How to work out how much concrete I need
Three steps, and the tool does all three, but they are worth knowing because the second and third are where it goes wrong. Length times width times depth gives the volume, in whatever unit you measured. Add waste, because ground is never as flat as it was measured and being short mid pour costs a cold joint rather than a trip to the merchant. Then turn the volume into the thing you actually buy, which is bags, or a load with a minimum on it.
The step people skip is the last one. Knowing you need 0.9 cubic metres does not tell you whether to mix it or order it, and the honest answer depends on how many bags that is, what they weigh, and how many mixer loads it works out at. Those are the three numbers below the volume here, and they are the reason this page exists rather than a box that multiplies three figures together.
The volume is the easy half
Length times width times depth. Every concrete calculator on the internet does that bit, and then stops, which is where they stop being useful.
The question you have is bags or ready-mix, and a figure in cubic metres does not answer it. So this one converts the volume into the three numbers that do.
How many bags, and what they weigh
A 3m × 3m shed base at 100mm is 0.9 cubic metres. With a sensible waste allowance that is about 107 twenty-kilo bags, which is 2.1 tonnes to get from the merchant into the van, and then out of the van and round the back of the house.
Almost nobody expects that number. It is the most useful output on this page.
How many mixer loads, and how long
A "90 litre" mixer does not give you 90 litres of concrete. A drum filled to the brim will not turn over, so about 40 litres of usable mix per load is realistic for a small belle-type mixer.
Which makes a cubic metre roughly 25 loads. At four or five minutes each: loading, mixing, barrowing away: that is a couple of hours of mixing before a single barrow gets laid, on your own, in whatever the weather is doing.
That figure changes more minds than any price comparison, and it is why the tool leads the recommendation with time rather than cost.
Whether you can stop
People often find this out afterwards. Concrete poured in two sessions has a cold joint through it: a plane where the first pour had gone off before the second arrived, which is a weakness and a crack waiting to happen.
If the pour has to be continuous and you are mixing it yourself, you are committed to several hours with no realistic break. That is a reason to order it that has nothing to do with money.
It has an honest middle band
Under about 0.25 m³, which is a few post holes or a small pad, use bags every time. Over about 1.5 m³, order it.
In between, the tool says either, honestly, because there is no real threshold there. It depends on your access, whether anyone is helping, and whether it has to go in one go. Inventing a cut-off to look decisive would make the tool feel better and serve you worse.
The minimum load trap
Ready-mix is not automatically dearer for a small pour, and it is not automatically cheaper either. Most suppliers have a minimum load and charge part-load rates on top of it, so half a cubic metre can cost close to what a full metre would.
Which cuts both ways: if you are near a metre, the extra may be nearly free, and running slightly over on a delivery is a much better problem than running out. The tool flags it under a cubic metre so you make the phone call rather than the assumption.
Post holes deduct the post
A small thing that adds up. A 100mm post sitting in a 300mm hole takes up about a seventh of it, and across ten fence posts that is a bag or two of difference.
Add several shapes at once, too. An L-shaped base is two rectangles, and the footings can go in the same sum as the slab so you order once rather than twice.
How thick the slab should be is a different question
It does not tell you how thick the slab should be, what mesh it wants, or what it can carry. Those are design questions, they depend on the ground and on what is going on top of it, and they belong to whoever is signing the job off.
This works out how much concrete the dimensions you gave it need. It does not choose the dimensions, and it will say so if you enter something that looks thin, without telling you what to put instead.
Next to it: what mix to order, aggregate and hardcore for the sub-base underneath, and rebar weights if there is steel in it. Nothing is uploaded and nothing is stored.
Common questions
How much concrete do I need for a 3m by 3m shed base?
At 100mm thick, 0.9 cubic metres, or 0.99 with a ten per cent waste allowance. That is about 2.4 tonnes of concrete, and if you buy it in 20kg bags, roughly 107 of them. Most people are surprised by the bag count, so this tool shows it.
How many bags of concrete do I need for a cubic metre?
About 108 twenty-kilo bags, or 87 twenty-five kilo bags, based on the yields typically printed on them. Bagged products do vary between brands, so check what yours says and change the figure: the tool lets you.
How much cement do I need?
Cement and concrete are not the same thing and ordering by the wrong one is an expensive afternoon. Cement is the grey powder that binds; concrete is cement plus sand plus aggregate plus water. If you are buying bagged concrete or ballast, the tool gives you bags directly. If you are buying the ingredients separately and mixing to a ratio, a 1:2:4 mix by volume works out at roughly 300 to 350 kg of cement per cubic metre of finished concrete, which is six or seven 50 kg bags, plus about 0.5 cubic metres of sharp sand and 0.8 of aggregate. The volumes add up to more than a cubic metre before mixing because the smaller particles fill the gaps between the larger ones.
How much ready-mix concrete do I need, and is it worth it?
The same volume, and then a separate question. Ready-mix is ordered by the cubic metre here and by the cubic yard in North America, most suppliers will not deliver less than one, and part-load charges above the minimum mean a small pour can cost close to what a full load would. The tool shows where you sit against that minimum. The reason to order rather than mix is often not money at all: concrete poured in two sessions has a cold joint through it, and a pour that takes one person all day is a pour that gets finished in two sessions.
Bags or ready-mix?
Under about a quarter of a cubic metre, bags, every time. Over about one and a half, order it. In between it genuinely depends on your access, whether you have help and whether the pour has to be continuous, and the tool says so rather than inventing a threshold to look decisive.
Why does it show mixer loads and mixing time?
Because that is what decides it, and price usually is not. A cubic metre is about 25 loads through a small mixer, which is a couple of hours of mixing before a single barrow gets laid. Seeing that number changes more minds than a cost comparison does.
Is ready-mix cheaper for a small pour?
Often not. Most suppliers have a minimum load and charge part-load rates above it, so half a cubic metre can cost close to what a full metre would. It is worth a phone call rather than an assumption, in either direction.
Does it tell me how thick my slab should be?
No, deliberately. That is a design question and it depends on ground conditions and what is going on top, which is somebody else’s decision to make and sign. This tool works out how much concrete the dimensions you give it need. It does not choose the dimensions.