The quick answer: length x width x thickness
Concrete for a shed base is a volume in cubic metres. Take the length and width of the base in metres and multiply by the thickness, also in metres. For the standard 100 mm slab the thickness is 0.1 metres, so the sum is length x width x 0.1. Then add around 10 percent on top, because some concrete is always lost to spillage, an uneven sub-base and slightly over-deep digging. That 10 percent is cheap insurance against the worst outcome on the day, which is running out of concrete part way through a pour.
One thing many people miss: size the base bigger than the shed itself. Make it roughly 100 mm larger on every side, so a shed with a footprint of 2.2 m by 1.6 m sits on a base nearer 2.4 m by 1.8 m. The overhang gives you a dry margin, keeps the timber walls clear of splashing rain, and leaves room for error in positioning. To turn dimensions straight into a volume and bag estimate, the concrete calculator handles the multiplication and the waste allowance for you.
A full worked example
Take a common 8 ft by 6 ft shed, which has a footprint of about 2.4 m by 1.8 m once you allow the 100 mm overhang each way. You are pouring a standard 100 mm slab. The core volume is straightforward:
- Base volume: 2.4 x 1.8 x 0.1 = 0.432 cubic metres.
- Add 10 percent for waste: 0.432 x 1.1 = 0.475, so order about 0.48 cubic metres.
- At 150 mm thick instead: 2.4 x 1.8 x 0.15 = 0.648 cubic metres, about 0.71 with waste.
So a typical small shed base is just under half a cubic metre of concrete at the standard thickness. Notice how much the thickness matters: stepping up from 100 mm to 150 mm adds half as much again to the order. That is the right call for a heavy workshop or soft ground, but it is money you do not need to spend on a light storage shed on firm soil. Remember the slab sits on a compacted hardcore sub-base of 50 to 75 mm that you work out and buy separately, so dig deep enough to fit both the hardcore and the concrete below your finished level.
The C20 mix and what goes in it
The standard concrete for a domestic shed base is a C20 mix, meaning it reaches roughly 20 newtons per square millimetre of strength after curing, which is ample for a garden building. Mixed by volume, C20 is about 1 part cement to 2 parts sharp sand to 4 parts coarse aggregate, written 1:2:4. If you buy all-in ballast, which already blends sand and stone, the recipe simplifies to roughly 1 part cement to 5 parts ballast.
Get the water right, because it is where most home pours go wrong. You want a mix that is workable and holds together when you ball it in a gloved hand, not a soupy one. Too much water weakens the finished slab significantly, so add it gradually. As a rough planning figure, a cubic metre of concrete takes on the order of six 25 kg bags of cement plus the matching sand and aggregate, but for the half cubic metre in our example, weights add up fast, which is the natural lead-in to whether you should be mixing it yourself at all. Lay the wet concrete onto the hardcore between timber shuttering, level it off with a straight board, and let it cure slowly, keeping it damp and covered in hot or freezing weather.
Bags, mixing your own, or ready-mix
There are three ways to get concrete into a shed base, and the right one depends almost entirely on volume. For a small slab under about 0.2 cubic metres, bagged ready-mixed concrete that you just add water to is simplest: no separate materials, no ratios, just tip, mix and pour. The catch is yield. A 20 kg bag makes only about 0.009 to 0.01 cubic metres, so even a modest base swallows dozens of them.
Run the example. The 0.48 cubic metre base, at roughly 0.01 cubic metres a bag, needs about 48 to 50 bags of 20 kg ready-mixed concrete. That is around a tonne of bags to haul, open and mix one or two at a time, which is slow and hard work. This is exactly the point where mixing your own from bulk cement, sand and aggregate with a hired mixer becomes much cheaper, and where ordering ready-mix delivered by truck starts to make sense despite minimum load charges. As a rule of thumb, up to about 0.3 cubic metres, bags or a small mixer are fine; beyond that, get a few quotes for ready-mix, because the labour saved and the consistent quality of a single continuous pour usually justify it. Whichever route you pick, work the volume out first so you are ordering to a real number rather than guessing. The concrete calculator gives you the cubic metres and a bag estimate so you can compare the options on cost before you commit.