Concrete Calculator
Volume, bags and mix materials for slabs, footings, columns and steps.
How to use the Concrete Calculator
- 1Choose a slab, footing, column or steps and enter the dimensions.
- 2Add a few percent extra for spills and uneven ground.
- 3Read the volume for ready-mix, the number of premixed bags, or cement, sand, gravel and water to mix it yourself.
About Concrete Calculator
The Concrete Calculator works out how much concrete you need for a slab, a strip footing, a round or square column, or a set of solid steps with an optional top landing. Enter the dimensions, the number of identical pieces and a few percent extra for spills and uneven ground. The result comes in cubic meters or cubic yards, with the amount to order as ready-mix rounded up to the next quarter and the weight of the placed concrete.
If you would rather use bags, the calculator shows how many 25 kg, 40 kg, 60 lb or 80 lb bags of premixed concrete you need and how much each bag yields. To mix it yourself, choose a cement : sand : gravel ratio such as 1 : 2 : 3, 1 : 1.5 : 3, 1 : 2 : 4 or 1 : 3 : 6, or enter a custom ratio. You get the cement in kilograms or pounds and bags, the sand and gravel by volume and weight, and the water from the water–cement ratio.
This concrete calculator suits patios, garden paths, shed bases, fence post footings, tube-form piers and garden steps. For foundations and other structural work, use it to estimate the volume, then order ready-mix to the strength class your plans specify.
Frequently asked questions
How many bags of concrete make a cubic yard?
About 45 bags of 80 lb or 60 bags of 60 lb. In metric, one cubic meter takes about 85 bags of 25 kg or 54 bags of 40 kg premix.
What is a good concrete mix ratio?
1 part cement, 2 parts sand and 3 parts gravel is a common general-purpose mix. 1 : 1.5 : 3 is a richer mix for stronger work and 1 : 2 : 4 a leaner one for footings and bases. For structural concrete, order ready-mix to the strength your plans specify.
Why is the dry volume larger than the concrete volume?
Sand and cement fill the gaps between the gravel, so the dry ingredients shrink by about a third once mixed. The usual factor is 1.54.
How thick should a concrete slab be?
Patios and garden paths are usually about 10 cm (4 in) thick, and driveways 12 to 15 cm (5 to 6 in). Slabs that carry heavy loads or form part of a building should follow your plans or local building rules.
How do I calculate cubic yards of concrete?
Multiply length × width × thickness in feet to get cubic feet, then divide by 27. A 10 × 10 ft slab that is 4 in thick is 10 × 10 × 0.333 = 33.3 ft³, or about 1.23 yd³ before the extra for waste.
More in Home & DIY
Work out how much paint you need for walls and ceilings, and which cans to buy.
Count tiles, planks and boxes, with grout, adhesive and the right waste for your pattern.
Plan risers, treads, stringer length and headroom, with a side-view drawing.