Working out how much concrete you need comes down to two steps: find the volume, then convert it to the units concrete is actually sold in — bags or cubic yards of ready-mix.
Step 1 — find the volume
Multiply length × width × thickness, with the thickness converted from inches to feet:
volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12A 10 × 10 ft slab at 4 inches is 10 × 10 × (4 ÷ 12) = 33.3 ft³. Concrete is
priced by the cubic yard, and there are 27 cubic feet in a yard, so that’s
33.3 ÷ 27 = 1.23 cubic yards.
Step 2 — convert to bags or yards
A bag of standard concrete mix lists its yield — the cubic feet it fills:
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
So our 33.3 ft³ slab needs 33.3 ÷ 0.60 = 56 bags of 80 lb mix. These yields come
straight from the QUIKRETE and Sakrete data sheets and are the same for any
standard mix — but fast-setting and specialty mixes yield differently.
Always add a waste factor
Subgrade is never perfectly flat and some concrete is always lost, so add 10% (a 5–15% range is normal). Running short means a cold joint or an expensive second trip — so round up, never down. With 10% waste our slab needs 62 bags, or 1.5 cubic yards of ready-mix.
Bags or ready-mix?
- Bags are simplest for small pours — under about half a cubic yard.
- Ready-mix delivered by truck is cheaper and far less work at a cubic yard or more. Watch for a 1 cubic yard minimum and short-load fees below that.
A ~1.2 yard slab like our example sits right at the crossover; most people order ready-mix.
Worked examples (4-inch slab)
| Slab | Volume | 80 lb bags (10%) | Ready-mix |
|---|---|---|---|
| 10 × 10 | 33.3 ft³ (1.23 yd³) | 62 | 1.5 yd³ |
| 12 × 12 | 48 ft³ (1.78 yd³) | 88 | 2.0 yd³ |
| 20 × 20 | 133 ft³ (4.94 yd³) | 245 | 5.5 yd³ |
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard? 45. A cubic yard is 27 ft³ and an 80 lb bag yields 0.60 ft³ (27 ÷ 0.60 = 45). It’s 60 bags for 60 lb, or 90 bags for 40 lb.
How thick should a slab be? 4 inches is standard for patios, sheds, and walkways; driveways and anything bearing heavy loads are usually 5–6 inches over a compacted base.
How much does concrete cost? Bagged mix runs a few dollars a bag; delivered ready-mix is roughly $125–200+ per cubic yard, plus short-load fees on small orders. Prices vary a lot by region.
Get the exact count for your project with the Concrete Calculator — or jump to a common size like 10×10 or 12×12.