Concrete Calculator
Calculate how many cubic yards of concrete you need for a slab, patio, driveway or footing, plus how many 80 lb, 60 lb, 50 lb or 40 lb bags to buy.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Concrete needed
- 1.36 cu yd
- 36.7 cu ft including 10% waste
- Without waste
- 1.23 cu yd
- 33.3 cu ft
- 80 lb bags to buy
- 62 bags
- Approximate weight
- 2.5 tons
- At 150 lb per cubic foot
Concrete formula
Cubic feet = Length (ft) x Width (ft) x Thickness (in) / 12
Cubic yards = Cubic feet / 27
Order quantity = Cubic yards x (1 + waste %)
Bags = Cubic feet / bag yield (80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft, 40 lb = 0.30 cu ft)Worked example
A 12 x 12 ft patio poured 4 in thick: 12 x 12 x (4 / 12) = 48 cubic feet. 48 / 27 = 1.78 cubic yards. Add 10% waste for 1.96 yards, so order 2 cubic yards of ready-mix.
In bags: 48 x 1.1 = 52.8 cubic feet. 52.8 / 0.60 = 88 bags of 80 lb mix, or 118 bags of 60 lb mix.
Quick reference
| Thickness | Cubic feet | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 4 in | 33.3 | 1.23 | 56 | 75 |
| 5 in | 41.7 | 1.54 | 70 | 93 |
| 6 in | 50.0 | 1.85 | 84 | 112 |
| 8 in | 66.7 | 2.47 | 112 | 149 |
How to calculate concrete
Concrete is sold by the cubic yard. Multiply length by width by thickness (all in feet) to get cubic feet, then divide by 27 to convert to cubic yards. A 10 ft by 10 ft slab at 4 inches thick is 10 x 10 x 0.333 = 33.3 cubic feet, or 1.23 cubic yards.
Always add 5 to 10 percent for waste. Subgrade is never perfectly level, forms bow slightly, and some concrete stays in the mixer or chute. Running short mid-pour is far more expensive than a little extra.
Bags vs ready-mix
Bagged concrete makes sense for small jobs under about half a cubic yard: fence posts, small pads, steps. An 80 lb bag yields 0.60 cubic feet, so one cubic yard takes 45 bags. For anything larger, ready-mix delivered by truck is cheaper per yard and far less work.
Typical thickness: 4 inches for patios, walkways and shed slabs; 5 to 6 inches for driveways and garage floors; 8 inches or more where heavy trucks will park.
Key facts
- One cubic yard of concrete is 27 cubic feet and weighs about 4,050 lb (2 tons).
- One cubic yard covers 81 sq ft at 4 in thick, 65 sq ft at 5 in, and 54 sq ft at 6 in.
- An 80 lb bag of concrete mix yields 0.60 cubic feet, so 45 bags make one cubic yard.
- A 60 lb bag yields 0.45 cubic feet, so 60 bags make one cubic yard.
- Standard slab thickness is 4 in for patios and walkways and 5 to 6 in for driveways.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
45 bags. Each 80 lb bag yields about 0.60 cubic feet and a cubic yard is 27 cubic feet.
How much does a cubic yard of concrete cover?
At 4 inches thick, one cubic yard covers 81 square feet. At 6 inches it covers 54 square feet.
How thick should a concrete slab be?
4 inches is standard for patios, sidewalks and shed floors. Use 5 to 6 inches for driveways and anywhere vehicles will drive or park.
How much does a yard of concrete weigh?
About 4,050 pounds, or roughly 2 tons, at the standard density of 150 pounds per cubic foot.
Sources and references
Next steps for this project
The usual order for a concrete & masonry project. See all concrete & masonry calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.