Concrete slab calculator
Enter the slab size and thickness. You’ll get cubic yards for a ready-mix order and bag counts if you’re mixing it yourself.
Concrete needed
1.36 cubic yards
- 80 lb bags
- 62 bags
- 60 lb bags
- 82 bags
- 40 lb bags
- 123 bags
- Volume
- 36.7 cubic feet
How it works
Volume is length × width × thickness, with thickness converted from inches to feet. The calculator adds your extra percentage, then divides by 27 to get cubic yards, which is how ready-mix concrete is ordered.
Bag counts use the yield printed on standard premix bags: an 80 lb bag makes about 0.6 cubic feet, a 60 lb bag 0.45, and a 40 lb bag 0.3.
Example
With slab length 10 ft, slab width 10 ft, thickness 4 in, extra for spillage and uneven ground 10%, the answer is 1.36 cubic yards.
Tips before you buy
- 4 inches is standard for patios, sidewalks and shed floors. Driveways that carry heavier vehicles are commonly poured 5–6 inches thick.
- Ground is rarely perfectly flat. 5–10% extra keeps you from running short with half a slab poured.
- Mixing more than about a cubic yard by hand is a long, hard day. Compare the cost of ready-mix delivery, and ask about short-load fees.
- Check your local building department before pouring. Some slabs need a permit, rebar or a minimum thickness.
Common questions
How many 80 lb bags of concrete are in a yard?
45. A cubic yard is 27 cubic feet and each 80 lb bag yields about 0.6 cubic feet. It takes 60 bags at 60 lb.
How much area does an 80 lb bag of concrete cover?
About 1.8 square feet at 4 inches thick, or 2.4 square feet at 3 inches.
How thick should a concrete patio be?
Four inches is the usual thickness for a patio on well-compacted ground. Go thicker if it will hold a hot tub or other heavy load.