Concrete Slab Calculator
Work out the concrete needed for a rectangular or circular slab, in cubic yards, cubic metres, litres or bags, and add an optional sub-base layer underneath.
Results
Enter your project measurements to see your results.
What the slab calculator does
A slab is the most common domestic pour, and it is also the one where small measurement errors cost the most. This page takes a single rectangular or circular pad, works out its surface area, multiplies by the thickness you enter and reports the result in every unit a supplier might quote in. Adding the optional sub-base section gives you the aggregate order in the same visit.
How to measure a slab
- Measure the length and width at the formwork, not the finished paving line.
- For a circle, run the tape across the centre to get the true diameter.
- Check the thickness at several points. A base that dips by 20 mm across a 4 m slab adds noticeably to the pour.
- Decide whether the thickness you type includes any haunching or thickened edge. If it does not, add that as a separate section on the main concrete calculator.
The slab formula explained
Surface area comes first: length × width for a rectangle, or π × radius² for a circle. That area is multiplied by the number of identical slabs, then by the thickness, which gives the net volume. Waste is added last so you can see exactly how much of the order is allowance rather than slab.
Metric example
A 4 m × 3 m slab at 100 mm thick has an area of 12 m² and a net volume of 1.2 m³. A 10% allowance brings it to 1.32 m³, which is 1,320 litres or 1.73 cubic yards.
Imperial example
A 12 ft × 12 ft slab at 4 in thick covers 144 square feet. At one third of a foot deep that is 48 cubic feet, or 1.78 cubic yards. With 10% waste the order figure is 1.96 cubic yards, which is 1.5 m³ in metric terms.
Sub-base, gravel base and basecourse
The layer under a slab has different names in different markets: sub-base in the UK and Ireland, gravel base in the United States, granular base in Canada and basecourse in New Zealand. The maths is identical. Compacted depth is what you want in the ground, and the compaction factor converts that into the looser volume you actually order. Densities vary a great deal between crushed stone, recycled aggregate and gravel, so the density box is yours to fill from supplier information rather than a value we invent.
Common mistakes
- Mixing the slab thickness unit with the plan-view unit and entering 100 m instead of 100 mm.
- Measuring a circle's radius but typing it into the diameter box.
- Ordering the compacted sub-base volume instead of the loose volume.
- Ignoring the thickened edge that many slabs need around the perimeter.
When to get professional input
Slab thickness, reinforcement, joints and ground preparation are design decisions. Where the slab supports a building, retains soil, carries vehicles or sits in ground subject to frost heave or movement, speak to an engineer or your local building control authority before pouring. Requirements vary by use, soil, climate and local building rules, and nothing on this page substitutes for that advice.
Frequently asked questions
- How thick should my slab be?
- This calculator will not answer that, because the right thickness depends on what the slab carries, the ground beneath it, the climate and the rules where you live. The quick-select buttons are common sizes people search for, not recommendations. Ask a local professional or your building control authority when the slab supports a structure or vehicles.
- What is the difference between the compacted and loose sub-base figures?
- Compacted volume is the space the material fills once it has been rolled or whacked down. Loose volume is what you order, because aggregate arrives fluffed up and reduces in depth as it is compacted. The compaction factor you enter is the ratio between the two.
- Why does a circular slab need the diameter rather than the radius?
- Because a tape measure across the full circle is far easier to take accurately than a measurement from a centre point you have to guess. The calculator halves the diameter internally before using π × radius² × depth.
- Does the reinforcement figure tell me what mesh to buy?
- No. It only reports the slab area so you can talk to a supplier about sheet sizes, laps and spacing. Mesh type, cover and laps are design decisions that depend on the slab's purpose and local requirements.
- Can I calculate several slabs at once?
- Yes, if they are identical: set the number of slabs and the calculator multiplies the volume before waste is applied. For slabs of different sizes, use the main concrete calculator and add a section for each one.
Related calculators
Working on something other than a flat pad? These tools use the same conversion core.