How the Concrete Calculator Works
Before you pour a slab, a patio or a path, you need to know how much concrete to buy. Too little, and you have to stop in the middle of a pour, which leaves a weak joint. Too much costs money. The Concrete Calculator works out the volume of the slab, and then the number of bags of ready-mix that fill it.
What you enter
- The shape: a rectangle (length and width), a circle (diameter), or an area that you already know.
- The thickness of the slab, in inches or centimeters.
- Extra for waste (optional), as a percent.
- The price of a bag and of a delivered cubic yard (optional), to compare the costs.
The formulas
volume (cubic feet) = length x width x thickness (inches) / 12 cubic yards = cubic feet / 27 bags = cubic feet / yield of one bag, rounded up circle area = 3.1416 x (diameter / 2) x (diameter / 2)
A cubic yard is 27 cubic feet. The number of bags is always rounded up, because you cannot buy part of a bag. If you enter an extra percent, the volume is multiplied by 1 plus that percent (1.10 for 10%).
Worked example: a patio of 10 ft by 10 ft, 4 inches thick
| Step | Calculation | Result |
|---|---|---|
| Area | 10 x 10 | 100 sq ft |
| Volume | 100 x 4 / 12 | 33.33 cubic feet |
| Cubic yards | 33.33 / 27 | 1.23 cubic yards |
| 80 lb bags | 33.33 / 0.6 = 55.6 | 56 bags |
| 60 lb bags | 33.33 / 0.45 = 74.1 | 75 bags |
How much does a bag make?
The volume of concrete that a bag makes (its yield) depends on its size. These are the usual figures that are printed on the bags:
| Bag | Yield (cubic feet) | Bags for 33.33 cubic feet | Rounded up |
|---|---|---|---|
| 80 lb | 0.6 | 55.6 | 56 |
| 60 lb | 0.45 | 74.1 | 75 |
| 50 lb | 0.375 | 88.9 | 89 |
| 40 lb | 0.3 | 111.1 | 112 |
Each block of the chart is about 2.8 bags. The smaller the bag, the more bags you need, but the total weight of concrete is about the same.
80 lb ████████████████████ 56 bags 60 lb ███████████████████████████ 75 bags 50 lb ████████████████████████████████ 89 bags 40 lb ████████████████████████████████████████ 112 bags
Chart: the effect of the thickness
For 100 square feet. A thicker slab needs more concrete in a straight line: 6 inches needs 50% more than 4 inches. Each block is about 0.06 cubic yards, and the 4-inch slab is green.
2 in ██████████ 0.62 yd³ 3 in ███████████████ 0.93 yd³ 4 in ████████████████████ 1.23 yd³ 5 in █████████████████████████ 1.54 yd³ 6 in ██████████████████████████████ 1.85 yd³ 8 in ████████████████████████████████████████ 2.47 yd³
How thick should it be?
| Use | Usual thickness |
|---|---|
| Walkway, patio, shed pad | 4 inches (10 cm) |
| Driveway, garage floor | 5 to 6 inches (12 to 15 cm) |
| Heavy loads (a big truck) | 6 inches or more, with steel bars |
These are common values. Your building code and your soil decide what is right for you.
Common jobs
| Job | Area (sq ft) | Cubic yards | 80 lb bags |
|---|---|---|---|
| Walkway 3 ft x 20 ft, 4 in | 60 | 0.74 | 34 |
| Shed pad 8 ft x 10 ft, 4 in | 80 | 0.99 | 45 |
| Patio 10 ft x 10 ft, 4 in | 100 | 1.23 | 56 |
| Patio 12 ft x 12 ft, 4 in | 144 | 1.78 | 80 |
| Garage 20 ft x 20 ft, 5 in | 400 | 6.17 | 278 |
Each block is about 0.15 cubic yards.
Walkway 3 ft x 20 ft █████ 0.74 yd³ Shed pad 8 ft x 10 ft ██████ 0.99 yd³ Patio 10 ft x 10 ft ████████ 1.23 yd³ Patio 12 ft x 12 ft ████████████ 1.78 yd³ Garage 20 ft x 20 ft ████████████████████████████████████████ 6.17 yd³
Bags or ready-mix delivery?
Bags are best for small jobs, up to about 1 cubic yard (about 45 bags of 80 lb). You mix them yourself, and each bag weighs 80 lb, so a big job is a lot of lifting and mixing. For larger jobs, a truck delivers mixed concrete by the cubic yard. Suppliers often have a minimum load, and they usually charge for a short load, so the calculator rounds the delivery up to the next quarter of a cubic yard. Enter both prices, and it shows the cheaper choice.
Tips
- Add 5% to 10%. The ground is never perfectly flat, the forms move a little, and some concrete is spilled.
- Prepare the base. A layer of 4 inches of compacted gravel under a slab helps it to last.
- Weight: concrete weighs about 145 pounds per cubic foot (about 4,000 pounds per cubic yard). Do not forget it if you carry it or if it goes on a structure.
- Order the extra before you start. A second trip for a few bags is expensive.
What this calculator does not include
- Footings, walls, columns and stairs, which have other shapes. Work out their volume and enter it as an area with a thickness of 12 inches, or use a special calculator.
- Steel bars, wire mesh, forms, gravel and the labor.
- Bags of other sizes or makes. Check the yield printed on your bag.
This tool gives an estimate for planning. Check the amount with your supplier before you order, and follow your local building rules.