How Much Concrete Do I Need?
Multiply length × width × thickness in feet, then divide by 27 to convert cubic feet to cubic yards. For a slab measured in inches of thickness: (length ft × width ft × thickness in ÷ 12) ÷ 27 = cubic yards.
A 10 × 12 patio at 4 inches works out to 1.48 cubic yards. Order 5–10% extra so you don't come up short.
| Slab size | Square feet | At 4 in | At 5 in | At 6 in |
|---|---|---|---|---|
| 8 × 8 | 64 | 0.79 yd³ | 0.99 yd³ | 1.19 yd³ |
| 10 × 10 | 100 | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ |
| 10 × 12 | 120 | 1.48 yd³ | 1.85 yd³ | 2.22 yd³ |
| 12 × 12 | 144 | 1.78 yd³ | 2.22 yd³ | 2.67 yd³ |
| 16 × 16 | 256 | 3.16 yd³ | 3.95 yd³ | 4.74 yd³ |
| 20 × 20 | 400 | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ |
| 24 × 24 | 576 | 7.11 yd³ | 8.89 yd³ | 10.67 yd³ |
| 30 × 30 | 900 | 11.11 yd³ | 13.89 yd³ | 16.67 yd³ |
Volumes are exact geometry with no waste added. Add 5–10% before you order.
The formula
Concrete is sold by volume, so every estimate comes down to the same three measurements. Convert everything to feet first, because thickness is almost always quoted in inches:
- Cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
- Cubic yards = cubic feet ÷ 27
There are 27 cubic feet in a cubic yard, which is where that second division comes from. A cubic yard covers 81 square feet at 4 inches thick, so a quick sanity check is to divide your square footage by 81.
Worked example: a 10 × 12 patio
Say you are pouring a 10 ft × 12 ft patio at the standard 4 inches:
- Thickness in feet: 4 ÷ 12 = 0.333 ft
- Volume in cubic feet: 10 × 12 × 0.333 = 40 ft³
- Volume in cubic yards: 40 ÷ 27 = 1.48 yd³
- With 10% waste: 1.63 yd³ — order 1.75 or 2 yards
Suppliers deliver in quarter-yard increments, so round up to the next quarter yard rather than hoping the truck driver is generous.
Shapes that aren't rectangles
Footings, post holes, and columns use the same idea with different geometry. Break complicated pours into simple shapes, work out each one, then add them up.
- Footings: length (ft) × width (in ÷ 12) × depth (in ÷ 12) ÷ 27
- Round holes and columns: π × radius (ft)² × depth (ft) ÷ 27
- Steps: treat each step as a box and stack the totals
The concrete calculator has a preset for each of these, including footings, post holes, and steps.
Bags or ready-mix?
Under about half a cubic yard, bags win — you mix what you need and there is no delivery minimum. Above roughly one cubic yard, ready-mix is usually cheaper and far less work, even after the short-load fee.
One cubic yard takes 45 bags of 80 lb mix. That is more than a full pallet, and mixing it by hand is a long day. See how many bags of concrete are in a yard for the full breakdown.
Estimating notes
- Measure in several places and use the largest dimension — subgrade is never perfectly flat.
- Add 5% waste on a tidy, well-formed pour and 10% on rough or uneven ground.
- Deduct large voids like sleeves or block-outs only if they are big enough to matter.