About the Concrete Calculator
Concrete Calculator tells you how much concrete a job needs. Choose a slab, a footing or a round column, enter the sizes in metres and centimetres or in feet and inches, and add a waste allowance. The tool shows the volume in cubic metres, cubic yards and cubic feet and the number of premixed bags you must buy, with a choice of six common bag sizes from 20 kg to 80 lb. It also gives a rough recipe of cement, sand and gravel if you plan to mix your own. You can enter several identical pours, for example four fence-post footings, and the volumes are added together. It is a planning aid for garden paths, patios, steps and small structural work, and it runs in your browser.
Updated September 2026
How to use the Concrete Calculator
- 1Choose metres and centimetres, or feet and inches.
- 2Pick what you are pouring: a slab, a footing or a round column.
- 3Enter the length and width and the thickness, or the diameter and height for a column.
- 4Set the number of identical pours, the waste allowance and the bag size.
- 5Read the volume, the bags of premix and the rough mix recipe.
Working out the volume
Concrete is sold by volume, so every job starts with a volume sum. For a slab or a footing, multiply the length, the width and the thickness. Take care with units: measure the length and width in metres or feet, but the thickness is easier in centimetres or inches, so the tool converts it for you. For a round column, use the diameter and the height.
If you have several identical pieces, such as four post footings, enter the number and the tool multiplies the volume. For an irregular shape, split it into rectangles, work out each and add them.
Allowing extra
- Add 5 to 10 percent for small, well-made forms.
- Add 10 to 15 percent for ground that is uneven or has been dug by hand.
- Round up to the next whole bag or the next quarter cubic metre.
- Order slightly more than the tool shows if the ground is soft and may sink.
Bags, mixing your own and ready-mix
Each bag makes a set amount of wet concrete once you add water, and the yield is printed on the bag. The tool uses typical yields for six bag sizes, so choose the one you will buy, and check the label since brands vary slightly. If you mix your own from cement, sand and gravel, the recipe shown is a rough guide for about C25 concrete, and you should follow the supplier's mix instructions.
For anything larger than a few bags, ready-mix is quicker and more consistent. The supplier will ask for the volume, so give them the figure with the waste allowance included.
Related tools
Volume Calculator and Area Calculator work out other shapes, Unit Converter changes cubic metres, yards and litres, Paint Calculator plans the finish for walls and Percentage Calculator handles waste and costs. Ratio Calculator scales mix proportions.
Features
- Slab, footing and round column shapes
- Metric and imperial input
- Volume in m³, cubic yards and cubic feet
- Number of bags for six common bag sizes
- Several identical pours added together
- Waste allowance you can change
- Rough cement, sand and gravel quantities
- Runs in your browser
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length by width by thickness, all in the same unit, to get the volume. A slab of 4 by 3 metres and 10 centimetres thick is 4 × 3 × 0.1 = 1.2 cubic metres. Add about 10 percent for spills, uneven ground and over-digging, which gives 1.32 cubic metres.
How many bags of concrete do I need?
Divide the volume you need by the yield of one bag. An 80 lb bag makes about 0.6 cubic feet and a 25 kg bag about 0.012 cubic metres. For 1.32 cubic metres you need about 113 bags of 25 kg, which is why large jobs are usually ordered as ready-mix.
How thick should a concrete slab be?
Paths and patios usually use 75 to 100 millimetres (3 to 4 inches). Driveways and garage floors use 100 to 150 millimetres (4 to 6 inches). Sheds and heavy loads may need more, and reinforcement. Follow local building rules and take advice for structural work.
How is the volume of a round column worked out?
It is pi times the radius squared times the height. A column with a diameter of 30 centimetres and a height of 2 metres has a radius of 0.15 metres, so its volume is 3.1416 × 0.0225 × 2, which is about 0.14 cubic metres.
What is a cubic yard of concrete?
It is a cube 3 feet on each side, which is 27 cubic feet or about 0.765 cubic metres. Ready-mix in the United States is sold by the cubic yard, and in most other countries by the cubic metre.
Why add extra for waste?
Ground is never perfectly flat, forms bulge slightly, and some concrete is spilled or left in the mixer. Ten percent is a typical allowance for small jobs. Running short in the middle of a pour is far worse than having a little left, because a joint between batches weakens the slab.
Should I mix it myself or order ready-mix?
Bags suit jobs of up to about half a cubic metre, such as post holes and small pads. Beyond that, mixing by hand is hard work and the batches may vary. Order ready-mix from a supplier, and give them the volume the tool shows, plus the slump or strength class they recommend.