Concrete Calculator
Work out how much concrete you need for a slab, footing, wall, column, round slab, stairs or post holes, in cubic yards, cubic meters and bags.
Results
This is the result for your previous entries. Finish entering your numbers to update it.
Enter your numbers to see the answer.
Cubic yards
–
Cubic feet
–
Cubic meters
–
Approximate weight
–
Premixed bags needed
| Bag size | Yield per bag | Bags |
|---|
Bag counts include the waste allowance and are rounded up.
How it's worked out
Result look wrong? Tell us and we'll check it.
How to use the concrete calculator
- Choose the shape you are pouring: a slab, footing or wall, a round column, a circular slab or tube, stairs, or fence post holes.
- Enter the measurements. Each one has its own unit menu, so you can mix feet and inches or meters and centimeters.
- Set how many identical pieces you need and how much extra to allow for waste (10% is a sensible default).
- Read the volume in cubic yards, cubic feet and cubic meters, and the number of premixed bags in each size.
Concrete volume formulas
Every measurement is converted to the same unit first, then the volume is worked out for the shape:
- Slab, footing or wall: length × width × thickness.
- Round column or pier: π × (diameter ÷ 2)² × height.
- Circular slab or tube: π ÷ 4 × (outer diameter² − inner diameter²) × height.
- Stairs: width × rise × (run × (steps − 1) × steps ÷ 2 + platform depth × steps). Seen from the side, each step is a block one run deep and as tall as all the steps below it, and the top step is as deep as the platform.
- Post holes: the volume of the hole minus the part of the post that sits in it.
The result is multiplied by the quantity and increased by the waste percentage. One cubic yard is 27 cubic feet, and one cubic meter is 35.31 cubic feet. The weight estimate uses 150 lb per cubic foot (2,400 kg per cubic meter), a typical figure for normal-weight concrete.
Worked example: a 10 × 10 ft patio
- Thickness 4 inches = 4 ÷ 12 = 0.333 ft.
- Volume: 10 × 10 × 0.333 = 33.33 cubic feet.
- In cubic yards: 33.33 ÷ 27 = 1.23 yd³.
- With 10% waste: 33.33 × 1.10 = 36.67 cubic feet, or 1.36 yd³.
- 80 lb bags at 0.60 cubic feet each: 36.67 ÷ 0.60 = 61.1, so buy 62 bags.
Bags per cubic yard and cubic meter
| Bag size | Yield per bag | Bags per yd³ | Bags per m³ |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | 118 |
| 50 lb | 0.375 ft³ | 72 | 95 |
| 60 lb | 0.45 ft³ | 60 | 79 |
| 80 lb | 0.60 ft³ | 45 | 59 |
| 20 kg | 0.0094 m³ | 82 | 107 |
| 25 kg | 0.0117 m³ | 66 | 86 |
| 30 kg | 0.0140 m³ | 55 | 72 |
Yields are the figures printed on standard premixed concrete bags; metric bags use the same dry-mix-to-volume ratio. Counts are rounded up to whole bags.
How far one cubic yard goes
| Slab thickness | Area covered by 1 yd³ |
|---|---|
| 3 inches | 108 ft² |
| 4 inches | 81 ft² |
| 5 inches | 64.8 ft² |
| 6 inches | 54 ft² |
| 8 inches | 40.5 ft² |
Frequently asked questions
How much concrete do I need for a 10 × 10 slab?
A 10 × 10 ft slab that is 4 inches thick holds 10 × 10 × 0.333 = 33.3 cubic feet, which is 1.23 cubic yards. With 10% extra for waste that is 1.36 cubic yards, or 62 bags of 80 lb premixed concrete.
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. At the yields printed on standard premix bags, that is 45 bags of 80 lb (0.60 cu ft each), 60 bags of 60 lb (0.45 cu ft) or 90 bags of 40 lb (0.30 cu ft).
How thick should a concrete slab be?
Four inches is the usual thickness for patios, walkways and shed bases. Driveways that carry cars and light trucks are commonly 5 to 6 inches. Footings and structural slabs should follow your local building code or an engineer’s drawings.
How much extra concrete should I order?
Add 5% to 10% to cover spillage, uneven ground and forms that bow slightly. The calculator adds 10% by default; change the waste percentage if your base is very flat and well formed, or raise it for rough ground.
Should I use bags or order ready-mix?
Bags are practical for small jobs such as post holes, steps and small pads, roughly up to a cubic yard. Beyond that, mixing by hand gets slow and the concrete may not be placed evenly, so ready-mix from a truck is usually easier. Ask your supplier about minimum orders and short-load fees.