How our calculations work
Every estimate should make a distinction between mathematical geometry and the assumptions of real materials.
Canonical units and verified conversions
Calculations use metres, square metres, cubic metres and kilograms internally. International feet and inches convert by exactly 0.3048 metres per foot and 0.0254 metres per inch. One avoirdupois pound is exactly 0.45359237 kilograms. One US liquid gallon is exactly 3.785411784 litres; a UK imperial gallon is different and is not used in this website.
Area and volume are derived from these exact length factors. A US cubic yard is 0.764554857984 cubic metres, and a US short ton is 2,000 pounds. These conversion definitions can be checked against the US National Institute of Standards and Technology guidance: https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8
What is mathematically certain?
For a rectangular area, length multiplied by width gives area. Area multiplied by uniform depth gives geometric volume. For example, 5 m × 3 m × 0.05 m = 0.75 m³. These results are deterministic if the measured dimensions and shape are correct.
The same certainty does not extend to the weight of gravel or topsoil, actual paint coverage, usable wallpaper drops with a complex pattern, cuts reclaimed from a patio slab or the final structural performance of a fence. Those factors require explicit assumptions or professional checking.
Supplier-specific values and purchase rounding
Editable assumptions include loose material density, bag weight, bag volume, wet concrete yield, paint coverage, stock board length, product area per pack and wallpaper roll dimensions. The values initially shown in fields are illustrative starting points, never supplier specifications. Use the product label or a credible technical data sheet for the exact item being bought.
Purchase quantities are rounded upwards to complete bags, rolls, tins, packs, slabs and boards. Waste allowances are applied to the relevant measured or preliminary quantity before purchase rounding. Intermediate calculations are not rounded prematurely. The result panel may round a displayed number for readability while internal calculations retain more precision.
Where the estimates are deliberately simplified
Patio slabs use a rectangular row-and-column grid with joints; decking counts board rows and stock lengths without crediting offcut reuse; wallpaper counts full-height drops by wall group; a fence uses one straight run and at most one gate. Each limitation is explained on its calculator page.
Concrete estimates concern rectangular slab volume only. Structural dimensions, reinforcement, drainage, sub-base, foundations, planning permission and local code requirements are not verified by these calculators. Always obtain suitable independent technical guidance for safety-critical work.
Testing, corrections and source discipline
The software repository includes independent known-answer test fixtures for all ten calculators, conversion tests and validation tests. Automated checks provide useful evidence, but are not proof that every real-world installation scenario has been captured.
Reference material should be used for measurement definitions and individual product claims, not as decoration. We do not reproduce manufacturer charts or make unsupported assertions about a product. Formula corrections and test results can be recorded as the project evolves.