Concrete Calculator
Find how many cubic yards of concrete, or how many bags, you need for a slab, footing, or pad. Enter your measurements for an instant result.
Tip: for anything above about 1 cubic yard, ready-mix delivery is usually cheaper than bags. Add 5 to 10% for spillage and uneven sub-grade. Thickened edges and footings need a separate calculation.
How to calculate concrete
Concrete is ordered by volume, in cubic yards for ready-mix or in bags for small jobs. You measure the job in feet and inches, then convert to cubic yards. The formula is:
Cubic yards = Length (ft) × Width (ft) × Thickness (ft) ÷ 27
Thickness is usually in inches, so divide by 12 to get feet. There are 27 cubic feet in a cubic yard. The calculator above handles the conversion and adds a waste allowance.
Worked example
A slab 10 ft by 10 ft, poured 4 inches thick:
| Area | 10 × 10 = 100 ft² |
|---|---|
| Thickness | 4 in ÷ 12 = 0.333 ft |
| Volume | 100 × 0.333 = 33.3 ft³ |
| Cubic yards | 33.3 ÷ 27 = 1.23 yd³ |
With a 10% allowance that is about 1.36 cubic yards, or roughly 62 eighty-pound bags.
Bags per cubic yard
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
Frequently asked questions
How many bags of concrete are in a cubic yard?
It takes about 45 eighty-pound bags, 60 sixty-pound bags, or 90 forty-pound bags to make one cubic yard of concrete.
How much concrete do I need for a 10x10 slab?
A 10 by 10 foot slab at 4 inches thick needs about 1.23 cubic yards of concrete, or roughly 1.36 yards with a waste allowance. Use the calculator above for other thicknesses.
Should I order bags or ready-mix?
For small jobs under about 1 cubic yard, bags are convenient. Above that, ready-mix delivery is usually cheaper and faster than mixing dozens of bags by hand.
How thick should a concrete slab be?
Patios and walkways are usually 4 inches thick. Driveways and areas carrying vehicles are typically 5 to 6 inches thick.