How this is worked out
Volume is length × width × depth, with the depth converted from millimetres to metres. A round hole uses π × radius² × depth. The waste percentage is added on top.
For bags, we use about 0.009 m³ per 20kg bag. Tarmac says a 20kg bag of its Blue Circle Postmix covers about 0.12 m² at 75mm deep, or 0.09 m² at 100mm, and both work out at about 0.009 m³. Other brands of 20kg ready-to-use concrete are close to this, but check the bag.
The weight figure assumes normal concrete at about 2.4 tonnes per m³. That matters if you’re carrying bags or checking what a van can take.
Worked example
A 3m × 2.4m shed base at 100mm deep is 3 × 2.4 × 0.1 = 0.72 m³. With 5% added that’s about 0.76 m³, or 84 bags of 20kg concrete.
That many bags is a long day of mixing. For anything over about half a cubic metre, compare the price of a small ready-mix load or a volumetric mixer that mixes on site and charges for what it pours.
Bags or ready-mix?
Bags make sense for post holes, a small step or a slab for a water butt. Once you’re past 40 or 50 bags, a ready-mix lorry or volumetric truck usually costs about the same and saves your back. Ask about access first: a lorry needs to get close, or you’ll need barrows or a pump.
What this doesn’t cover
It doesn’t design the slab. Garages, extensions and anything holding up a wall need a proper specification, and often building control approval. The hardcore sub-base under a slab is separate: use the gravel calculator for tonnes of stone.
Questions people ask
How many bags of concrete do I need for 1 m³?
About 111 bags of 20kg ready-to-use concrete, based on Tarmac’s coverage for a 20kg bag of about 0.009 m³. At that point a ready-mix delivery is far less work than mixing bags.
How thick should a shed base be?
Many shed and summerhouse makers ask for 75 to 100mm of concrete on a compacted hardcore base. Check the instructions for your building, because larger or heavier ones need more.
How much extra should I order?
Five to ten per cent covers spillage, uneven ground and the mix left in the barrow. Go towards ten per cent if the dig is rough or the formwork is not square.
Can I work in feet and inches?
Yes. Type 12ft 6in, 12' 6" or 3.8m in the length and width boxes, and 4in or 100 in the depth box. The calculator converts it.
Sources
Trade review pending. A named tradesperson will check this calculator, and their name and qualifications will appear here.
Figures last checked 25 October 2026. Read how we calculate and our disclaimer. Found a mistake? Tell us.