How to use the concrete calculator
- Length and width of the slab in feet. For inches, divide by 12 (6 in = 0.5 ft).
- Thickness in inches — the depth of concrete, not the height of the forms above the ground.
- Extra for waste — a percentage added to cover spillage, low spots in the base and forms that bulge under the weight of wet concrete.
The concrete volume formula
cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
cubic yards = cubic feet ÷ 27, because a cubic yard is 3 ft × 3 ft × 3 ft.
Multiply by 1 + waste ÷ 100 to add the allowance. To count bags, divide the cubic feet by the bag's yield and always round up — a partial bag still has to be bought.
Worked example: a 10 × 10 ft patio
- Volume: 10 × 10 × 4 ÷ 12 = 33.33 cubic feet.
- Add 10%: 33.33 × 1.10 = 36.67 cubic feet, or 36.67 ÷ 27 = 1.36 cubic yards.
- 80-lb bags: 36.67 ÷ 0.60 = 61.1, so 62 bags.
- 60-lb bags: 36.67 ÷ 0.45 = 81.5, so 82 bags.
Sixty-two 80-lb bags weigh 4,960 pounds, all of which has to be loaded, carried and mixed. At this size, get a quote for ready-mix delivered by truck too; many suppliers have minimum orders and short-load fees, so ask before you decide.
Bag yields
| Bag size | Approximate yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
These are the yields commonly printed on standard premix bags. Check the bag you buy, since specialty mixes such as fast-setting or high-strength concrete can differ.
How thick should the slab be?
Four inches is the usual thickness for patios, sidewalks and shed floors. Driveways that carry cars are commonly poured 5 to 6 inches thick, and slabs under heavy vehicles or equipment thicker still. Local building codes and the design of the project come first, especially for footings and anything structural. Every inch adds a quarter more concrete to a 4-inch slab:
| Thickness | Cubic yards per 100 sq ft | 80-lb bags per 100 sq ft |
|---|---|---|
| 4 in | 1.23 | 56 |
| 5 in | 1.54 | 70 |
| 6 in | 1.85 | 84 |
The table has no waste allowance; add your percentage on top.
Why order extra
A base is rarely perfectly level. Over a 10 × 10 ft slab, every extra half inch of depth needs another 4.2 cubic feet — about seven more 80-lb bags. Running short in the middle of a pour leaves a cold joint where fresh concrete meets concrete that has started to set, which is weaker and more likely to crack. Leftover bags of dry mix keep if stored dry; a shortfall is much harder to fix.
Round holes and columns
For post holes, footings or round columns, use the cylinder formula with every measurement in feet: volume = π × (diameter ÷ 2)² × depth. A hole 10 inches across and 3 feet deep holds π × (0.417)² × 3 ≈ 1.64 cubic feet — three 80-lb bags. If you are setting a post, the post itself takes up some of that space.
Frequently asked questions
How many bags of concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab is 33.3 cubic feet. That is 56 bags of 80 lb with no allowance, or 62 bags with 10% extra. With 60-lb bags, it is 75 bags without the allowance and 82 with it.
How do I convert cubic feet to cubic yards?
Divide by 27. 54 cubic feet is 2 cubic yards.
How many 80-lb bags make a cubic yard?
About 45, since each bag yields roughly 0.60 cubic feet and a cubic yard is 27 cubic feet. With 60-lb bags it takes about 60.
What is the difference between cement and concrete?
Cement is the powder that binds concrete together. Concrete is cement mixed with sand, gravel and water. Premix concrete bags already contain the cement, sand and gravel — you only add water.
How much extra concrete should I buy?
Five to ten percent is a common allowance for a flat slab on a well-prepared base. Go toward the higher end if the ground is uneven or the forms are long and could bow.
Last updated: October 8, 2026