in depth, formwork may not be perfectly straight, and some material may remain in a mixer, chute or pump line. A useful estimate therefore shows both the exact geometric volume and a separate allowance for normal site variation.

What Can the Concrete Calculator Show?

Concrete volume Cubic metres, cubic yards and cubic feet based on the entered dimensions.
Adjusted quantity The calculated volume after applying the selected additional allowance.
Bag requirement The approximate number of complete bags based on the selected bag yield.
Ready-mix order A practical quantity that can be discussed with a local concrete supplier.
Material cost An estimate based on the price per bag, cubic metre or cubic yard.
Project comparison Separate calculations for slabs, footings, walls, columns and other sections.

How to Use the Concrete Calculator

Follow these steps before buying concrete bags or requesting a ready-mix quotation.

  1. Select the measurement system. Choose metric measurements such as metres and millimetres, or imperial measurements such as feet and inches.
  2. Choose the project shape. Select the option that best represents the slab, footing, wall, column, trench, circular base or post hole.
  3. Enter the project dimensions. Add the required length, width, height, thickness, depth or diameter. Use the internal dimensions of the formwork or the actual excavation dimensions.
  4. Add the number of identical elements. Enter the quantity when calculating repeated items, such as twelve pad footings or twenty fence-post holes.
  5. Check variable depths. Measure an excavation or sub-base in several places. Use an average depth when the surface is not consistently level.
  6. Select an additional allowance. Apply a suitable percentage for uneven excavation, measurement variation, placement loss or irregular formwork.
  7. Choose bagged concrete or ready-mix. Consider the total volume, access, available workers, mixing equipment and required placing speed.
  8. Enter the local material price. Add the price per bag, cubic metre or cubic yard when a material cost estimate is needed.
  9. Review the purchase quantity. Keep the exact geometric volume separate from the adjusted amount that may be ordered.

Understanding Your Concrete Calculator Results

Net concrete volume
The theoretical volume calculated directly from the entered dimensions. It assumes that the excavation and formwork match those measurements.
Additional allowance
A percentage added for ordinary measurement variation, irregular ground, placement loss or other site conditions.
Adjusted volume
The net volume after the selected allowance has been applied.
Purchase quantity
The quantity rounded to complete bags or to an ordering increment accepted by the ready-mix supplier.
Finished bag yield
The approximate volume of usable concrete produced by one bag after it is mixed according to the product instructions.
Estimated material cost
The purchase quantity multiplied by the entered unit price. Delivery, labour, pumping and equipment may need to be added separately.
Adjusted volume = Net concrete volume × (1 + Allowance percentage)

Allowance calculation example

A project has a net volume of 10 cubic metres. Applying a 7.5% allowance gives:

10 × 1.075 = 10.75 m³

When a supplier accepts orders only in 0.5 m³ increments, the practical order may be rounded to 11 m³. Confirm minimum-load rules and accepted ordering increments directly with the supplier.

The Percentage Calculator can be used to check percentage additions or compare two project estimates.

How Is Concrete Volume Calculated?

Concrete is measured by volume because it fills a three-dimensional space. The correct formula depends on the shape of the element being poured.

Rectangular section

Used for slabs, walls, rectangular columns, trenches and pad footings.

Volume = Length × Width × Depth

Circular section

Used for circular slabs, post holes, round columns and cylindrical bases.

Volume = π × Radius² × Depth

Hollow tube or ring

The inner cylindrical volume is deducted from the outside volume.

Volume = π × (Outer radius² − Inner radius²) × Height

Keep all measurements compatible

Length, width and depth must be expressed in compatible units before multiplication. When length and width are entered in metres, a thickness shown in millimetres must first be divided by 1,000.

A 150 mm slab thickness is therefore entered as 0.15 m when calculating manually:

150 ÷ 1,000 = 0.15 m

For imperial measurements, divide inches by 12 before multiplying values measured in feet.

4 inches ÷ 12 = approximately 0.333 foot

Concrete Calculations for Different Project Types

Concrete slabs

Slabs include building floors, driveways, patios, warehouse floors, equipment bases, concrete yards and walkways.

Measure the internal length and width of the formed area together with the actual slab thickness. Calculate beams, thickened edges and machinery bases separately.

