Updated June 17, 2026
Multiply length (ft) x width (ft) x depth (ft), then divide by 27 to get cubic yards. One cubic yard of concrete fills about 81 square feet at 4 inches thick.
The formula step by step
Convert your depth to feet first (4 inches = 0.333 ft, 6 inches = 0.5 ft). Then:
- Volume (cu ft) = length x width x depth
- Cubic yards = volume / 27
- Add 5 to 10 percent for waste and uneven subgrade
Example: a 10 x 20 ft slab at 4 inches thick is 10 x 20 x 0.333 = 66.6 cu ft / 27 = about 2.5 cubic yards. Order 2.7 yards to cover the waste factor. Use the Concrete Calculator to skip the arithmetic.
Common slab size reference
| Slab size | Thickness | Cubic yards | 80-lb bags (approx) |
|---|---|---|---|
| 4 x 8 ft | 4 in | 0.40 | 24 |
| 10 x 10 ft | 4 in | 1.23 | 59 |
| 12 x 12 ft | 4 in | 1.78 | 85 |
| 20 x 20 ft | 4 in | 4.94 | 237 |
| 20 x 20 ft | 6 in | 7.41 | 356 |
| 10 x 20 ft driveway | 4 in | 2.47 | 119 |
How much will 1 yard of concrete pour?
One cubic yard covers about 81 sq ft at 4 inches thick, 54 sq ft at 6 inches, or 40 sq ft at 8 inches. A standard ready-mix truck holds 8 to 10 yards, so small jobs are usually better done with bagged concrete mixed on site. See how many bags per yard to compare options.
Footings and columns
For round footings, the volume in cubic feet is 3.14 x radius^2 x depth. A 12-inch diameter footing (6-inch radius) at 12 inches deep is 3.14 x 0.25 x 1 = 0.785 cu ft, or about 0.03 yards. Multiply by the number of footings. See the full walkthrough in estimating concrete for footings.
What is the 1/2/3 rule for concrete?
The 1:2:3 mix ratio refers to the proportions of cement, sand, and aggregate (gravel) by volume used to make concrete by hand. It produces a general-purpose mix. Modern bagged concrete already has this blended in, so you just add water. If you are batching from scratch, 1 part cement, 2 parts sand, and 3 parts gravel is the traditional starting point.
Why the waste allowance matters
Concrete waste happens in three ways: spillage during the pour, slightly uneven subgrade that eats volume in low spots, and the small residue left in a mixer drum or wheelbarrow. On a flat, well-prepared surface you might get away with 5 percent. On rough ground or poured footings with irregular soil, use 10 percent. Running short on a slab is a serious problem because a cold joint (where fresh concrete meets partially cured concrete) is a structural weak point. Always round up to the nearest quarter yard when ordering ready-mix.
Depth conversion quick reference
| Depth (inches) | Depth (feet) | Cu ft per 10 x 10 ft area | Cu yd per 10 x 10 ft area |
|---|---|---|---|
| 2 in | 0.167 ft | 16.7 | 0.62 |
| 3 in | 0.250 ft | 25.0 | 0.93 |
| 4 in | 0.333 ft | 33.3 | 1.23 |
| 6 in | 0.500 ft | 50.0 | 1.85 |
| 8 in | 0.667 ft | 66.7 | 2.47 |
Irregular shapes and L-shaped slabs
Break irregular footprints into rectangles. Measure each rectangle separately, calculate the cubic yards for each, then add them together. For example, an L-shaped patio that is 10 x 16 ft plus 8 x 6 ft at 4 inches thick is (10 x 16 x 0.333)/27 + (8 x 6 x 0.333)/27 = 1.97 + 0.59 = 2.56 cubic yards. Order 2.8 yards with the waste allowance.
Bagged vs ready-mix: which makes sense for your project?
For jobs under about 0.5 cubic yards (roughly 22 80-lb bags), bagged concrete is practical to mix by hand with a hoe or drill mixer. Above 1 cubic yard, hand-mixing becomes exhausting and slow, increasing the risk of cold joints if you cannot keep up with the pour rate. Most contractors call for ready-mix at anything over 1 yard. Ready-mix is priced by the yard (typically $150 to $200 per yard in 2025), while bagged concrete runs $9 per 80-lb bag at hardware stores, or about $405 per yard in material cost alone before labor and mixer rental. The break-even where ready-mix becomes cheaper is roughly 0.75 to 1 yard depending on your area.
Planning your order: questions to ask before you buy
Before you call a ready-mix plant or load a cart at the hardware store, nail down the following. First, confirm your exact dimensions in feet and double-check by measuring twice. Second, decide your slab thickness based on what the surface will carry (see concrete slab thickness by use). Third, ask the ready-mix plant about minimum load size: most plants have a minimum of 1 to 1.5 cubic yards and charge a short-load fee below that. Fourth, confirm the date and time of your pour and make sure you have enough helpers; a typical 4-yard patio pour needs three to four people to screed and finish before the concrete sets. Fifth, check the weather forecast: avoid pouring in rain or when temperatures will drop below 40 degrees Fahrenheit within 24 hours of the pour.
How concrete is sold: volume vs weight
Ready-mix concrete is sold by the cubic yard. Bagged concrete is sold by weight, but you buy a fixed yield per bag regardless of the bag weight. One 80-lb bag always yields about 0.60 cubic feet. The weight of the bag refers to the dry ingredients before water is added. When you add the required water (roughly 0.5 to 0.6 gallons per 80-lb bag), the final mixed volume is 0.60 cubic feet regardless of the exact water amount within the recommended range. This is why the bag yield in cubic feet is the number to use in your calculations, not the bag weight.