Concrete Calculator
Work out cubic yards, bags and cost for any pour. The first area is an example, so replace it with your own numbers.
How much concrete do I need?
Multiply length × width × thickness, with every measurement in feet, then divide by 27. The answer is cubic yards, which is how ready-mix is sold in the United States.
For a 10 ft × 10 ft slab that is 4 inches thick: 10 × 10 × 0.333 = 33.3 cubic feet, and 33.3 ÷ 27 = 1.23 cubic yards. Add 10% for waste and you order 1.5 yards, or 62 bags of 80 lb mix.
The concrete calculator above does this for you. It also handles shapes that are harder to work out by hand, and it adds several areas into one order.
How to use the concrete calculator
- Pick the shape of your pour: slab, L-shape, round slab, footing, wall, post hole, steps or curb.
- Enter your measurements. Each field has its own unit, so you can mix feet, inches, yards, meters and centimeters.
- Check the drawing. It is drawn to scale and shows your numbers, so a wrong entry is easy to spot.
- Add another area if your job has more than one pour, such as a patio plus a walkway.
- Read the order panel: cubic yards, bag counts for four bag sizes, the ready-mix order and the cost.
Concrete formulas for each shape
Every formula gives cubic feet when the measurements are in feet. Divide the result by 27 for cubic yards.
| Shape | Formula |
|---|---|
| Slab, pad or patio | length × width × thickness |
| L-shaped slab | (overall length × overall width − cut-out length × cut-out width) × thickness |
| Round slab | 3.1416 × (diameter ÷ 2)² × thickness |
| Footing | length × width × depth |
| Wall | length × height × thickness |
| Post hole or round column | 3.1416 × (diameter ÷ 2)² × depth |
| Steps | width × rise × run × n × (n + 1) ÷ 2, where n is the number of steps |
| Curb and gutter | length × (curb depth × (curb height + flag thickness) + gutter width × flag thickness) |
To convert inches to feet, divide by 12. Four inches is 0.333 ft, 5 inches is 0.417 ft and 6 inches is 0.5 ft.
How many bags of concrete do I need?
Divide your volume in cubic feet by the yield of one bag and round up. The yields below are from the QUIKRETE Concrete Mix data sheet.
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 50 lb | 0.375 cu ft | 72 |
| 40 lb | 0.30 cu ft | 90 |
Bag counts always round up, because you cannot buy part of a bag. Other brands and specialty mixes can yield slightly more or less, so check the bag if you are using something other than standard concrete mix.
Concrete needed for common slab sizes
All figures are for a 4-inch slab. The last two columns include 10% for waste.
| Slab size (ft) | Square feet | Cubic yards | Cubic yards + 10% | 80 lb bags + 10% |
|---|---|---|---|---|
| 8 × 8 | 64 | 0.79 | 0.87 | 40 |
| 10 × 10 | 100 | 1.23 | 1.36 | 62 |
| 10 × 12 | 120 | 1.48 | 1.63 | 74 |
| 12 × 12 | 144 | 1.78 | 1.96 | 88 |
| 12 × 16 | 192 | 2.37 | 2.61 | 118 |
| 16 × 16 | 256 | 3.16 | 3.48 | 157 |
| 20 × 20 | 400 | 4.94 | 5.43 | 245 |
| 24 × 24 | 576 | 7.11 | 7.82 | 352 |
One cubic yard covers 81 square feet at 4 inches thick, 65 square feet at 5 inches and 54 square feet at 6 inches.
Bags or ready-mix?
For small pours, bags are simpler: you buy what you need and mix as you go. Post holes, a few steps and pads under about 1 cubic yard are typical bag jobs.
Above 1 cubic yard, most people order ready-mix. One yard is 45 bags of 80 lb, which is 3,600 lb to carry, open and mix. A truck places the same amount in minutes, and the concrete is mixed evenly.
Price decides the rest. Enter what your store charges per bag and what your local supplier charges per yard, and the calculator shows both totals side by side. Ask the supplier about a short-load fee, which many charge on small orders.
How thick should the concrete be?
Four inches is the usual thickness for patios, walkways and shed pads. Driveways are commonly 4 to 6 inches, with the thicker end for heavier vehicles. Footings and structural slabs are set by your local building code, so check with your building department before you pour.
Thickness changes the volume quickly. Going from 4 inches to 6 inches adds 50% more concrete to the same slab.
Why add a waste allowance?
The ground is never perfectly flat, forms bow outward under the weight, and some concrete is always spilled or left in the wheelbarrow. Running short in the middle of a pour leaves a weak joint in the slab, so a little extra is cheap insurance. Use 5% for simple, well-formed pours and 10% or more when the subgrade is rough.
More concrete calculators
- Concrete slab calculator – patios, pads and driveways, by dimensions or square feet
- Concrete yard calculator – cubic yards for any pour, with a coverage chart
- Concrete bag calculator – how many 40, 50, 60 or 80 lb bags you need
- Bags of concrete per yard – the bags-per-yard table for every bag size
- Concrete weight calculator – weight of a slab, footing or column in pounds
- Sonotube concrete calculator – piers and tube forms from 6 to 18 inches
- Fence post concrete calculator – bags per post for 4 × 4 and 6 × 6 posts
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length by width by thickness, with every measurement in feet, to get cubic feet. Divide by 27 to get cubic yards. A 10 ft by 10 ft slab that is 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards.
How much concrete do I need for a 10×10 slab?
A 10 ft by 10 ft slab at 4 inches thick needs 1.23 cubic yards. With 10% added for waste, order 1.5 cubic yards of ready-mix or buy 62 bags of 80 lb mix.
How many bags of concrete are in a cubic yard?
One cubic yard is 27 cubic feet. That takes 45 bags of 80 lb mix, 60 bags of 60 lb mix, 72 bags of 50 lb mix or 90 bags of 40 lb mix.
How many 80 lb bags of concrete do I need?
Divide the volume in cubic feet by 0.60, because one 80 lb bag makes about 0.60 cubic feet, and round up. A 33.3 cubic foot slab needs 56 bags, or 62 bags with 10% waste.
How many square feet does a yard of concrete cover?
One cubic yard covers 81 square feet at 4 inches thick, 65 square feet at 5 inches and 54 square feet at 6 inches.
Should I use bags or order ready-mix?
Bags suit small jobs such as post holes, steps and pads under about 1 cubic yard. Above that, ready-mix is usually easier because one yard means mixing 45 bags of 80 lb by hand. Compare both totals in the calculator before you decide.
How much extra concrete should I order?
Add 5% to 10% for most jobs. Uneven ground, forms that bow and spillage all use more than the drawing says. Use the higher figure when the subgrade is rough.
How much does a yard of concrete weigh?
About 4,050 lb. The calculator uses 150 lb per cubic foot for normal-weight concrete, and a cubic yard is 27 cubic feet.
How long does concrete take to cure?
Keep new concrete damp for 5 days in warm weather and 7 days in cooler weather. Bagged concrete mix reaches its rated strength, typically 4,000 psi, at 28 days.
Is a cement calculator the same as a concrete calculator?
Yes, for most people. Cement is the powder that binds concrete, and concrete is the finished mix of cement, sand, gravel and water. When you search for a cement calculator for a slab, you are working out concrete volume.