Slab volume = Length × Width × Thickness

Strip footings and trenches

Measure the total trench length, average width and average depth. Split the trench into separate sections where its size changes.

Strip footing volume = Total length × Width × Depth

Pad footings

Calculate one footing and multiply the result by the number of identical units. Create a new group for every different footing size.

Total volume = Length × Width × Depth × Quantity

Concrete walls

Measure wall length, height and thickness. Large door, window or service openings may be deducted from the gross wall volume.

Wall volume = Length × Height × Thickness

Rectangular columns

Multiply the column width by its depth and height, then multiply by the number of identical columns.

Column volume = Width × Depth × Height × Quantity

Circular columns and piers

Enter the column diameter and height. The radius used in the formula is half of the measured diameter.

Column volume = π × Radius² × Height × Quantity

Post holes

Calculate one hole as a cylinder and multiply it by the number of holes. The embedded volume of a large post can be deducted when it materially changes the result.

Post-hole volume = π × Radius² × Depth × Quantity

Circular slabs

This method can be used for tank bases, round equipment pads, circular foundations and other cylindrical pours.

Circular slab volume = π × Radius² × Thickness

Concrete stairs

Stairs may contain individual steps, a supporting waist slab, landings and beams. Divide simple solid stairs into measurable sections. Structural stairs should be measured from approved drawings.

Kerbs, curbs and channels

Calculate the cross-sectional area of the kerb or channel and multiply it by the full installed length.

Volume = Cross-sectional area × Total length

Thickened slab edges

Calculate the normal slab first. Measure the extra depth and width of each thickened edge separately, then add that additional volume to the slab total.

Equipment and machine bases

Local foundations below plant or machinery often have greater depth than the surrounding floor. Treat every base as a separate concrete section.

Metric and Imperial Concrete Measurements

Mixing measurement systems is a common cause of incorrect results. A slab may be measured in metres while its thickness is shown in millimetres, or measured in feet while its depth is shown in inches.

Common metric concrete conversions
Measurement Equivalent Typical use
1 metre 1,000 millimetres Lengths, widths and heights
100 millimetres 0.10 metre Thin slabs and paths
125 millimetres 0.125 metre Slab thickness conversion
150 millimetres 0.15 metre Slab thickness conversion
200 millimetres 0.20 metre Deeper structural sections
1 cubic metre 1,000 litres Ready-mix volume
Common imperial concrete conversions
Measurement Equivalent Typical use
1 foot 12 inches Lengths, widths and depths
4 inches Approximately 0.333 foot Slab thickness conversion
6 inches 0.5 foot Slab thickness conversion
1 cubic yard 27 cubic feet Ready-mix ordering
1 cubic metre Approximately 1.308 cubic yards Metric-to-imperial conversion

How Much Additional Concrete Should You Allow?

A geometric calculation assumes that the finished excavation and formwork are exact. Actual construction conditions may require a slightly larger quantity. There is no single percentage that is suitable for every project.

Why the actual quantity may be higher

  • The excavation is deeper or wider than the drawing dimensions.
  • The sub-base contains high and low areas.
  • Formwork edges are irregular or move during placement.
  • Concrete is lost during unloading, pumping or placing.
  • Material remains inside a mixer, chute, pump line or placing equipment.
  • Dimensions were rounded during the site measurement.
  • Additional filling is required while the pour is in progress.
  • The ground absorbs or displaces part of the placed material.
General allowance examples for concrete planning
Site condition Example allowance Points to consider
Accurately formed work Approximately 5% Formwork, dimensions and levels have been carefully checked.
Typical site conditions Approximately 7.5% to 10% Allows for ordinary measurement variation and minor placement loss.
Irregular excavation 10% or more Depth, width or ground level varies across the pour.
Complex shapes or uncertain dimensions Project-specific review Divide the work into smaller sections and recheck every measurement.

These percentages are general planning examples rather than fixed rules. Large, high-value or structural pours should be reviewed by the project estimator, site manager and supplier before an order is confirmed.

How to Calculate Multiple, Uneven or Irregular Concrete Sections

Multiple project sections

A construction project often contains more than one concrete element. A warehouse floor, for example, may include the main slab, thickened perimeter edges, internal bases, loading-bay ramps and external paths.

