How we calculate
This page lists every formula and every default figure our calculators use. It's built from the calculators' own code and figures, so it can't drift out of step with them.
Three kinds of figure
- Typical value: a planning figure from a published source, such as a typical density for gravel. Your supplier's figure takes precedence.
- Our assumption: an allowance we chose, such as 10% extra for waste. Change it to suit your job.
- Common UK size: a size products often come in, such as a 25 kg bag. Check the size you're buying.
Figures marked "Not yet checked against maker data" are still waiting for our second check against manufacturer or supplier data.
Rounding
Quantities are worked out in metric units and rounded once, at the end. Anything you buy whole (bags, tins, boards, packs) is always rounded up.
We don't test products
We don't test products and our calculators don't recommend brands. A calculator gives a quantity estimate, not advice.
Formulas and figures by trade
- Groundworks (7 calculators)
- Concrete and mortar (5 calculators)
- Brickwork (5 calculators)
- Plastering (5 calculators)
- Roofing (6 calculators)
- Flooring (4 calculators)
- Tiling (3 calculators)
- Decorating (4 calculators)
- Landscaping (12 calculators)
- Decking and fencing (4 calculators)
- Carpentry (7 calculators)
- Electrics (10 calculators)
- Plumbing (5 calculators)
- Heating (8 calculators)
- Insulation and ventilation (5 calculators)
- Site safety (6 calculators)