Concrete Calculator

Say you are pouring a base for a garden shed, 3 metres square, 100 mm deep, and you add 5% for spillage and an uneven sub-base. Enter those and the calculator returns 0.95 m³ of concrete, 10 bags of cement at 40 kg, 0.64 tonnes of sand, 1.27 tonnes of gravel and about 170 litres of water.

The step that surprises people is the third one. Your dimensions come to 0.9 m³, and 5% waste takes that to 0.945 m³ — but the dry materials you buy add up to 1.455 m³, which is 1.54 times more. That is not an error and it is not waste. Dry sand, gravel and cement are full of air; mixing and compacting drives the air out, so a heap of dry material always makes less wet concrete than it measured.

From there the mix ratio splits it. M20 is 1:1.5:3, which is 5.5 parts in total, so 1.455 m³ becomes 0.265 m³ of cement, 0.397 of sand and 0.794 of gravel. The cement figure is turned into bags at 1440 kg per m³ and 40 kg a bag, rounded up, because nobody sells two-thirds of a bag.

Switch the shape to Circular Column and enter the hole rather than the post. A typical hole 300 mm across and 600 mm deep is 0.042 m³ — call it one bag of cement, 0.03 tonnes of sand, 0.06 of gravel and about 8 litres of water.

That is per hole. The calculator does one column at a time, so multiply by your post count: eight posts is 0.34 m³, which is where a job that felt like nothing becomes a third of a cubic metre and worth mixing properly. Enter the HOLE size, not the post — a 100 mm post in a 300 mm hole is mostly concrete, and sizing the pour off the timber would leave you short by a factor of nine.

Ten metres by one, at 100 mm deep, with 5% waste for an uneven sub-base: 1.05 m³, 11 bags of cement and 0.71 tonnes of sand.

Worth noticing how quickly a path outgrows bagged mixing. Eleven bags is a morning of work with a mixer, and the pour has to be continuous or you get a cold joint every time you stop. If the path is much longer than this, price it both ways before deciding — the Supply dropdown will show you the ready-mix version.

Thirty-six square metres at 150 mm is 5.4 m³. Switch Supply to ready-mix and the answer stops being ingredients and becomes what you actually order: 5.4 m³, one load at 6 m³ per truck.

The working adds the line that matters at this size — the last load carries 5.4 m³ of a 6 m³ truck, and plants usually charge a part-load fee for the shortfall. Nudging a pour up to a full load, or splitting it differently, is often cheaper than paying for air. Above about a cubic metre, bags stop being the sensible answer at all.

Ordering the wet volume of dry materials. If you had taken that 0.945 m³ and asked for 0.945 m³ of sand and gravel in the right ratio, you would have arrived on site with roughly a third too little and no way to finish the pour. Concrete cannot be left half-poured while somebody fetches more — the joint between yesterday and today is a weakness you cannot undo.

This is why the calculator applies the 1.54 dry factor for you, and why the working shows it as its own line. If a number here ever disagrees with a supplier, that line is usually where the two of you are talking about different things.

For a bagged, mixed-on-site job, order by the three quantities: cement in bags, sand and gravel in tonnes. Merchants sell aggregate by weight and you need it by volume, which is why the calculator converts at 1.6 tonnes per m³ rather than leaving you to. Ask what their sand and gravel actually weigh if you want to be exact — it moves with the quarry and with how wet the pile is.

For anything much above a cubic metre, switch the Supply dropdown to ready-mix. You then order cubic metres and truckloads instead of ingredients, which is how it is actually sold at that size, and the bag count stops being a useful thing to quote.

Open "Show the working" under the results. Every line is the arithmetic in order, with the constants named — the 1.54 dry factor, the 1440 kg per m³ cement density, the 40 kg bag, the 1.6 tonnes per m³ aggregate. If one of those differs from what your supplier or your engineer uses, you can see exactly which figure to argue with rather than having to distrust the whole answer.

The one thing worth saying plainly: these are nominal mixes for estimating, not designed mixes. M20 as 1:1.5:3 is the conventional proportion, and it is the right basis for working out what to buy. It is not a substitute for a specified mix on anything structural.