Concrete Calculator
Find out how many bags of concrete to buy for a slab, footing or column. Free, with no signup.
Enter dimensions to see how many bags to buy.
Concrete needed
Bags to buy
Check the yield printed on your bag.
Default yields are for standard mixes. Some mixes differ.
Estimates only. Check your bag and local requirements before ordering.
How the calculation works
First the calculator finds the volume in cubic feet:
Slab: Length × Width × Thickness ÷ 12
Footing: Length × (Width ÷ 12) × (Depth ÷ 12)
Column: 3.1416 × (Diameter ÷ 24)² × Height
Length, slab width and column height are in feet. Thickness, footing width, depth and diameter are in inches. Then it adds waste, divides by what one bag makes, and rounds up, because you can't buy part of a bag.
Bags = Volume × (1 + Waste ÷ 100) ÷ Bag yield
What one bag makes
| Bag size | Yield (cu ft) |
|---|---|
| 80 lb | 0.60 |
| 60 lb | 0.45 |
| 40 lb | 0.30 |
Example: a 10 × 10 ft slab, 4 inches thick
With 10% waste you need 62 bags of 80 lb, 82 bags of 60 lb or 123 bags of 40 lb.
More concrete answers
Questions
How do I calculate how many bags of concrete I need?
Multiply the volume in cubic feet by 1 plus your waste percentage, divide by the yield of one bag, and round up.
How much will a 60 lb bag of concrete cover?
A standard 60 lb bag makes about 0.45 cubic feet. At 4 inches thick, that covers about 1.35 square feet.
How many 80 lb bags do I need for a 10x10 slab?
At 4 inches thick you need 62 bags with 10% waste, or 56 bags with no waste.
How much extra concrete should I buy?
The calculator starts at 10% waste. You can change it from 0 to 30%.