How the volume is worked out
The slab is treated as a rectangular block: length multiplied by width multiplied by thickness. The thickness is converted to the same unit as the length before multiplying, so a thickness in millimetres or inches never mixes with metres or feet.
The waste allowance is added afterwards, as a percentage. Five per cent covers spillage, an uneven sub-base and the small overfill most slabs need at the edges.
Volume is then converted to weight at 2,400 kg per cubic metre, which is about 150 lb per cubic foot. Wet concrete is heavier than the dry mix, so this is the figure to use for a delivery.
Worked example
A slab 4 metres by 3 metres at 100 mm thick is 1.2 cubic metres. Adding 5 per cent for waste gives 1.26 cubic metres, which weighs 3.02 tonnes and needs 101 bags of 25 kg mix.
Change any input and every output moves with it. The bag count always rounds up, because you cannot buy part of a bag.
What a bag of mix actually yields
Bagged mix is sold by weight, not by volume, so the yield depends on the product: a 25 kg bag of concrete mix makes about 0.0125 cubic metres, which is 80 bags to the cubic metre. A stronger mix yields slightly less because it contains more cement and less aggregate.
The calculator uses that stated yield. If your bag says something different, the volume and weight stay right but the bag count will be out by the same proportion.
For anything larger than about two cubic metres, ready-mix delivered by lorry is usually cheaper than bags, and it arrives at a consistent strength. Bag counts above a hundred are a sign to price ready-mix as well.
Limits of this calculator
It works on a rectangular slab at one thickness. It does not handle stepped footings, a slab with thickened edges, reinforcement, or a base that falls to a drain.
It does not design the mix. Strength, reinforcement and sub-base specification are engineering decisions, and a garden slab and a vehicle crossing need different ones.
It does not include a price. Bag and ready-mix prices vary by region and by order size, so enter your own supplier quote against the volume.
For the sub-base under the slab, use the UK aggregate calculator.
Inputs for the concrete calculator
Length
The long side of the slab, in metres.
Width
The short side of the same slab, in metres.
Thickness
The slab depth in millimetres. A path is usually 75 mm or 3 in, a driveway 150 mm or 5 in.
Waste allowance
Extra material for spillage and an uneven base. Five per cent is a fair default.
CALCULATOR QUESTIONS
UK concrete calculator questions
How many bags make a cubic metre or yard?
About 80 bags of 25 kg make a cubic metre, and about 45 bags of 80 lb make a cubic yard. The exact figure depends on the mix.
Should I add waste?
Yes, for anything poured by hand. Five per cent covers spillage and an uneven base; add more if the ground is rough.
Is bagged mix as strong as ready-mix?
It can be, if you buy the right product and mix it properly. Ready-mix is more consistent on a large pour.
Does this include reinforcement or a sub-base?
No. It measures concrete only. Mesh, membrane and hardcore are separate quantities.