Quick answer: To find how much concrete you need, multiply length × width × thickness in feet for cubic feet, then divide by 27 for cubic yards, and add 5–10% for waste. Example: a 10 × 10 ft slab, 4 inches thick = 100 × 0.33 ÷ 27 ≈ 1.23 cubic yards.
Quick answer: To find how much concrete you need, multiply length × width × thickness (in the same units) to get volume, then convert to cubic yards (divide cubic feet by 27) or cubic metres. For a 10 ft × 10 ft slab at 4 inches thick, that’s about 1.23 cubic yards — and you should add 5–10% extra for spillage and uneven ground.
Order too little and your pour fails; order too much and you pay for waste. This guide gives the simple formula, the bag math for small jobs, and a quick reference table.
The basic formula
Concrete is sold by volume, so every estimate comes down to length × width × depth:
- Measure length and width in feet, and thickness in inches.
- Convert thickness to feet (divide inches by 12).
- Multiply the three numbers for cubic feet.
- Divide cubic feet by 27 for cubic yards (how ready-mix is ordered).
Example: a 10 × 10 ft slab, 4 inches thick → 10 × 10 × (4÷12) = 33.3 cubic feet → 33.3 ÷ 27 = 1.23 cubic yards. The free Concrete calculator does this for slabs, footings, columns and stairs automatically.
Quick reference: common slabs at 4 inches thick
| Slab size | Cubic feet | Cubic yards | Cubic metres |
|---|---|---|---|
| 8 × 8 ft | 21.3 | 0.79 | 0.60 |
| 10 × 10 ft | 33.3 | 1.23 | 0.94 |
| 12 × 12 ft | 48.0 | 1.78 | 1.36 |
| 20 × 20 ft | 133.3 | 4.94 | 3.77 |
How many bags for a small job?
For small repairs and posts, bagged concrete is easier than ordering ready-mix. Approximate yields:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | ≈ 0.30 cu ft | ≈ 90 |
| 60 lb | ≈ 0.45 cu ft | ≈ 60 |
| 80 lb | ≈ 0.60 cu ft | ≈ 45 |
Rule of thumb: ordering more than about 1 cubic yard? Ready-mix delivery is usually cheaper and faster than mixing dozens of bags.
Don’t forget the waste factor
Always add 5–10% to your estimate. Reasons: subgrade is never perfectly level, forms bulge slightly, and you cannot stop a pour to mix more. For structural footings, round up generously — running short mid-pour is far more costly than a little excess.
Measuring an irregular area first? Use the square footage calculator, and for the base layer try the gravel calculator.
Frequently asked questions
How much concrete do I need for a 10×10 slab?
At 4 inches thick, a 10 ft × 10 ft slab needs about 1.23 cubic yards (0.94 m³). Add 5–10% for waste, so order around 1.3 cubic yards.
How do I calculate cubic yards of concrete?
Multiply length × width × thickness in feet to get cubic feet, then divide by 27. Convert thickness from inches to feet first (inches ÷ 12).
How many 80 lb bags are in a cubic yard?
About 45 bags of 80 lb concrete make one cubic yard, since each yields roughly 0.6 cubic feet.
Should I order extra concrete?
Yes. Add 5–10% for spillage and uneven subgrade. Running out during a pour is far more expensive than a small surplus.
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