Volume, cement, sand and gravel for slabs, footings, and columns.
How thick you are pouring, not how deep you dug.
Mixed on site means you buy the ingredients. Ready-mix arrives mixed and you buy it by the cubic metre.
M15 suits most residential slabs; M20 for load-bearing work. A nominal mix for estimating, not a designed grade.
Extra to order for spillage and uneven subgrade. 5% is typical.
Nothing to work out yet
Enter values for Length, Width, Slab Depth.
Volume follows the shape you pick — a slab is length × width × depth, a column is π r² × height, and stairs sum every step plus the platform on top. Your waste factor is added, then the volume is raised by 1.54 to allow for dry materials bulking before they are mixed, and split by the mix ratio. Cement becomes 40 kg bags at 1440 kg/m³; sand and gravel are both weighed at 1.6 tonnes per m³. Water is quoted at 180 litres per cubic metre so you know what to have standing by; treat it as a guide rather than a mix design, since the real figure depends on the slump you want and how wet the sand already is. The mix ratios are nominal volumetric mixes for estimating: a specified grade comes from a designed mix, so where an engineer has specified the concrete, order to their design rather than to this ratio.
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.
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.