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.
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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In plain language
How Much Concrete Do I Need? Formula, Bags and Slab Chart can be understood by starting with the definition on this page, keeping every input in the same unit, and checking the result against a reasonable estimate. Short sentences and one-step checks make the method easier to follow and reduce avoidable mistakes.
Limitations and assumptions
Important: This guide about How Much Concrete Do I Need? Formula, Bags and Slab Chart is for education. It uses the definitions and assumptions shown here. Dates, rates, rules, health facts, and local conditions can change. Verify important decisions with current primary sources or a qualified professional.
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