Yardage is the one piece of quoting math you cannot fudge. Price too low and you lose margin. Order too low and you are staring at a half-finished slab while the plant decides whether it can get you another truck today. Here is the math, done right.
The formula
Cubic yards = length (ft) x width (ft) x thickness (ft), divided by 27.
The only trap is thickness. It is measured in inches but the formula needs feet, so divide the inches by 12 first. Four inches is 0.333 feet. Six inches is 0.5 feet. That is the whole trick.
Two worked examples
A 20 x 24 slab at 4 inches:
- 20 x 24 = 480 square feet
- 480 x 0.333 = about 160 cubic feet
- 160 / 27 = about 5.9 yards
The same slab at 6 inches:
- 480 x 0.5 = 240 cubic feet
- 240 / 27 = about 8.9 yards
Notice what thickness does. Going from 4 to 6 inches is half again more concrete on the identical footprint, which is exactly why a driveway pour should never be priced like a patio. The mud is the biggest hard cost on the job, and it scales with thickness even though the customer only sees the square footage.
Yardage by common slab size
Same formula, run for the sizes people ask about most. These are the raw numbers before the ordering cushion in the next section, so treat them as your floor, not your order.
| Slab size | Square feet | Yards at 4 in | Yards at 6 in |
|---|---|---|---|
| 10 x 10 | 100 | 1.2 | 1.9 |
| 12 x 12 | 144 | 1.8 | 2.7 |
| 20 x 20 | 400 | 4.9 | 7.4 |
| 20 x 24 | 480 | 5.9 | 8.9 |
| 24 x 24 | 576 | 7.1 | 10.7 |
Read across any row and the 6 inch column is always half again more mud than the 4 inch column on the exact same footprint. That gap is the whole reason a driveway pour costs more than a patio of the same size.
Order a cushion, not the exact number
The formula assumes a perfectly flat, perfectly compacted subgrade, and no jobsite has one. Low spots drink concrete. So do spillage, the pump line, and the last wheelbarrow that tips. Contractors commonly round up to the next half yard and carry a little extra beyond that on bigger or rougher pours. There is no magic percentage. The judgment call is simple: an extra half yard is cheap insurance, and a short pour with a cold joint in the middle of a driveway is not.
Small pours live in short load territory
If your math comes out to a couple of yards, you are well under a full mixer truck, and most plants will add a fee for bringing one out anyway. That fee can be a real chunk of a small job’s price, so it belongs in the quote, not in your margin. Read short load concrete fees explained before you price the next small pour.
Yardage tells you what the job costs you in material. What you charge is square footage times your rate, and that side of the math should be just as fast. The concrete slab calculator shows it live with example rates, and PriceDesk runs the same calculator with yours. Free for 14 days.