Concrete calculator
Enter the size and thickness of your slab, footing, or post holes to get cubic yards of ready-mix or the number of 40, 60, or 80 lb bags.
Last updated: · Free tool by InsightSpatial
Private by design. This tool runs entirely in your browser. What you enter stays on your device. It is not uploaded to us or stored.
How it is calculated
Volume = area × depth × how many, plus the extra percentage. In US units: cubic yards = length (ft) × width (ft) × depth (in) ÷ 12 ÷ 27, because a foot is 12 inches and a cubic yard is 27 cubic feet.
Example: 10 ft × 10 ft × 4 in = 100 sq ft × 0.33 ft = 33.3 cubic feet, or 1.23 cubic yards . With 10% extra: 1.36 cubic yards , about 2.72 tons at 2 tons per cubic yard , or 62 80 lb bags .
Fresh concrete weighs about 4,000 pounds (2 tons) per cubic yard.
Bag yields are typical for standard concrete mix: 0.6 cu ft for 80 lb, 0.45 for 60 lb, and 0.3 for 40 lb. Check the bag.
Typical depths
| Use | Depth |
|---|---|
| Sidewalks, patios, shed floors | 4 in |
| Driveways for cars | 4 to 5 in |
| Driveways for trucks or RVs | 5 to 6 in |
| Fence post holes | About one third of the post length, below the frost line |
Typical values. Footings, structural slabs, and frost depth are set by your local building code.
Tips
- For post holes, choose Circle, enter the hole diameter and depth, and the number of holes. Subtract the post if you want an exact figure; most people ignore it and keep the extra.
- Add 5 to 10% for spillage, uneven ground, and forms that bow. Running short in the middle of a pour is worse than a few leftover bags.
- Around a cubic yard or more, ready-mix delivered by truck is usually easier than mixing 45 or more 80 lb bags by hand. Ask local suppliers about minimum orders.
Questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet, so it takes 45 bags of 80 lb (0.6 cu ft each), 60 bags of 60 lb, or 90 bags of 40 lb.
How much concrete do I need for a 10 by 10 slab?
A 10 ft × 10 ft slab, 4 inches thick, needs 1.23 cubic yards (33.3 cubic feet). With 10% extra, plan on about 1.36 cubic yards or 62 bags of 80 lb.
How thick should a concrete slab be?
Patios and walkways are usually 4 inches thick; driveways 4 to 6 inches depending on the vehicles. Your local code may require more.