  1. Give each section a clear name.
  2. Record the dimensions of every section separately.
  3. Calculate the net volume of each section.
  4. Add the separate volumes together.
  5. Apply the selected allowance to the combined result.
  6. Check whether the sections will be poured together or on different dates.

Uneven or sloped excavations

One depth reading may underestimate an uneven excavation. Divide the area into a simple measurement grid and record the depth at several points.

  1. Measure the depth at the corners.
  2. Take additional readings through the centre of the area.
  3. Add all depth measurements together.
  4. Divide the total by the number of readings.
  5. Use the resulting average depth in the volume calculation.
Average depth = Total of all depth measurements ÷ Number of measurements

Irregular concrete areas

Divide an irregular plan into smaller rectangles, triangles or circular sections. This is usually more reliable than applying one average length and width to the whole area.

  1. Draw a simple plan of the area.
  2. Divide the plan into standard geometric shapes.
  3. Label every section with its dimensions.
  4. Calculate the volume of each section.
  5. Add all section volumes together.
  6. Subtract large voids, openings or unpoured sections.
  7. Apply the selected additional allowance.

Bagged Concrete or Ready-Mix Concrete?

Bagged concrete can be practical for small jobs and restricted-access locations. Ready-mix is commonly used for larger, continuous or time-sensitive pours. The lowest material price does not always produce the lowest complete project cost.

Bagged concrete and ready-mix comparison
Factor Bagged concrete Ready-mix concrete
Typical use Small, staged or restricted-access work Medium, large or continuous pours
Preparation Mixed with water on site Delivered ready for placement
Labour More lifting, handling and mixing Less manual mixing on site
Placing speed Usually slower Usually faster
Consistency Depends on batching and water control Produced under batching-plant controls
Access Bags can be carried into confined areas Requires truck, chute, conveyor or pump access
Purchasing unit Complete bags Cubic metres or cubic yards
Possible extra costs Mixer hire, labour, water and handling Delivery, small-load, waiting-time and pump charges
Unused material Dry unopened bags may be stored correctly Excess wet concrete needs an immediate plan

How to calculate concrete bags

The number of bags depends on the adjusted project volume and the finished yield of the selected product.

Bags required = Adjusted concrete volume ÷ Finished yield per bag

Always round the answer up to a complete bag. A result of 47.2 bags means that at least 48 complete bags are required.

Before buying concrete bags

  • Check the finished yield printed on the bag.
  • Do not estimate finished volume from bag weight alone.
  • Confirm how many bags the mixer can handle in each batch.
  • Estimate how quickly the workforce can mix and place each batch.
  • Provide a reliable supply of clean water.
  • Use a consistent water quantity for every batch.
  • Keep unused bags protected from moisture and direct ground contact.

Ready-mix ordering checks

  1. Confirm the required concrete grade and project specification.
  2. Check the net and adjusted concrete volumes.
  3. Confirm the supplier’s minimum-load policy.
  4. Ask which order increments the supplier accepts.
  5. Check truck access, turning space and ground condition.
  6. Decide whether discharge by chute is possible or a pump is required.
  7. Arrange enough workers for placement, compaction and finishing.
  8. Prepare a suitable concrete washout area.
  9. Confirm delivery times and the planned pour sequence.
  10. Review waiting-time, cancellation and additional-load charges.

How to Estimate Concrete Project Cost

A useful cost estimate separates concrete material from delivery, labour and equipment. This helps construction companies and developers compare supplier or contractor quotations more clearly.

Estimated material cost = Purchase quantity × Unit price

Costs that may need to be included

  • Price per concrete bag
  • Price per cubic metre or cubic yard
  • Delivery charge
  • Small-load surcharge
  • Concrete pump hire
  • Mixer hire
  • Labour for placement
  • Labour for finishing
  • Plant and equipment hire
  • Concrete testing charges
  • Local tax or other charges
  • Additional material allowance

Concrete cost example

An adjusted order contains 15.5 m³ of concrete. The entered price is 120 per cubic metre.

15.5 × 120 = 1,860

Delivery, pump hire, workers, equipment, testing and tax would be added separately where applicable.

