Concrete Volume Formula
Multiply length × width × thickness — then add a 10% waste factor.
V = L × W × T × 1.10
How it works
The calculation itself is only length × width × thickness, but two things trip people up: the thickness is almost always quoted in inches while the other two are in feet, and ready-mix concrete is sold by the cubic yard. Convert the thickness first, then divide the cubic feet by 27. A slab 20 feet by 10 feet at 4 inches thick is 20 × 10 × 0.3333 = 66.67 cubic feet, which is 2.47 cubic yards. Then add a waste allowance. Subgrade is never perfectly flat, forms bow slightly under load, and some concrete is always lost in handling, so 5 to 10% is normal and 10% is the safer figure on a small pour. That takes the example to 2.72 cubic yards, and since most suppliers sell in quarter-yard increments you would order 2.75 or simply 3. The asymmetry matters here. Ordering too much wastes the price of a fraction of a yard. Ordering too little means the pour stops while a second truck is dispatched, and the join between concrete that has begun to set and fresh concrete is a cold joint — a permanent plane of weakness through the slab. Over-ordering is the cheap mistake. If you work in metric, one cubic yard is 0.7646 m³.
Variables
- VConcrete volume to order
- L, WSlab length and width
- TSlab thickness
Worked examples
| Input | Result |
|---|---|
| 20 ft × 10 ft × 4 in | = 200 ft² × 0.333 ft × 1.10 ≈ 73.3 ft³ ≈ 2.72 yd³ |
| 5 m × 4 m × 0.1 m | = 2 m³ × 1.10 = 2.2 m³ |
Practical applications
- Ordering ready-mix for a driveway, patio, garage floor or shed base
- Estimating footing and pier volumes, where the thickness is large relative to the area
- Costing a slab-on-grade before committing to a quote
- Checking a contractor's material estimate against your own numbers
Common mistakes
- Leaving the thickness in inches. Four inches is 0.3333 feet; using 4 gives twelve times too much concrete.
- Forgetting to divide cubic feet by 27 to reach cubic yards.
- Skipping the waste factor on a small pour, where a slightly dished subgrade is a larger proportion of the total.
- Rounding down to save money. Running short mid-pour creates a cold joint, which is a structural defect rather than a cosmetic one.
Used in
Use it in a tool
FAQs
Why the 10% waste factor?+
Forms move, sub-grade isn't perfectly flat, and you can't return concrete. 10% is the industry default; bump to 15% on small or irregular pours.
Cubic yards or cubic meters?+
US ready-mix is ordered in cubic yards (1 yd³ = 0.7646 m³). Metric markets order in m³.