Concrete Calculator
Enter the size of a slab, footing, post hole or round tube to get the concrete in cubic yards and cubic feet, and how many 40, 50, 60 or 80 lb bags to buy.
In feet. For 10 ft 6 in, enter 10.5.
A patio or walk is often 4 inches. Bagged concrete mix is made for 2 inches or more.
For 8 post holes of the same size, enter 8.
For spills, uneven ground and forms that bow. Enter 0 for the exact volume.
Concrete you need
1.36 cubic yards
That is 36.67 cubic feet, with 10% extra. In premixed bags: 62 bags of 80 lb, or 82 bags of 60 lb.
| One slab: 10 ft × 10 ft × 4 in | 33.33 ft³ |
|---|---|
| 10% extra for waste | +3.33 ft³ |
| Concrete to buy | 36.67 ft³ |
| ÷ 27 cubic feet in a cubic yard | 1.36 yd³ |
| Bag | Yield per bag | Bags you need |
|---|---|---|
| 80 lb | 0.60 ft³ | 62 |
| 60 lb | 0.45 ft³ | 82 |
| 50 lb | 0.375 ft³ | 98 |
| 40 lb | 0.30 ft³ | 123 |
Bag counts are rounded up to whole bags. Yields are the approximate figures on the QUIKRETE Concrete Mix and Sakrete High-Strength Concrete Mix data sheets.
How this is calculated
Volume is length × width × thickness, with every measurement in feet. Thickness is usually given in inches, so it is divided by 12. A 10 × 10 ft slab that is 4 inches thick holds 10 × 10 × 4 ÷ 12 = 33.33 cubic feet. With 10% extra that is 36.67 cubic feet, and divided by 27 it is 1.36 cubic yards.
Round shapes use the area of a circle, π × radius². A post hole 12 inches across and 4 feet deep has a radius of 0.5 ft: 3.1416 × 0.5² × 4 = 3.14 cubic feet. A hollow tube takes the inner circle away from the outer one.
| Shape | Formula |
|---|---|
| Slab | length ft × width ft × thickness in ÷ 12 |
| Footing or wall | length ft × width in ÷ 12 × depth in ÷ 12 |
| Round column or hole | π × (diameter in ÷ 24)² × height ft |
| Round tube | π × ((outside in ÷ 24)² − (inside in ÷ 24)²) × height ft |
Bags of premixed concrete
The bag counts use the yields printed on the manufacturers' data sheets for QUIKRETE Concrete Mix (No. 1101) and Sakrete High-Strength Concrete Mix. Both list the same yields for the 40, 60 and 80 lb bags; the 50 lb bag is QUIKRETE's. A cubic yard is 27 cubic feet:
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 50 lb (QUIKRETE) | 0.375 ft³ | 72 |
| 40 lb | 0.30 ft³ | 90 |
Both makers call these yields approximate. Each also sells a 90 lb bag in some regions (QUIKRETE about 0.675, Sakrete about 0.66 cubic feet). Other products, such as fast-setting mixes, can yield differently, so check the bag you buy. Bag counts are rounded up to whole bags.
Extra for waste
The extra covers spills, ground that is not perfectly level and forms that bow out. The calculator starts at 10%; enter 0 to see the exact volume. It does not subtract a post or rebar, so a post hole gets a little more concrete than it needs. Ready-mix concrete from a truck is ordered in cubic yards.
Sources
Checked against the sources below on .
Questions people ask
How many bags of concrete do I need for a 10x10 slab?
At 4 inches thick, a 10 × 10 ft slab is 33.33 cubic feet. That is 56 bags of 80 lb (about 0.60 cubic feet each) without extra, or 62 with 10% extra for waste.
How many 80 lb bags of concrete make a yard?
45. A cubic yard is 27 cubic feet, and an 80 lb bag of QUIKRETE Concrete Mix or Sakrete High-Strength Concrete Mix yields about 0.60 cubic feet. With 60 lb bags it takes 60.
How much concrete do I need for a fence post?
For a hole 10 inches across and 3 feet deep: π × (10 ÷ 24)² × 3 = 1.64 cubic feet, about 3 bags of 80 lb. The post itself takes up some of that space.
How do I convert cubic feet to cubic yards?
Divide by 27, because a cubic yard is 3 × 3 × 3 feet. 54 cubic feet is 2 cubic yards.
How many cubic feet are in an 80 lb bag of concrete?
About 0.60 cubic feet, according to the QUIKRETE Concrete Mix and Sakrete High-Strength Concrete Mix data sheets. A 60 lb bag yields about 0.45 and a 40 lb bag about 0.30 cubic feet.