Concrete Calculator
Estimate volume for a rectangular concrete slab and an approximate bag count.
Calculate your materials
Your estimate
Enter your measurements and select Calculate estimate to see the results.
Estimates are for planning only; verify product specifications and local requirements.
How this concrete calculator works
Concrete is ordered by volume. For a basic rectangular slab, multiply length, width and thickness, then convert the result to cubic yards. Bag yield varies by brand and mix, so compare this estimate with the yield stated on the packaging before purchasing.
The calculation
Cubic feet = length × width × thickness in inches ÷ 12. Cubic yards = cubic feet ÷ 27. Apply waste percentage to order volume; 80 lb bag estimate = round up(order cubic feet ÷ bag yield).
Worked example
A 12 ft × 10 ft slab at 4 in thick is 40 cu ft or 1.48 cu yd before overage. With 5% extra, the estimated order is 1.56 cu yd and 70 bags at 0.6 cu ft per bag.
Before buying materials
- Verify slab thickness and reinforcement with a qualified contractor and applicable codes.
- Bagged mixes vary; read the manufacturer’s yield statement.
- Large pours may be more practical with ready-mix delivery.
Frequently asked questions
Why are cubic yards used?
Ready-mix concrete is commonly sold in cubic yards in the United States.
Does it include footings?
No. Calculate footings and thickened edges as separate shapes and add the volumes.
Is the bag count exact?
No. It is based on the yield you entered; actual packaging, waste and conditions affect the order.