The maths, explained
Measure a rectangular slab in one consistent unit system. Thickness is easy to misread: a drawing may show millimetres or inches while length and width use metres or feet. The calculator converts dimensions to metres before finding net cubic volume, adds your overage, and prices the order volume in the displayed cubic unit. Separate irregular sections before adding their volumes.
Net volume = length × width × thickness; order volume = net volume × (1 + overage / 100)
A worked example
The worked example below uses the calculator’s initial inputs and the same calculation function as the live interface. Replace example amounts with your own records; changing currency does not convert the numbers. Physical reference dimensions are stored in metric units, and the interface can display their imperial equivalents. Rounded display values may differ slightly from the full precision retained in the calculation.
| Your inputs | Editable example — not a market quote |
|---|---|
| Length | 6 m |
| Width | 4 m |
| Thickness | 100 mm |
| Order quantity with 10% example waste | 2.64 m³ |
Assumptions used
The result assumes simple geometry and the dimensions actually entered. Waste, unit price and allowances are editable assumptions, not verified regional market rates. An allowance is not a guarantee: compare the answer with the chosen product, cutting or laying plan, and delivery constraints. Tax is applied to the complete subtotal including contingency in this model; adjust the budget if your applicable tax base differs.
On your side of the Atlantic
Use metres, square metres and supplier quotations in pounds. UK VAT depends on the work and eligibility: 20% is a common standard-rate reference, not an assumption applied by this tool.
Common mistakes
Do not enter an inch thickness as feet. Check the formwork, base preparation, delivery minimum, pump cost and packaging with the supplier. The model estimates volume; it does not select concrete strength or reinforcement.
Before ordering or tendering, compare at least two scenarios: your base takeoff and a realistic alternative. Keep assumptions with the result so differences are explainable. Save an estimate only on a suitable device. A shared link contains the entered values, so avoid including confidential information. An exported estimate is a snapshot and does not update when a supplier changes a price.
Questions from the jobsite
What does this calculator measure?
What is the first mistake to avoid?
Are the example prices supplier quotes?
Which tax assumptions apply in United Kingdom?
How should I handle waste and contingency?
Can I use the result in a tender or estimate?
Planning estimate only. Not a substitute for a professional quote or local building-code compliance. Verify quantities, tax treatment and contract requirements.
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