Last reviewed October 16, 2026. General information, not legal or professional advice. Check the linked sources for the current rules.
The formula
Put every measurement in feet first. Thickness is where people slip, because it's usually given in inches. Divide the inches by 12: 4 inches is 0.333 feet, 6 inches is 0.5 feet.
Then length × width × thickness gives cubic feet, and cubic feet ÷ 27 gives cubic yards. Measure inside the forms, since that's the space the concrete fills.
Worked example: a driveway slab
Say the forms are 20 feet by 24 feet and the plans call for a 4-inch slab. The thickness should come from your plans or your contractor, not from this example.
- Area: 20 × 24 = 480 square feet.
- Volume: 480 × 0.333 = 160 cubic feet.
- Cubic yards: 160 ÷ 27 = 5.93.
- With a 4 to 10 percent margin: 5.93 × 1.04 = 6.16, and 5.93 × 1.10 = 6.52.
Why the margin matters
NRMCA recommends ordering 4 to 10 percent more than an estimate worked out from the plan dimensions, to cover contingencies (NRMCA CIP 31). On a real job those look like a base that dips in places, forms that bow a little under the weight, and concrete that ends up on tools and in the wheelbarrow.
A low base adds up faster than people expect. On that 480-square-foot driveway, a base that runs half an inch low across the whole area adds 20 cubic feet. That's about three-quarters of a yard. Check the depth in several spots before you order. The edge of the forms alone won't tell you.
Footers and post holes
A continuous footer works the same way: length × width × depth. A footer 1 foot wide and 1.5 feet deep that runs 60 feet is 90 cubic feet, or 3.33 cubic yards before the margin. If a slab has a thickened edge, work that part out separately as its own small footer and add it in.
Round holes use the circle formula, 3.1416 × radius × radius × depth. A hole 10 inches across has a 5-inch radius, which is 0.417 feet. At 3 feet deep that's 3.1416 × 0.417 × 0.417 × 3 = 1.64 cubic feet per hole. Twelve holes come to about 19.6 cubic feet, or 0.73 cubic yards.
How far one cubic yard goes
One cubic yard is 27 cubic feet, so the area it covers is 27 divided by the thickness in feet. Divide your slab's square footage by the number in the last column to get cubic yards. The 480-square-foot driveway at 4 inches: 480 ÷ 81 = 5.93.
| Slab thickness | In feet | Square feet per cubic yard |
|---|---|---|
| 4 inches | 0.333 | 81 |
| 5 inches | 0.417 | 64.8 |
| 6 inches | 0.5 | 54 |
| 8 inches | 0.667 | 40.5 |
When you're not sure, site-mix takes the pressure off
With a drum truck, the number you order is the number that gets batched. Come up short and the slab waits on another load. Order too much and the extra has to go somewhere.
A site-mix truck mixes at the forms (VMMB), so the final yardage gets settled during the pour. If the base ran low, the operator keeps mixing while there's material on board. If the estimate was high, what's left stays dry on the truck. You still need a decent estimate so the truck brings enough material, but a small miss doesn't stop the job. Short loads and the site-mix vs ready-mix guide have more.
Put the dimensions and thickness in your quote request, or have them handy when you call (321) 751-6666.
Questions people ask
Should I round up to the next full yard?
Use the 4 to 10 percent margin as your guide rather than a fixed rule (NRMCA CIP 31). With a site-mix truck, the part of the margin you don't use stays on the truck as dry material.
Do I measure inside or outside the forms?
Inside. That's the space the concrete fills.
What if my slab is thicker in some places?
Work each part out on its own and add them up. A thickened edge or a deeper section under a post is its own small volume: length × width × the extra depth.