Contractors comparing a material cost with a proposed selling price can use the Margin Calculator to calculate gross profit, markup and profit margin.

Worked Concrete Calculation Examples

Example 1: Commercial floor slab in metres

A construction company needs to pour a floor measuring 12 metres long, 8 metres wide and 150 millimetres thick.

  1. Convert the thickness: 150 ÷ 1,000 = 0.15 m.
  2. Calculate the net volume: 12 × 8 × 0.15 = 14.4 m³.
  3. Apply a 7.5% allowance: 14.4 × 1.075 = 15.48 m³.
  4. Round according to the supplier’s accepted order increments.
Commercial floor slab result
Net volume 14.4 m³
Allowance 7.5%
Adjusted volume 15.48 m³
Possible purchase quantity 15.5 m³, subject to supplier confirmation

Example 2: Patio in feet and inches

A patio measures 20 feet long, 12 feet wide and 4 inches thick.

  1. Convert the thickness: 4 ÷ 12 = approximately 0.333 ft.
  2. Calculate cubic feet: 20 × 12 × 0.333 = approximately 80 ft³.
  3. Convert to cubic yards: 80 ÷ 27 = approximately 2.96 yd³.
  4. Apply a 10% allowance: 2.96 × 1.10 = approximately 3.26 yd³.

Depending on supplier increments and actual site conditions, the contractor may consider an order of approximately 3.5 yd³.

Example 3: Twelve circular post holes

Each post hole has a diameter of 300 mm and a depth of 750 mm.

  1. Convert the diameter: 300 mm = 0.3 m.
  2. Calculate the radius: 0.3 ÷ 2 = 0.15 m.
  3. Calculate one hole: π × 0.15² × 0.75 = approximately 0.053 m³.
  4. Multiply by twelve holes: 0.053 × 12 = approximately 0.636 m³.
  5. Apply a 10% allowance: 0.636 × 1.10 = approximately 0.70 m³.

When the posts occupy a substantial part of each hole, deduct their embedded volume before applying the allowance.

Example 4: L-shaped driveway

Divide the driveway into two rectangular sections and calculate each one separately.

L-shaped driveway section calculation
Section Dimensions Thickness Volume
Main area 10 m × 5 m 0.15 m 7.5 m³
Side area 4 m × 3 m 0.15 m 1.8 m³
Combined net volume 9.3 m³
9.3 × 1.075 = 9.9975 m³

With a 7.5% allowance, the working requirement is approximately 10 m³ before checking the supplier’s ordering conditions.

For a driveway, road or parking project where asphalt is also being considered, compare material quantity using the Asphalt Calculator .

Using the Calculator on Construction and Development Projects

Construction companies

Use the result for preliminary material take-offs, supplier comparisons, procurement checks, variation estimates and planned pour quantities.

Real estate developers

Compare early foundation, floor, parking, road, boundary wall and external-work quantities during project budgeting and contractor review.

Quantity surveyors and estimators

Separate project elements, check contractor quantities and compare the calculated volume with drawings, schedules and bills of quantities.

Site managers and supervisors

Compare ordered quantities with actual pour progress and investigate unexpected increases or shortages during concrete placement.

Construction workers

Check the approximate number of bags required before mixing starts, especially for repairs, small pads, steps and post installations.

Concrete suppliers

Use clearly recorded dimensions and allowances as a starting point when discussing order quantities, load sizes and delivery arrangements with customers.

Common Concrete Calculation Mistakes

  1. Entering millimetres as metres. A thickness of 150 mm equals 0.15 m, not 150 m.
  2. Confusing area with volume. Square metres describe surface area. Concrete requires a cubic measurement that includes depth.
  3. Measuring depth at only one point. Several readings provide a more reliable average for uneven excavations.
  4. Forgetting repeated elements. Multiply one footing or post-hole result by the total number of identical units.
  5. Using diameter as radius. The radius of a circular section is half of its diameter.
  6. Rounding the purchase quantity down. Bags must be purchased as complete units, and suppliers may use fixed volume increments.
  7. Using bag weight instead of finished yield. Similar bag weights can produce different finished concrete volumes.
  8. Applying one thickness to every section. Beams, ramps, pits, thickened edges and machine bases should be calculated separately.
  9. Ignoring large openings. Significant wall openings and unpoured voids may need to be deducted.
  10. Forgetting placement loss. Some material may remain in the pump line, mixer, chute or other equipment.
  11. Adding the allowance twice. Check whether an entered supplier quantity already includes an additional percentage.
  12. Treating volume as structural design. The calculation does not determine concrete grade, reinforcement or structural dimensions.

