How We Verify Our Calculators
Every figure behind our 748+ calculators is checked against current UK primary sources, independently re-derived, and covered by an automated test suite. Here is exactly how we do it.
A calculator is only as good as the figures behind it. A wrong coverage rate, an out-of-date price cap, or a misread British Standard can send someone to the merchants with the wrong order — or worse. So we hold every number on this site to a single rule: verify against the primary source, never guess. This page explains what that means in practice.
We check the primary source, not a secondary copy
Every rate, limit and clause is taken from the authoritative document itself rather than a blog or aggregator. Depending on the calculator, that means:
- British Standards (BS & BS EN) — e.g. BS 7671 for electrical, BS 8500 for concrete, BS 5250 for roof ventilation, BS EN 1991 for structural loads.
- Approved Documents to the Building Regulations — Parts A, B, C, F, K, L, M, P and others, in their current in-force edition.
- Manufacturer technical data — coverage and mix figures taken from the current product data sheets (e.g. British Gypsum, Knauf), not rounded rules of thumb.
- Government & regulator figures — the Ofgem energy price cap, Boiler Upgrade Scheme grant levels, and planning-fee schedules, taken from gov.uk and Ofgem directly.
We cite the current in-force edition
Standards and regulations are revised. We track the edition that is actually in force in the UK and update figures when it changes — for example index-linked planning fees, the quarterly Ofgem price cap, and amendments to the Approved Documents. Where a figure depends on the edition, we name the edition so you can check it yourself.
Independent re-derivation, not rubber-stamping
Our calculators are audited by re-deriving each result from the underlying standard from scratch and comparing it back to the calculator's output — rather than assuming the existing figure is right because it has always been there. This adversarial approach is how we catch the subtle errors: an inverted rule, a clause read the wrong way round, or a coverage rate that under- or over-orders. When a check disagrees with the code, we fix the code.
Backed by an automated test suite
Every calculator has a set of worked examples with hand-derived expected answers, each cited to the UK standard it comes from. These run as part of an automated test suite of more than 14,800 checks across mobile and desktop browsers, so a future change can't silently break a result without a test failing first. We rebuild and re-run the full suite before every deployment.
Honest about what a calculator can and can't do
Our tools give you a sound, standards-based estimate — the right starting number for an order or a quote. They are not a substitute for site-specific design or sign-off. For structural, electrical, gas and fire-safety work in particular, the figures here should be confirmed by a suitably qualified professional and against the specific conditions of your job. Every calculator carries a last-updated date, and where a figure is an estimate or a provisional value, we say so.
Spotted something that looks off?We take accuracy seriously and would rather know. Tell us which calculator and what you expected, and we'll re-check it against the source.