How to Calculate Concrete Volume for a Slab
The three measurements you need, and the one formula behind every slab pour
The direct answer
Concrete volume for a rectangular slab is length × width × depth, with all three measurements converted to the same unit before you multiply. The result comes out in cubic feet (if you worked in feet) or cubic metres (if you worked in metres) — a raw volume, not yet a cubic-yard order quantity.
Why depth is where estimates go wrong
Length and width are usually measured in feet or metres without much confusion. Depth is almost always given in inches or centimetres — a 4-inch patio slab, a 6-inch driveway — and it's the one number that has to be converted before it goes into the formula. Multiplying 20 ft × 10 ft × 4 (as if it were 4 ft) overstates the volume by a factor of 12.
Convert depth to feet by dividing inches by 12 (4 in ÷ 12 = 0.333 ft), or work entirely in metres and convert centimetres by dividing by 100.
From cubic feet to cubic yards
Ready-mix concrete is ordered by the cubic yard in the US, and 1 cubic yard = 27 cubic feet. Once you have a volume in cubic feet, divide by 27 to get cubic yards. In metric regions, concrete is ordered directly in cubic metres, so no second conversion is needed.
Concrete volume for common slab sizes
The table works the formula through for common slab sizes. These are exact volumes: the concrete the forms hold, before any allowance for ordering.
| Slab size | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 10 × 10 ft | 33.33 ft³ (1.23 yd³) | 41.67 ft³ (1.54 yd³) | 50.00 ft³ (1.85 yd³) |
| 12 × 12 ft | 48.00 ft³ (1.78 yd³) | 60.00 ft³ (2.22 yd³) | 72.00 ft³ (2.67 yd³) |
| 10 × 20 ft | 66.67 ft³ (2.47 yd³) | 83.33 ft³ (3.09 yd³) | 100.00 ft³ (3.70 yd³) |
| 16 × 20 ft | 106.67 ft³ (3.95 yd³) | 133.33 ft³ (4.94 yd³) | 160.00 ft³ (5.93 yd³) |
| 20 × 20 ft | 133.33 ft³ (4.94 yd³) | 166.67 ft³ (6.17 yd³) | 200.00 ft³ (7.41 yd³) |
| 24 × 24 ft | 192.00 ft³ (7.11 yd³) | 240.00 ft³ (8.89 yd³) | 288.00 ft³ (10.67 yd³) |
Volume first, then the order quantity
The volume and the amount you order are two separate steps. Work out the exact volume first, then add an allowance for uneven subgrade, form movement and spillage. The concrete-waste guide covers that step, and How Much Concrete Do I Need? converts the volume to bags if you are mixing your own.
How thick should the slab be?
Thickness comes from your plans or the local building code, not from the volume formula. In jurisdictions that adopt the International Residential Code, concrete slab-on-ground floors must be at least 3½ in thick (Section R506.1, as reproduced in the 2025 California Residential Code; the model IRC text wasn't checked). Earlier or locally amended editions may differ.
That figure is a code minimum for one building type, not a design recommendation. Check the edition and amendments your jurisdiction uses.
Irregular slabs
An L-shaped or stepped slab isn't a single length × width × depth calculation. Split it into rectangular sections, calculate each section's volume separately, and add them together. A slab with a thickened edge (a common footing-and-slab combination) is two volumes: the main slab plus the extra depth of the thickened strip.
Worked Example
A 12 ft × 10 ft patio slab, 4 inches thick.
Depth: 4 in ÷ 12 = 1/3 ft (0.333 ft)
Volume = 12 × 10 × 4 ÷ 12 = 40 ft³
Cubic yards = 40 ÷ 27 = 1.48 yd³ — the exact volume, before any ordering allowance
Assumptions & Limitations
- The slab is a uniform, flat thickness — a sloped or crowned slab needs an average depth.
- No waste allowance is included in the raw volume — see the concrete-waste guide for that step.
FAQ
- Do I need to account for the subgrade or gravel base separately?
- Yes. Concrete volume is the concrete only. A compacted gravel base under the slab is a separate material and a separate calculation.
- What if my slab has a slight slope for drainage?
- Use the average of the shallow and deep edge measurements as your depth. For a slope of an inch or two over a typical slab run, the difference in volume is small enough that an average depth is standard practice.
- How many yards of concrete do I need for a 10×10 slab?
- At 4 in thick: 10 × 10 × 4 ÷ 12 = 33.3 ft³, ÷ 27 = 1.23 yd³. At 6 in thick it is 1.85 yd³. These are exact volumes; add your ordering allowance afterwards.
Related Calculators
Related Guides
From the Blog
Sources
NIST SP 811
Unit conversion reference (cubic feet, cubic yards, cubic metres).
NIST SP 811 — referenceInternational Residential Code, Section R506.1
Concrete slab-on-ground floors not less than 3½ in (89 mm) thick. Checked against the 2025 California Residential Code, which reproduces this IRC wording; the model IRC text was not checked, and editions and local amendments vary.
International Residential Code, Section R506.1 — reference