Concrete Pre-Order Checklist

  • Site dimensions have been measured and checked.
  • Different project sections have been calculated separately.
  • Millimetres, inches and other units have been converted correctly.
  • Variable excavation depths have been measured in several places.
  • Concrete grade and project specification have been confirmed.
  • The selected additional allowance is visible in the estimate.
  • Bag yield or ready-mix order increments have been checked.
  • Supplier minimum-load conditions are understood.
  • Delivery vehicle access has been reviewed.
  • Chute reach or concrete pump requirements have been checked.
  • Mixer, vibrating and placing equipment have been arranged.
  • Enough workers are available for placing and finishing.
  • Weather and ground conditions have been considered.
  • A suitable concrete washout area has been prepared.
  • Waiting-time and cancellation terms have been reviewed.
  • Curing materials and protection are available.

Frequently Asked Questions

How do I calculate how much concrete I need?

Measure the length, width and depth of the space. Convert all measurements into compatible units and multiply them together. Apply a suitable allowance before deciding how much concrete to purchase.

How do I calculate concrete for a slab?

Multiply the slab length by its width and thickness. A slab measuring 10 m × 5 m × 0.15 m has a net concrete volume of 7.5 m³.

How many bags of concrete do I need?

Divide the adjusted project volume by the finished yield of one bag. Round the answer up to the next complete bag and check the stated yield for the product being purchased.

Does the calculator include an additional allowance?

An allowance can be applied to the net concrete volume. The original volume, added percentage and adjusted purchase quantity should remain visible as separate figures.

How much additional concrete should I order?

The amount depends on measurement accuracy, excavation quality, formwork and the placement method. Carefully formed work may need a smaller allowance, while irregular excavation may require a larger one.

How do I calculate concrete for an irregular shape?

Divide the area into smaller rectangles, triangles or circular sections. Calculate each section separately, add the results, subtract large voids and then apply the selected allowance.

How do I calculate concrete for post holes?

Multiply π by the hole radius squared and the hole depth. Multiply that result by the number of holes. The embedded volume of a large post may be deducted where it materially reduces the requirement.

Should I use bagged concrete or ready-mix?

Bagged concrete is often practical for small or restricted-access work. Ready-mix is normally more practical for larger continuous pours. Compare material cost, labour, equipment, access and required placing speed.

Can I use metres and millimetres together?

Yes, when the calculator converts them correctly. For a manual calculation, convert millimetres to metres first. For example, 150 mm equals 0.15 m.

Can I calculate concrete using feet and inches?

Yes. Convert inches into feet before calculating cubic feet. Divide the cubic-foot result by 27 when a cubic-yard result is required.

What is the difference between cement and concrete?

Cement is a binding ingredient used to make concrete. Concrete normally contains cement, water, sand and stone aggregate in proportions determined by the required mix.

Does reinforcement reduce the concrete order?

Reinforcement is not normally deducted from a routine concrete order because its volume is usually small compared with normal site variation. Follow the project measurement method where a formal quantity calculation is required.

Does the calculator determine concrete strength?

No. It estimates volume, bags and material cost. Concrete strength, reinforcement, exposure class and structural dimensions must come from the project design and specification.

Can the calculator be used for commercial construction?

It can support preliminary estimates, procurement checks and site planning. Final commercial quantities should be checked against approved drawings, project documents and current site measurements.

Should the concrete quantity be rounded up or down?

Purchase quantities are normally rounded up. Bags must be bought as complete units, while ready-mix suppliers may accept orders only in set increments.

Check Before You Place the Concrete Order

Use the calculated result as an initial material estimate, then recheck the dimensions, project specification, access arrangements, placing method and supplier requirements. Keeping the exact volume separate from the adjusted purchase quantity makes the estimate easier to review and can help prevent a costly shortage or unnecessary surplus during the pour.