Job adverts quote annual, agencies quote daily, contracts quote hourly and payslips arrive monthly. Comparing two offers means putting them in the same unit first, and that conversion is where the errors live: a month is not four weeks, and there are twenty-six fortnightly pay runs in a year, not twenty-four.

Using it in three steps

1. 1 Type the figure you were given and pick what it is — hourly, monthly, annual and so on.
2. 2 Adjust hours, days and working weeks if yours are not the usual 40 / 5 / 52.
3. 3 Read all six figures at once. The one you typed is in the list too, so nothing has to be taken on trust.

Why a month is not four weeks

Fifty-two weeks divided by twelve months gives 4.33, not 4. Multiplying a weekly wage by four understates the monthly figure by about eight per cent — on a 1,000 weekly wage that is 333 missing every month, or 4,000 a year. The tool goes through the annual figure for exactly this reason: annual divided by twelve is the only monthly number that adds back up correctly.

Gross is not what lands in the account

This tool deliberately stops at gross. Net pay depends on the country, the year, your allowances, your pension and half a dozen other things that change annually — a built-in tax table would be wrong for most visitors on the day it shipped, and quietly wrong for everyone else a year later. Converting units is a question with one correct answer; calculating net pay is not, and the two do not belong in the same box.