How to calculate concrete volume
Concrete is sold by volume, so every job comes down to length times width times depth, with all three in the same unit. The catch is that slabs are measured in feet but their thickness in inches, so divide the inches by 12 first. The result in cubic feet divided by 27 gives cubic yards, the unit ready-mix suppliers in the US use. In metric, measure in meters, turn centimeters into meters by dividing by 100, and the result is in cubic meters.
Example: a 10 ft by 10 ft patio, 4 inches thick, is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. A post hole 10 inches wide and 3 feet deep holds about 1.6 cubic feet, minus the small space the post itself takes up.
How many bags of concrete
Premix bags list their yield on the back. Typical values are about 0.60 cubic feet for an 80 lb bag, 0.45 for 60 lb and 0.30 for 40 lb. Divide the volume by the yield and round up. The calculator uses these typical yields, and for metric bags about 0.012 cubic meters per 25 kg bag and 0.01 per 20 kg bag. The yield of the brand you buy may differ slightly, so compare it with the bag.
| Slab | Cubic feet | Cubic yards | Cubic meters | 80 lb bags |
|---|---|---|---|---|
| 10 × 10 ft, 4 in | 33.3 | 1.23 | 0.94 | 56 |
| 12 × 12 ft, 4 in | 48.0 | 1.78 | 1.36 | 80 |
| 10 × 20 ft, 4 in | 66.7 | 2.47 | 1.89 | 112 |
| 20 × 20 ft, 4 in | 133.3 | 4.94 | 3.78 | 223 |
| 12 × 24 ft, 6 in | 144.0 | 5.33 | 4.08 | 240 |
The table has no waste added. Past a cubic yard or two, mixing bags by hand becomes hard work, and ready-mix from a local plant is often the easier choice.
How thick should concrete be?
- Patios, walkways and shed floors: 4 inches (about 10 cm) is the usual thickness.
- Driveways and garage floors for cars: often 5 to 6 inches, thicker where heavy trucks or campers will park.
- Footings: width and depth depend on the load and on how deep the ground freezes where you live. Your local building department sets the minimum.
- Fence and deck posts: a common rule of thumb is a hole about three times as wide as the post and deep enough to reach below the frost line.
Local codes and the soil on your site always come first, so check them before you dig. Whatever the thickness, order a little extra. A slab that is 4 inches on paper often turns out deeper in low spots, and running out halfway through a pour leaves a weak cold joint. The waste setting adds 5 or 10 percent to the volume and the bag count.
How to use the concrete calculator
- 1Choose US units or metric, then pick the shape of the first item: a slab, a footing, or round columns and post holes.
- 2Enter the measurements. For post holes, enter the number of holes too. Add more items for the other parts of a project, like a slab plus its footings.
- 3Pick a waste allowance and read the total volume and the number of bags for each bag size.
- 4Optionally enter a price per bag or per cubic yard to see the cost, then copy the result for your shopping list.
Frequently asked questions
How many bags of concrete do I need for a 10 x 10 slab?
A 10 ft by 10 ft slab that is 4 inches thick holds 33.3 cubic feet, about 1.23 cubic yards. With typical yields that is 56 bags of 80 lb or 75 bags of 60 lb, before any waste. Add 5 to 10 percent extra so you do not run short in the middle of the pour.
How do I convert cubic feet to cubic yards?
Divide by 27, because one cubic yard is 3 ft by 3 ft by 3 ft. So 54 cubic feet is 2 cubic yards. One cubic yard is about 0.765 cubic meters, and one cubic meter is about 1.31 cubic yards.
How much does one bag of concrete make?
Typical premix yields are about 0.60 cubic feet for an 80 lb bag, 0.45 cubic feet for 60 lb and 0.30 cubic feet for 40 lb. Metric bags of 25 kg make roughly 0.012 cubic meters and 20 kg bags about 0.01. Brands differ a little, so check the yield printed on the bag you buy.
Should I use bags or order ready-mix?
Bags work well for post holes, small pads and repairs. A cubic yard already takes about 45 bags of 80 lb, which is a lot of mixing and lifting. For larger slabs, ask a local supplier about ready-mix delivery or a small trailer load, and compare the price per cubic yard.
How much extra concrete should I order?
Plan for 5 to 10 percent extra. Ground is rarely perfectly level, forms bow a little and some concrete stays in the mixer or wheelbarrow. Running out halfway leaves a weak joint, while a few leftover bags can be returned or used later.