How to calculate how much concrete you need
Concrete is sold by volume, so every calculation comes down to length × width × thickness. The one trick is keeping your units consistent: thickness is usually given in inches, so it has to be converted to feet before multiplying.
cubic feet = length(ft) × width(ft) × (thickness(in) ÷ 12)
cubic yards = cubic feet ÷ 27
Worked example
A 10 ft × 10 ft patio poured 4 inches thick: 10 × 10 × (4 ÷ 12) = 33.3 cubic feet. Divide by 27 and you get about 1.24 cubic yards. Add the standard 10% waste allowance and you should order roughly 1.36 cubic yards — which is why the calculator above rounds up rather than down.
How many bags of concrete are in a cubic yard?
Each bag lists how many cubic feet it yields once mixed. Since a cubic yard is 27 cubic feet, you divide 27 by the bag's yield to get bags per yard. These are the standard yields printed on most premixed bags:
| Bag size | Yield (cu ft) | Bags per cubic yard |
|---|
| 40 lb | 0.30 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
This table is also why bags stop making sense on bigger pours: much past half a cubic yard, the bag count climbs fast and a ready-mix delivery usually costs less per yard and saves hours of mixing.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
It takes 45 bags of 80 lb concrete mix to make one cubic yard, since each bag yields about 0.6 cubic feet and a cubic yard is 27 cubic feet.
How much does a cubic yard of concrete cost?
Ready-mix concrete delivered by truck typically runs about $125 to $175 per cubic yard, before any short-load or delivery fees. Buying premixed bags to fill the same yard usually costs more once you add up 45 to 60 bags.
How thick should a concrete slab be?
Patios and walkways are commonly poured 4 inches thick. Driveways and areas that carry vehicle weight are usually 5 to 6 inches. Always follow your local building code, which sets the minimum for your project.
Should I add extra concrete for waste?
Yes. Adding 5% to 10% covers uneven sub-grade, spillage, and slightly over-dug forms. Running short mid-pour creates a cold joint, so it is safer to order a little extra.