Concrete Calculator
Enter the size of the slab or the post holes and an allowance for waste. You get the volume to order, the weight and the number of bags, plus the cement, sand and aggregate if you mix it yourself.
This is an estimate. Check the yield printed on the bag, and order a little extra rather than run short mid pour.
Worked examples
A shed base
4 m long, 3 m wide, 100 mm deep, 10% waste, 25 kg bags
1.32 cubic metres, 3.17 tonnes, 125 bags
The bare slab is 1.2 cubic metres before the allowance for waste.
Four post holes
300 mm across, 600 mm deep, four holes
0.17 cubic metres before waste
Each hole is about 0.042 cubic metres.
About Concrete Calculator
A concrete calculator tells you how much to buy before you start, which matters because concrete cannot be sent back once it is mixed and a pour that runs short leaves a weak join. Enter the length, width and depth of a slab, base or footing, or the diameter and depth of round holes for posts or columns, and you get the volume in cubic metres, the weight in tonnes and the number of bags of ready-mix.
The calculator adds an allowance for waste, because ground is never perfectly flat and some mix is always left behind. If you prefer to mix your own, it also splits the volume into cement, sharp sand and coarse aggregate for the ratio you choose. All figures are estimates, and the yield printed on your bags is the best guide to how far each one goes.
How to use Concrete Calculator
- Choose a slab or footing, or round post holes.
- Enter the size, using metres and millimetres or feet and inches.
- Set the allowance for waste and pick your bag size, or type the yield printed on the bag.
- Read the volume, the bags and the cement, sand and aggregate for your own mix.
Common uses
- A shed or summerhouse base
- Fence and gate post holes
- A path, patio base or small drive
- Footings for a garden wall
Frequently asked questions
How much concrete do I need for a 4 m by 3 m slab 100 mm thick?
The slab holds 1.2 cubic metres. With 10% waste you should order 1.32 cubic metres. That is about 125 bags of 25 kg ready-mix, so for a slab this size a ready-mixed delivery is usually easier.
How many bags of concrete make a cubic metre?
A 25 kg bag of ready-mix makes about 0.0106 cubic metres, so a cubic metre takes about 94 bags. A 20 kg bag makes about 0.0085, so you need about 118. The yield on the bag is the figure to trust.
How thick should a concrete slab be?
A base for a shed or a path is often 75 to 100 mm. A drive for cars is 100 to 150 mm on a compacted base of crushed stone, and heavier use needs more. Ask a builder or your building control team for anything structural.
What is a 1:2:4 concrete mix?
It is one part cement, two parts sharp sand and four parts coarse aggregate by volume, a general purpose mix. The calculator allows for the dry materials taking up about 54% more space than the finished concrete, which is why the dry volumes add up to more than the pour.
Should I use ready-mixed or mix my own?
Mixing by hand suits jobs of a few bags such as a post hole. Above about a cubic metre, delivered ready-mix gives a more consistent result and saves hours of work, as long as you can pour it before it starts to set.