How We Calculate
Last reviewed: 9 September 2026
Most online calculators show you a number with no way to check where it came from. We'd rather show our working - what's derived, what's a sourced industry figure, and what we've had wrong before and fixed. This page is the general approach; each calculator's own FAQ and reference tables cite the specific figures that page uses.
Derived, not hardcoded, wherever a formula exists
If a figure can be calculated from the inputs you actually give us, we calculate it - we don't fall back to a single flat number and hope it's close enough.
Featheredge boards per metre is the clearest example. A single "10 boards per metre" figure is genuinely correct - for 125mm boards at a 25mm overlap. The same figure quoted for a 100mm board is wrong by 33%. Our close board fence calculator derives boards per metre from whatever board width and overlap you actually enter (1,000 ÷ (board width − overlap)), so the answer is correct for your boards, not for whichever width a competitor happened to assume.
Postcrete is the same idea applied to posts: bags per post vary by post type (a 100mm timber post needs less than a 100mm concrete post, since the concrete section itself displaces more of the hole), so our fence and close board calculators use a rate per post type with a stated hole-size basis, not one flat "2 bags a post" rule that happens to be right for exactly one post type.
Sourced figures, stated as sourced
Where a real formula doesn't exist - a material's bulk density, a settling allowance, a bag size - we use a sourced industry figure and say so on the page, rather than presenting an assumption as if it were a calculated fact.
MOT Type 1's compacted density (1,920 kg/m³) is a good example of why the distinction matters: compacted and loose density genuinely differ by around 7% for the same material, and conflating them is a real, common way to under-order. Our calculators state which basis a figure uses, and use the compacted figure consistently for tonnage (since mass doesn't change with compaction, only volume does) rather than mixing the two. See our MOT Type 1 standards page for the full detail.
Topsoil density is stated by moisture state for the same reason: screened topsoil ranges from around 1,200 kg/m³ dry to 1,800 kg/m³ soaked, with 1,500 kg/m³ ("damp, as delivered") as the realistic default - a single density figure with no stated moisture basis is a common source of the wide, seemingly-contradictory density figures you'll find quoted elsewhere.
We check our own numbers, including after they ship
Sourcing a figure correctly once isn't the end of it - implementation bugs happen after the fact, and we'd rather find and fix them ourselves than leave them for a visitor to discover. Two examples from this month:
- A floating-point rounding defect meant
Math.ceil()could silently over-order by one unit whenever a waste-adjusted quantity landed exactly on a whole number - found on one calculator, then confirmed present on ten, all fixed with the same shared rounding function. - Two "quick reference" tables (hand-typed summaries of common scenarios) had drifted out of step with the calculators they were supposed to summarise. We now generate those tables at build time from the same code the live calculator runs, rather than hand-typing a number that can silently go stale.
We're not claiming to have never been wrong - we're claiming to check, and to fix what we find.
Standards we've actually verified
Where a calculator cites a British Standard or a Specification for Highway Works clause, we've checked what that document actually says - what it covers and, just as importantly, what it doesn't - rather than repeating a citation we found somewhere else. See the standards reference for the ones we've verified so far.