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Concrete Calculator
Concrete for a slab, footing, column or steps in cubic metres or yards, and how many bags.
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Bags of pre-mixed concrete
Yields marked typical are common figures for pre-mixed concrete. The yield printed on your bag is the one to use, so type it in if it differs.
This works out volume and bag counts only. It does not size a slab, footing or column for a load, and it is not structural advice: thickness, reinforcement and footing size depend on the ground, the load and local building rules, so check those with a builder or engineer before you pour.
How to use it
- Pick Metric or Imperial.
- Choose what you are pouring: a slab, a strip footing, a column or a flight of steps, and enter its sizes.
- Set how many identical pieces there are and an allowance for waste.
- Pick a bag size or type the yield printed on your bag to see how many bags you need.
Questions
How is the volume of a slab worked out?
Length times width times thickness. A slab 3 m long, 2 m wide and 10 cm thick is 3 x 2 x 0.1 = 0.6 cubic metres. In imperial, 10 ft by 10 ft at 4 in thick is 10 x 10 x 4/12 = 33.3 cubic feet, about 1.23 cubic yards.
How are steps calculated?
Each step is treated as a solid block that runs down to the ground, so the lowest step is one rise high, the next two rises, and so on. The total is width x tread depth x rise x n(n+1)/2 for n steps. A landing at the top can be added as a separate slab.
How much extra should I order?
An allowance of 5 to 10 percent is common to cover uneven ground, spillage and formwork that bows. Running short in the middle of a pour is worse than a bag left over, so many people round up.
What does bag yield mean?
The volume of placed concrete one bag makes once mixed with water. It is printed on the bag. As a typical guide, an 80 lb bag of pre-mix makes about 0.6 cubic feet and a 25 kg bag about 11 to 12 litres, but brands differ.
When should I order ready-mix instead of bags?
Bags suit small jobs. Above roughly one cubic metre or one cubic yard the number of bags and the mixing time grow quickly, and many people order ready-mixed concrete by volume instead. Suppliers quote by the cubic metre or yard, which this calculator gives you.
About this tool
About the concrete calculator
Ordering too little concrete stops a pour halfway; ordering far too much wastes money and leaves bags to store. This calculator works out the volume for the most common jobs and how many bags of pre-mixed concrete that takes.
Shapes it covers
- Slab: length times width times thickness, for patios, shed bases and paths.
- Footing: a strip footing by length, width and depth.
- Column: round by diameter and height, or square and rectangular.
- Steps: each step treated as a solid block down to the ground, by number of steps, rise, tread depth and width.
Set how many identical pieces you are pouring and an allowance for waste and spillage. Results show cubic metres, litres, cubic yards and cubic feet.
Bags
Pick a bag size and its yield, the volume one bag makes once mixed. Common sizes are listed with typical yields, clearly marked as typical, and you can type the yield printed on your own bag. The bag count is rounded up to whole bags.
The calculator gives quantities only. It does not size anything for a load and is not structural advice; thickness, reinforcement and footings depend on the ground and local rules. Switch between metric and imperial at any time.
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