How Much Concrete Do I Need? (Formula, Examples & Chart)
To find how much concrete you need, multiply length × width × thickness (in feet) to get cubic feet, then divide by 27 for cubic yards. Add 5–10% for waste. A 10×10 ft slab at 4 inches needs about 1.23 cubic yards, or roughly 56 eighty-pound bags. For anything over one cubic yard, ready-mix delivery is usually cheaper than bags.
Ordering concrete is unforgiving: too little and you halt the pour and risk a weak cold joint; too much and you've paid for material you'll shovel into a wheelbarrow to dispose of. Getting the volume right the first time is the whole game.
This guide gives you the simple formula, a couple of worked examples, and a coverage chart so you can figure out exactly how much concrete you need for a slab, footing, or set of post holes — then decide whether to buy bags or order ready-mix.
Key takeaways
- Concrete volume = length × width × thickness (in feet) ÷ 27 for cubic yards.
- Always add 5–10% for waste and uneven subgrade.
- A cubic yard ≈ 45 eighty-pound bags or 60 sixty-pound bags.
- Under ~1 yd³, bags are convenient; above that, order ready-mix.
- Use 4-inch thickness for patios, 5–6 inches for driveways.
Skip the math
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The basic formula
Concrete is ordered by volume. Measure your slab in feet, convert the thickness from inches to feet (divide by 12), then multiply all three dimensions for cubic feet. Divide by 27 to get cubic yards — the unit ready-mix is sold in. Then add 5–10% for waste.
Formula: Cubic yards = (Length ft × Width ft × Thickness ft) ÷ 27 × waste factor.
Worked example
Say you're pouring a 10 ft × 10 ft patio at 4 inches thick. Convert 4 inches to feet (4 ÷ 12 = 0.333 ft). Multiply: 10 × 10 × 0.333 = 33.3 cubic feet. Divide by 27 = 1.23 cubic yards. Add 10% waste and you'd order about 1.36 cubic yards — or roughly 56 eighty-pound bags if you're mixing by hand.
Concrete needed by slab size
| Slab size (4" thick) | Cubic yards | 80 lb bags |
|---|---|---|
| 8 × 8 ft | 0.79 yd³ | ~36 |
| 10 × 10 ft | 1.23 yd³ | ~56 |
| 12 × 12 ft | 1.78 yd³ | ~80 |
| 20 × 20 ft | 4.94 yd³ | ready-mix |
Bags vs ready-mix
- Under ~1 cubic yard: bagged concrete is convenient.
- 1 cubic yard or more: ready-mix delivery is usually cheaper and faster.
- A cubic yard ≈ 45 eighty-pound bags or 60 sixty-pound bags.
- Mixing many bags risks cold joints between batches — order a truck instead.