Concrete calculator
Length, width and thickness give the volume, the bags and the ready-mix order.
- Runs in your browser — nothing you enter is uploaded
- No sign-up, no quotas, free to use as often as you like
How it works
Concrete is unforgiving about quantity: it has to be poured in one go, and a slab finished in two pours has a cold joint through it. Working out the volume first, with a margin, is the whole job.
Volume is the only real number
Length times width times thickness. Everything else — bags, weight, cubic yards — is a conversion of it. Thickness is the one people get wrong, because it is entered in the same unit as the rest: a 100 mm slab is 0.1 metres, not 100.
Bags or ready-mix
Bags make sense for small jobs, up to roughly a quarter of a cubic metre. Beyond that, mixing by hand becomes a very long day and ready-mix delivered is usually cheaper as well as better. Suppliers sell in steps, commonly 0.1 cubic metres, and will not take back what you over-order.
Always order extra
Ground is never perfectly level and formwork bulges slightly, so the hole is almost always bigger than the drawing. Ten per cent is the usual allowance and it is rarely wasted.
An honest caveat about bag yields
The yield per bag varies by product and by how wet you mix it, so the bag count here is an estimate from typical figures. The number printed on the bag is authoritative — check it and adjust.
Concrete calculator — questions
How thick should a slab be?
It depends entirely on what it carries — a garden path is not a driveway. Check local building guidance; this tool calculates volume, not structural adequacy.
How many bags per cubic metre?
Roughly 60 bags of 25 kg, though it varies by product. Always check the yield printed on the bag.
Should I use bags or ready-mix?
Bags for small jobs up to about a quarter of a cubic metre. Beyond that, ready-mix is usually cheaper and far less work.
How much extra should I order?
About ten per cent. Ground is never level and formwork bulges.
Does it work in feet?
Yes, and results are given in cubic metres, cubic feet and cubic yards.