How it works
A slab or footing is a rectangular box, so its volume is length × width × thickness. Convert the thickness from inches to feet, multiply, then divide cubic feet by 27 to get cubic yards, the unit ready-mix trucks sell.
Add an allowance for waste, uneven subgrade, and spillage. For bagged concrete, divide the cubic feet by the yield of one bag and round up: an 80 lb bag yields about 0.60 ft³ and a 60 lb bag about 0.45 ft³.
Worked example
A 20 ft × 10 ft patio slab poured 4 inches thick with a 10% allowance:
- Volume = 20 ft × 10 ft × (4 in ÷ 12) × 1 = 66.67 ft³
- With 10% allowance = 66.67 × 1.1 = 73.33 ft³
- Cubic yards = 73.33 ÷ 27 = 2.72 yd³
- 80 lb bags = 73.33 ÷ 0.60 = 122.2, rounded up to 123; 60 lb bags at 0.45 ft³ = 163
| Input | Value |
|---|---|
| Length | 20 ft |
| Width | 10 ft |
| Thickness or depth | 4 in |
| Number of identical pieces | 1 |
| Waste and over-excavation allowance | 10 % |
| Result | Value |
|---|---|
| Concrete to order (with allowance) | 2.72 yd³ |
| Volume with allowance | 73.3 ft³ |
| Volume, no allowance | 66.7 ft³ |
| 80 lb bags | 123 bags |
| 60 lb bags | 163 bags |
Assumptions and limits
- Rectangular shapes only. For a round pier or an irregular shape, work out the volume separately and use this for the cubic yards conversion.
- Bag yields are the manufacturer’s nominal figures for standard concrete mix; other products differ, so check the bag.
- This calculates quantity, not structural adequacy: thickness, reinforcement, and subbase come from your plans and local code.
Common questions
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb concrete or 60 bags of 60 lb, at yields of 0.60 and 0.45 ft³ per bag.
How much extra concrete should I order?
Many builders add 5 to 10 percent for uneven subgrade, spillage, and forms that move. It is a judgment call; this tool lets you set it.
How do I calculate a footing?
Use the footing’s length, width, and depth. For a continuous footing, enter the total run as the length and the footing width. For several identical footings, use the count field.
How thick should a slab be?
That depends on the use, soil, and local code, so this calculator does not pick it for you. Check your plans or your building department.
Sources
- Volume of a rectangular prism; 1 cubic yard = 27 cubic feet.
- Quikrete Concrete Mix data sheet (yields of about 0.60 ft³ per 80 lb bag and 0.45 ft³ per 60 lb bag).
- Read them: Quikrete Concrete Mix data sheet
Updated 2026-09-30