Every statutory figure on this site comes from primary sources: legislation.gov.uk for the law itself, and GOV.UK, Acas or nidirect for the current thresholds.
The rules are stored as versioned data rather than written into the calculators. Each tax year is a separate record with its own source references and a date recording when the figures were last checked. That is why a calculation for a past date uses that year's limits rather than today's.
Every calculator shows its workings. If you cannot see how a figure was reached, treat that as a fault and tell us.
Where the law is genuinely unclear — the classification of irregular hours workers is the clearest example — the tool says so rather than picking an interpretation quietly.
Nothing here is legal or financial advice.