About this calculator
This calculator helps estimate how much concrete a floor slab or foundation slab needs — just enter the length, width, and thickness. It works for a house's floor slab as well as for a garage or patio pour, when the shape is rectangular.
Formula
The calculator converts the dimensions (length A, width B, thickness C) from nanometres to millimetres, then:
area = A × B
net volume = area × C
total volume = net volume + round(net volume × waste% / 100)
perimeter = 2 × (A + B)
Dimensions are converted to millimetres before multiplying, rather than computed directly in nanometres — otherwise a 100 m × 100 m slab's volume in cubic nanometres would exceed the safe integer range. The waste percentage is a user input (default 10%), added to the net volume and rounded to the nearest cubic millimetre. Perimeter is computed for informational purposes (e.g. to estimate formwork) and has no effect on the volume calculation itself.
Worked example
Defaults: length 5 m, width 4 m, thickness 150 mm, waste 10%.
area = 5 m × 4 m = 20 m²
net volume = 20 m² × 0.15 m = 3.0 m³
total volume = 3.0 m³ + (3.0 m³ × 10 / 100) = 3.3 m³
perimeter = 2 × (5 m + 4 m) = 18 m
For these inputs the calculator returns exactly this: net volume 3.0 m³, total volume (incl. waste) 3.3 m³, area 20 m², and perimeter 18 m.
FAQ
How much waste should be allowed when pouring a slab? The calculator suggests 10% by default, but you can change this percentage from 0 to 50 — it's your own input, not a standard-based recommendation. The real requirement depends on formwork quality, subgrade evenness, and the pouring method.
Do I need to calculate formwork separately? The calculator shows the perimeter, which helps estimate the formwork length, but it does not calculate formwork materials itself — only the sum of the slab's own edge lengths.
Why does the calculator warn about a thin slab? When the thickness is below 100 mm, the calculator shows a warning — such a thickness may need different reinforcement or a use-case check that this calculator does not perform.
Is the result suitable for ordering concrete from a plant? The total volume (incl. waste) is a reasonable starting point for an order, but the final quantity is always worth confirming with your supplier, especially when the slab shape isn't simply rectangular.
Assumptions and limits
The calculator assumes the slab is rectangular and of uniform thickness across its whole
area — for a more complex shape or a stepped thickness, the result will only be
approximate. No standard was found for this calculation, and none is attempted — it's a
plain volume formula (length × width × thickness), not an engineering calculation, so
meta.standards is deliberately left empty.
The waste percentage is a user input with a default value, not a constant derived from a standard or manufacturer data.
This calculator is the only one on this platform that returns an items[] list — a single
concrete (C25 grade) material line with the net volume and its own separate waste factor.
This means the net quantity and waste factor are passed to the partner budget system
separately, not pre-multiplied — the system can apply its own waste policy.