How to calculate concrete for a slab
- Measure the length and width of the slab in feet.
- Choose the thickness in inches and divide by 12 to get feet.
- Multiply length × width × thickness for cubic feet, then divide by 27 for cubic yards.
- For premixed bags, divide the cubic feet by the yield of one bag and round up.
Example: a 10 × 10 ft patio, 4 inches thick. 10 × 10 × 0.333 = 33.33 cu ft = 1.23 cubic yards. With 80 lb bags at 0.6 cu ft each that is 55.6, so buy 56 bags.
Concrete slab calculator: common sizes
Cubic yards and the number of 80 lb bags for common slab sizes, before any extra for waste.
| Slab size | 4 in: cu yd | 4 in: 80 lb bags | 6 in: cu yd | 6 in: 80 lb bags |
|---|---|---|---|---|
| 4 × 4 ft | 0.2 | 9 | 0.3 | 14 |
| 10 × 10 ft | 1.23 | 56 | 1.85 | 84 |
| 10 × 12 ft | 1.48 | 67 | 2.22 | 100 |
| 12 × 12 ft | 1.78 | 80 | 2.67 | 120 |
| 12 × 20 ft | 2.96 | 134 | 4.44 | 200 |
| 20 × 20 ft | 4.94 | 223 | 7.41 | 334 |
How many bags of concrete in a cubic yard
Premixed concrete is sold in bags that each yield a set volume. These are typical label values; always check the bag you are buying.
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb bag | 0.3 cu ft | 90 |
| 60 lb bag | 0.45 cu ft | 60 |
| 80 lb bag | 0.6 cu ft | 45 |
Footings, piers and round holes
For a round post hole or pier, use the Circle tab: enter the diameter and the depth. For a long trench footing, use the Rectangle tab with the trench length, width and depth.
Bags or ready-mix?
Bags suit small jobs. For larger pours, ready-mix concrete is ordered by the cubic yard, which is the first number this calculator gives you. Ask the supplier about minimum loads and delivery charges.
How much extra should you order?
Subgrades are rarely perfectly level and some mix is lost in the forms, so many people add roughly 5–10% to the total. Enter it in the options to see the adjusted yards and bags.
Working in other units? The cubic yard calculator gives cubic feet, yards and cubic meters. For stone, use the gravel calculator.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness in feet to get cubic feet, then divide by 27 for cubic yards. A 10 × 10 ft slab that is 4 inches thick is 10 × 10 × (4 ÷ 12) = 33.33 cu ft, or 1.23 cubic yards.
How many bags of concrete are in a cubic yard?
Using typical label yields, a cubic yard takes about 45 bags of 80 lb mix, 60 bags of 60 lb mix or 90 bags of 40 lb mix. Yields vary slightly by brand, so check the bag.
How many 80 lb bags of concrete do I need for a 10x10 slab?
A 10 × 10 ft slab at 4 inches thick needs 33.33 cu ft, which is 56 bags of 80 lb mix at 0.6 cu ft per bag. At 6 inches thick it is 84 bags.
How thick should a concrete slab be?
Patios and walkways are commonly poured about 4 inches thick, and driveways and garage floors thicker. The right thickness depends on the load and local building rules, so check with your supplier or building department.
How do I calculate concrete for a round footing or post hole?
Choose the Circle tab, enter the diameter and the depth, and the calculator uses π × (diameter ÷ 2)² × depth.
Should I order extra concrete?
Yes. Ground is rarely perfectly level and some mix is lost in forms, so many people add roughly 5–10%. Enter it in the waste box and the bag count updates.
Does this calculator upload my measurements?
No. The maths runs in your browser. Nothing is sent to a server, and it works offline once the page has loaded.
Written by Prakhar Singh, software engineer and creator of K4Converter · Last updated November 2026