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Concrete & screed · m³ · loads
How much concrete do I need?
Volume for a slab, base or raft, with waste, rounded the way the batching plant sells it. Under-order and you are paying for a second part-load and a cold joint.
1
Measure it
Every box has its own unit next to it.
Change the unit beside any box and the number converts. The (i) beside each question says what the box wants and where the figure usually comes from. Anything marked per cent is a percentage of the quantity above it.
2
Check the rates
Rates and assumptions used
These are the figures the calculator works to. Change any of them and the order list updates — your merchant’s pack sizes and your own waste allowances beat the defaults every time.
How this one works
Net volume = L × W × thickness plus thickenings and downstands
Order = net × (1 + waste %), rounded up to the plant’s order step
Loads = order ÷ full load size
The figures the calculator works to. Every one can be changed in step 2, and your merchant’s pack sizes and your own waste allowances beat these defaults every time.
Rate
Default
Why
Full load
6 m³
A full mixer. Some plants run 8 m³ wagons.
Concrete density
2.4 t/m³
Normal-weight concrete.
Order step
0.25 m³
What the plant batches in.
Barrow volume
80 L
A full builder’s barrow. You will not fill it every time.
A worked example
With the defaults the page opens on:
How long is the slab? 8 metres
How wide is it? 5 metres
How thick is it? 150 millimetres
Any extras to add on? 0.4 cubic metres
Spare for waste 7.5 per cent
Volume
Slab area: 40 m²
Net volume: 6.4 m³ — area × thickness + extras
Waste allowance: 7.5 %
Order quantity: 7 m³— rounded up to 0.25 m³
Weight: 16.8 t — × 2.4 t/m³
Deliveries
Full loads: 1 × 6 m³
Part load: 1 — part loads usually carry a surcharge
Barrow runs: 88 barrows — if it is wheeled rather than pumped
Change any figure above and every line recalculates; the small print under each row is the sum that produced it.
Questions people ask
How much concrete do I need for a slab?
Length × width × thickness, plus any thickenings or downstands, plus waste. A 8 m × 5 m slab at 150 mm is 6 m³ before extras; the calculator adds your waste allowance and rounds up to the quarter-metre most plants batch in.
How much waste should I allow on ready-mix?
The default is 7.5%. Use 5% on a clean, level formation and 10% on hardcore, because the undulations in a hardcore bed swallow more than people expect.
What is a full load of ready-mix?
The calculator assumes a 6 m³ mixer; some plants run 8 m³ wagons. Anything left over after the full loads is shown as a part load, which usually carries a surcharge - it is often cheaper to add a thickening than to pay for a part load.
How many barrows in a cubic metre?
About 13, at 80 litres to a full builder’s barrow. You will not fill it every time, so the calculator’s barrow count is a floor, not a promise.
Read before you order. These work out quantities, not structural design — spans, lintels, rebar and foundation depths need an engineer and Building Control sign-off. Waste allowances are defaults, not promises: awkward cuts, long runs and bad weather all eat more than 5%. Where a merchant sells whole packs, the figure is rounded up to whole packs.
