Concrete Calculator
Concrete volume, premix bags and cement, sand and aggregate.
What does the concrete calculator do?
This concrete calculator works out how much concrete you need for common jobs: a slab or floor, rectangular footings, round columns, walls and solid steps. It gives the volume in cubic metres, cubic yards and cubic feet, adds a waste allowance, counts how many bags of premixed concrete you need, and estimates cement, sand and coarse aggregate if you mix on site with a nominal mix.
How to calculate concrete
- Choose metric or imperial units and pick the shape you are pouring.
- Enter the dimensions. Lengths are in metres or feet, and thickness, depth and diameter are in centimetres or inches.
- Set the waste percentage. Five to ten percent covers spillage, uneven ground and formwork movement.
- Read the volume, then the number of premix bags for several bag sizes.
- Choose a nominal mix to see cement bags, sand and aggregate quantities, and copy the estimate.
Shapes and formulas
- Slab: length x width x thickness.
- Footing: length x width x depth x number of footings.
- Round column: pi x radius squared x height x number of columns.
- Wall: length x height x thickness.
- Solid stairs: width x rise x run x n(n + 1) / 2 for n steps resting on the ground.
Premix bags and site mixing
Bag yields used are about 11.5 litres for a 25 kg bag, 18.5 litres for a 40 kg bag, 0.45 cubic feet for a 60 lb bag and 0.60 cubic feet for an 80 lb bag. Brands differ, so check the yield printed on your bag. For site mixing, wet volume is multiplied by 1.54 to get the dry volume of materials, because dry materials shrink when water is added and voids fill. That dry volume is split by the mix ratio. A 1:2:4 mix, roughly M15, is common for footings and floors, and a 1:1.5:3 mix, roughly M20, for general reinforced work. Cement is taken as 1,440 kg per cubic metre, so one 50 kg bag is about 0.035 cubic metres.
Tips for ordering concrete
For anything larger than about one cubic metre, ready-mix delivery is usually cheaper and more consistent than bags. Order a little more than you need, since running short in the middle of a pour causes weak joints. Check the slab thickness at several points, because uneven ground can add a lot of volume. Structural elements such as columns and suspended slabs should always follow an engineer’s design and mix specification.
Frequently asked questions
How many bags of concrete are in a cubic metre?
With the yields used here, about 87 bags of 25 kg, 55 bags of 40 kg, or 59 bags of 80 lb make one cubic metre. Check your brand's stated yield.
Why is the dry volume 1.54 times the wet volume?
Dry cement, sand and aggregate contain air gaps that fill when mixed with water, and some material is lost, so about 54 percent more dry material is needed to make a given wet volume.
Are the cement, sand and aggregate amounts exact?
No. They are estimates for nominal volume mixes. Actual quantities depend on moisture in the sand, aggregate size and the mix design your engineer specifies.