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Concrete Volume Calculator: Slabs, Foundations

How to Calculate Concrete Volume: Slabs, Foundations and Columns (with Free Calculator)

Ordering the wrong amount of ready-mix is costly, whether you run short mid-pour or pay for concrete you cannot use. Learn the formulas for slabs, strip foundations, pads and columns, how much wastage to add, and how many trucks you need, with worked examples and a free calculator.

Last updated 7 November 2026

Why getting the volume right matters

Ready-mix concrete is ordered by the cubic metre and delivered on a fixed schedule. If you order too little, you stop mid-pour, risk a cold joint and pay for a second delivery. If you order too much, you pay for concrete that ends up as waste.

A short calculation before you order avoids both problems. KaelSite’s free Concrete Volume Calculator works out the volume, adds wastage, rounds up truck loads and can estimate the cost.

The basic formulas

All volumes are in cubic metres (m³), so convert every dimension to metres first.

  • Slab, pad, trench, strip foundation and footing: length × width × depth
  • Circular column: π × (diameter ÷ 2)² × height

A slab is a broad horizontal pour such as a floor. A strip foundation is a continuous pour in a trench under a wall. A pad is an isolated rectangle under a column or pier. The maths is the same rectangular formula, and the shape you choose mostly changes the labels and prompts.

For identical elements, calculate one and multiply by the quantity.

Adding wastage

Real pours use more concrete than the drawing suggests. Spillage, uneven ground, over-dig in trenches and pump or barrow losses all take a share. Most site teams add 5 to 10%. The calculator applies your percentage to give a total to order.

Use the higher end where trenches are dug by machine in soft ground, and the lower end for neat, formed pours.

Worked examples

1. Ground-floor slab
A slab 6 m × 4 m × 0.15 m deep:
6 × 4 × 0.15 = 3.6 m³
With 10% wastage: 3.6 × 1.10 = 3.96 m³
With a typical 8 m³ truck, that is 1 load. As an example only, at £120 per m³ the concrete would be about £475. Use your supplier’s actual price.

2. Strip foundation
A trench 20 m long, 0.6 m wide and 0.9 m deep:
20 × 0.6 × 0.9 = 10.8 m³
With 10% wastage: 10.8 × 1.10 = 11.88 m³
With a 6 m³ truck: 11.88 ÷ 6 = 1.98, so 2 loads. This is close to the limit, so check ground conditions before relying on it.

3. Six circular columns
Each column is 0.4 m diameter and 3 m high:
π × 0.2² × 3 = 0.377 m³ each
Six columns: 0.377 × 6 = 2.26 m³
With 10% wastage: 2.49 m³

How many trucks do you need?

Divide the total volume, including wastage, by the mixer capacity. Common mixers carry roughly 6 to 8 m³. Always round up to a whole load. The calculator does this for you.

Two practical points:

  • Suppliers may charge extra for small or part loads, so ask about minimum loads.
  • Ready-mix has limited working time after batching, so plan access, pump or barrow capacity and labour so each load is placed promptly.

Common mistakes

  • Mixing units. Entering a depth of 150 as metres instead of millimetres gives a result 1,000 times too large.
  • Forgetting wastage. The calculated volume is the minimum, not what to order.
  • Using the design depth for a trench. Over-dig and uneven bases add volume.
  • Ignoring blinding and sub-base. Check whether these are concrete or aggregate, and calculate them separately.
  • Ordering the exact figure. Small errors in dimensions can leave you short.
  • Not rounding to loads. A 12 m³ pour in 6 m³ trucks needs two full loads and leaves no margin.

UK specification and testing

You order concrete by specifying the mix, for example a designated mix or designed mix to BS 8500, along with strength class, consistency and exposure conditions. Samples are taken and tested for consistency and strength, with test methods in the BS EN 12350 and BS EN 12390 series. The volume calculation is only one part of the order, so confirm the mix with your supplier and designer.

Frequently asked questions

How do I calculate concrete volume?
Multiply length by width by depth in metres for rectangular pours. For a column, use π × radius² × height.

What wastage allowance should I use?
Typically 5 to 10%. Use more for trenches and rough ground.

How many concrete trucks do I need?
Divide the total volume including wastage by the truck capacity (often 6 to 8 m³) and round up.

Can I estimate the cost?
Yes. Multiply the total volume by the price per m³. This excludes labour, formwork, pumping, delivery surcharges and VAT unless already in your price.

What is the difference between a slab, strip foundation and pad?
A slab is a broad floor pour, a strip is a continuous pour under a wall, and a pad is an isolated base under a column.

Related calculators

Concrete is rarely planned on its own. These tools work well alongside it:

See all of them in our complete guide to free construction calculators.

Keep your pour records in one place

Ordering concrete is one step. Pour inspections, cube test results and photos also need recording. KaelSite helps UK site teams manage inspections, safety and reporting from one mobile-first app. Explore KaelSite features.

References