How to calculate concrete
Concrete is sold by volume. Multiply length × width × thickness, making sure every measurement is in feet, to get cubic feet. Divide by 27 to get cubic yards, the unit ready-mix suppliers use.
10 ft × 10 ft × (4 ÷ 12) ft = 33.3 ft³ ÷ 27 = 1.23 yd³
For bags, divide the cubic feet by the bag yield: 33.3 ÷ 0.6 = 56 bags of 80 lb mix. Always round up.
Post holes
A round hole is a cylinder: π × radius² × depth. The post takes up some of that space, so the calculator subtracts the post's volume. A 10-inch hole 30 inches deep with a 4×4 post needs about 1.2 cubic feet, or two 80 lb bags.
Typical slab thickness
| Project | Usual thickness |
|---|
| Walkway, patio, shed base | 4 in |
| Hot tub pad, light garage | 4–6 in |
| Driveway | 5–6 in |
| Footing | Per local code, often 8–12 in |
Tips before you pour
- Add 5–10% extra. Running short in the middle of a pour leaves a weak joint.
- An 80 lb bag is heavy. For more than about 40 bags, compare the price of ready-mix delivery.
- Lay a compacted gravel base first. The gravel calculator can size it.
- Check local codes for thickness, reinforcement and permits on driveways and structural footings.
Last reviewed October 2026.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10 × 10 ft slab that is 4 inches thick needs about 33.3 cubic feet, or 1.23 cubic yards, of concrete. That is about 56 bags of 80 lb premix, 75 bags of 60 lb or 112 bags of 40 lb, before adding extra for waste.
How much concrete is in an 80 lb bag?
An 80 lb bag of standard premixed concrete makes about 0.6 cubic feet. A 60 lb bag makes about 0.45 cubic feet and a 40 lb bag about 0.3 cubic feet. Check the label of your brand, as yields vary slightly.
How much extra concrete should I order?
Add 5–10% for spillage, uneven ground and forms that bow out. This calculator adds 10% by default. You can change it to 0% if you want the exact volume.
When should I order ready-mix instead of bags?
For about 1 cubic yard or more (roughly 45 bags of 80 lb), a ready-mix truck is often easier and can be cheaper once you count your time. Many suppliers have a minimum order or a short-load fee for small amounts, so get a quote.
How thick should a concrete slab be?
Four inches is common for patios, walkways and shed floors. Driveways and garage floors are usually 5–6 inches. Local building codes and the load the slab will carry decide the right thickness, so check them before you pour.