Material figures
Densities, bag yields, coverage rates, standard sizes and building regulation limits come from UK sources: manufacturers' data sheets, builders' merchants, British Standards summaries, the Energy Saving Trust, CIBSE guidance and the Approved Documents on GOV.UK. Where sources disagree we use the most widely used UK figure and say "about". Material figures were last checked on 6 November 2026.
Costs
Cost ranges come from published UK cost guides (for example Which?, Checkatrade, MyBuilder and the Federation of Master Builders), the Energy Saving Trust, Ofgem and GOV.UK. We show a low, typical and high figure because real quotes vary with access, finish and the trade's workload. Regional adjustments are our own estimates, built from the regional figures those guides publish. Costs were last checked on 6 November 2026. Time-limited figures, such as the Ofgem price cap or grant amounts, show their dates on the page.
Rounding
- Things you buy whole, such as bags, rolls, panels and packs, are always rounded up.
- Volumes and areas are shown to two or three decimal places.
- Money is rounded to a sensible step (£5, £50 or £100) so ranges don't look more precise than they are.
Waste allowances
Most calculators add a waste allowance you can change. The defaults follow common UK trade practice for that material, for example 10% for concrete and tiles laid straight.
Testing
Each calculator has automated tests with hand-worked examples. Every update to the site runs them, and the site won't publish if any fail.
Limits
Results are estimates for planning and ordering. They are not a design, a survey or a quote. Heat-loss figures are a room-by-room guide, not a full heat-loss survey. Planning tools describe the national rules for houses in England; your council has the final say.