About proratacalc

proratacalc builds single-purpose pro rata calculators that always show the formula with your own numbers, so you can see exactly why a figure comes out the way it does.

"Pro rata" just means in proportion — but the same words hide a lot of small choices: how you count days, whether both endpoints count, which divisor a payroll or insurer uses. Those choices are usually why a number you're given doesn't match the one in your head. Most calculators give you a single figure and stop. We show the day-count basis, the counted days, and the line-by-line derivation, so you can check the result rather than trust it.

How we calculate

Every calculator on this site runs on one small, tested arithmetic engine. The headline figure and the steps shown beneath it come from the same computation — the steps are never a re-typed approximation. The day-count basis selector (Actual/actual, Actual/365, Actual/360, 30/360) and the inclusive/exclusive endpoint toggle map directly to how the math is done, so changing them changes both the answer and the shown working in lockstep.

The calculations happen entirely in your browser. Nothing you type is sent to a server — see the privacy policy for detail.

Not advice

These are estimation tools, not financial, tax, insurance, or legal advice. See the terms of service.

Who's behind it

Miguel Lin — Software engineer · builds and verifies the calculators.

Miguel Lin is a software engineer who builds and maintains the calculators on proratacalc. His focus is the thing the whole site is built around — getting the math right and showing every step, so a figure can be checked rather than trusted. Every formula runs on a unit-tested engine and is cross-checked against authoritative worked examples (GOV.UK and ACAS guidance, US payroll references, IRS publications) before it ships. For legal, tax, and insurance specifics, proratacalc cites primary sources and notes a qualified reviewer on those pages; the calculators are estimation and transparency tools, not professional advice.

Have a correction or a calculator you'd like to see? Use the contact page.