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How does a concrete calculator work out volume?
Procon 24/7, a UK concrete supplier, sets out the method plainly: concrete is ordered in cubic metres, so measure the pour in metres, multiply the dimensions together, then add an allowance for wastage. It says this is the same arithmetic any supplier runs for you.
Its worked examples are:
| Shape | Formula | Example |
|---|---|---|
| Slab or base | length × width × depth | 4 m × 3 m × 0.1 m = 1.2 m³ |
| Strip footing | length × width × depth | 10 m × 0.6 m × 0.3 m = 1.8 m³ |
| Column or pad | π × (diameter ÷ 2)² × height | 3.14 × 0.15² × 1 m = 0.07 m³ |
Procon also gives a rule of thumb for what one cubic metre covers: 10 m² at 100 mm deep, 6.7 m² at 150 mm, or 5 m² at 200 mm.
Why do millimetres cause so many mistakes?
Depths are usually given in millimetres, but the volume is worked out in metres. Procon points out that this is where most ordering mistakes start. Divide millimetres by 1,000 to get metres: 100 mm is 0.1 m, 150 mm is 0.15 m and 300 mm is 0.3 m. Procon warns that getting this wrong by one decimal place orders ten times the concrete you need.
How much extra concrete should you allow?
Even a careful pour wastes some concrete. Heidelberg Materials UK recommends adding 5 to 10% to the calculated volume to cover spillage, uneven surfaces and other factors. Procon’s calculator adds 10% for wastage and spillage.
For the 4 m × 3 m × 100 mm slab above, 1.2 m³ plus 10% is 1.32 m³. If you are buying bags instead of ready mixed concrete, bag sizes vary, so check how much the maker says one bag makes before you divide.
What does a calculator not decide for you?
A calculator works out a volume from the sizes you give it. It does not tell you what those sizes should be.
Heidelberg Materials notes that preparing the ground comes first. For strip footings, it says excavation is generally down to undisturbed ground, while for pad foundations and floor slabs you need to allow enough depth for the sub-base and damp-proof membrane as well as the concrete.
How thick a slab must be, how deep a footing goes and which concrete mix to use depend on what you are building and the ground it sits on. For anything structural, such as foundations for an extension or a wall, get advice from a builder, a structural engineer or your local building control team before you order.
Questions people ask
How do I calculate concrete for a slab?
Multiply length × width × depth, all in metres, to get cubic metres. Then add 5 to 10% for waste, as Heidelberg Materials recommends.
How much does one cubic metre of concrete cover?
Procon’s rule of thumb is about 10 m² at 100 mm deep, 6.7 m² at 150 mm and 5 m² at 200 mm.
How do I work out concrete for a post hole?
Use π × radius² × depth. Procon’s example is a 0.3 m column, 1 m high, which needs about 0.07 m³. A post hole 0.3 m wide and 1 m deep takes the same volume.
How thick should my slab be?
That depends on the job and the ground. A calculator cannot decide it, so check with a builder or building control.
Where DIY Material Calculator helps, and where it does not
DIY Material Calculator is our Android app for estimating materials for home improvement jobs. For concrete, it estimates the bags needed for slabs, footings and posts. It also covers mulch, gravel, topsoil, sand and paint. You pick a material, enter your project dimensions, choose a bag size or bulk delivery, and get an estimate of the bags needed and the total volume, with an expandable formula breakdown showing how it was worked out. You can enter local prices for a cost estimate, and switch between imperial and metric units, different currencies and regional bag sizes. Google Play lists the app as containing ads and offering in-app purchases.
What it is good for: a fast first estimate, with the working shown, so you can sanity check an order.
What it will not do: its results are estimates for planning. Actual needs vary with waste, settling and product differences, and the app itself suggests buying 10 to 15% extra. It cannot tell you how deep or thick your concrete should be, or which mix to use. For structural work, get professional advice.
Questions or bugs: Email us.
Sources
- Ready mixed concrete calculator and FAQsHeidelberg Materials UK heidelbergmaterials.co.uk
- Concrete calculator UKProcon 24/7 proconcrete247.co.uk