How to Calculate Concrete Yardage
Published 2026-09-04
Multiply length by width by thickness — all in feet — then divide by 27, because a cubic yard is 27 cubic feet. A 10 × 12 slab at 4 inches thick is 10 × 12 × 0.333 ÷ 27, or 1.48 cubic yards.
“Yardage” on a job site means cubic yards, and it is the unit every ready-mix plant quotes in. The arithmetic is three multiplications and a division. The step that catches people is the thickness.
The formula
cubic yards = length ft × width ft × thickness ft ÷ 27The 27 is exact — NIST Handbook 44’s general tables of units of measurement defines a cubic yard as 27 cubic feet exactly — so the only places error creeps in are your measurements and the thickness you actually pour. Background on mix design and placing is published by the Portland Cement Association.
Thickness is almost always given in inches and must be converted first:
| Thickness | In feet |
|---|---|
| 3 in | 0.25 |
| 4 in | 0.333 |
| 5 in | 0.417 |
| 6 in | 0.5 |
| 8 in | 0.667 |
| 12 in | 1.0 |
Using 4 instead of 0.333 makes the answer twelve times too big. It is the single most common error in a concrete estimate.
Change any measurement to update your estimate.
- 40 lb bags
- 112 bags
- 50 lb bags
- 89 bags
- 60 lb bags
- 75 bags
- 80 lb bags
- 56 bags
Worked example: a 10 × 12 slab
At 4 inches:
- Area: 10 × 12 = 120 ft²
- Thickness in feet: 4 ÷ 12 = 0.333
- Cubic feet: 120 × 0.333 = 40 ft³
- Cubic yards: 40 ÷ 27 = 1.48 yd³
Add 5–10% and order 1.6 yd³ — in practice, 1.75 yards, since plants sell in quarter-yard increments.
Square yards is a different thing
A square yard is 9 square feet of flat area. It tells you nothing about how much concrete to order, because it has no thickness in it. Contractors sometimes price finishing by the square yard, which is where the confusion starts.
square yards = ft² ÷ 9 (area — for finishing labour)
cubic yards = ft³ ÷ 27 (volume — for ordering concrete)That 10 × 12 slab is 13.3 square yards and 1.48 cubic yards. Order 13.3 yards of concrete and nine trucks will show up.
Bags or ready-mix
An 80 lb bag of concrete mix yields about 0.6 ft³, so a cubic yard takes 45 of them.
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
The 10 × 12 slab at 4 inches is 40 ft³ — 67 bags of 80 lb mix. That is the point where bagging stops making sense. Past about 50 bags, order ready-mix: hand-mixing that volume takes a full day and risks cold joints between batches, which show as permanent seams in the finished surface.
The concrete calculator does this for slabs, and there are fixed-size slab pages if yours is a standard footprint.
Footings and odd shapes
Break the pour into rectangles and add the volumes:
| Element | Dimensions | Cubic feet |
|---|---|---|
| Slab | 10 × 12 × 0.333 | 40.0 |
| Thickened edge | 44 lin ft × 0.5 × 0.5 | 11.0 |
| Total | 51.0 ft³ = 1.89 yd³ |
For a round footing or sonotube, use π × radius² × depth. A 12-inch tube 4 feet deep is 3.14 × 0.5² × 4 = 3.14 ft³, so nine of them is just over a cubic yard.
Why you always round up
You cannot pause a pour to make more concrete. A short load means a cold joint you will see forever; a surplus is a wheelbarrow of waste. Order to the next quarter yard, dampen the subgrade before the truck arrives, and set your forms dead level — concrete finds every low spot and fills it at your expense.