Explainer
How to Calculate Pro Rata: The Formula With Worked Examples (2026)
Pro rata turns up on a job posting, a first rent invoice, or an insurance refund, and it always means the same thing: in proportion. Once you see the single formula underneath it, every one of those situations becomes the same quick calculation. You can follow along live as you read — open the pro rata calculator and plug in your own figures, or work through the examples below first.
What is the pro rata formula? Most basic pro rata calculations follow one core formula: full amount × (used ÷ total) — the per-unit value times the units you actually used.
- Worked 6 of 12 months? Multiply by 6/12.
- Occupied 21 of 30 days? Multiply by 21/30.
- Used 183 of 365 policy days? Multiply by 183/365.
Whatever the unit — months, days, or policy days — the arithmetic is identical; only the fraction changes, which is why a single method handles pay, a rent bill, and a refund alike.
The One Pro Rata Formula
Strip away the context and pro rata is a two-step idea. First, find the value of one unit — divide the full amount by the total units in the period. Then multiply by the units you actually used. Written out, that’s full amount × (used ÷ total), and it’s the whole method.
The “unit” is just whatever the situation counts in: months, days, hours, or pay periods. Salary counts in working days or pay periods; rent and a refund count in calendar days; a part-time role counts in days or hours per week. The formula never changes — only the unit and how you count it do. (Occupy 21 of 30 days of a £1,500 rented month and you pay £1,050 — the shape is identical.) That single fact is why the same arithmetic can handle pay, a rent bill, and a refund.
Worked Example 1: Pro Rata Salary
Say a role is advertised at a full-time figure but you work 3 of the 5 standard days a week. Your pay is the full salary × (3 ÷ 5). On a £35,000 full-time salary that is £21,000 — three-fifths of the headline number. The same logic scales an hourly arrangement: 22.5 of 37.5 hours a week on the same full-time figure also comes to £21,000.
For an ongoing part-time role, use the working pattern stated in your contract. The UK pro rata salary calculator accepts days or hours and shows the annual and monthly gross figures. If you start or leave part-way through a leave year, the annual leave calculator handles the separate question of leave accrued during that period.
Worked Example 2: Pro Rata Refund
Cancel a yearly policy or subscription early and a pro rata refund returns the unused part: the full premium × (unused days ÷ total days). A £1,200 annual premium with 182 of 365 days unused produces a proportional baseline of about £598. The agreement or policy decides which days and adjustments count, so treat the result as a figure to compare with the provider’s calculation rather than a guaranteed refund.
How to Calculate Pro Rata Step by Step
Whatever the context, the calculation comes down to three steps. First, name the unit — decide whether you are counting calendar days, working days, months, or pay periods, because that choice fixes everything after it. Second, find the per-unit value by dividing the full amount by the total units in the period. Third, multiply that per-unit value by the number of units you actually used. Spelled out one step at a time, that is the entire formula: full amount × (used ÷ total).
Step one is where people usually go wrong. Mixing calendar days with working days, or counting the final day twice, is what makes two people calculate pro rata from the same numbers and land on different figures — it is almost always a unit or an inclusive-versus-exclusive day-count difference, not the arithmetic. Pin the unit down first; the relevant calculator then shows the dates or working pattern used in its result.
Pick the Right Day Count
Most disagreements over a pro rata figure aren’t about the formula — they’re about how the units were counted. For UK salary work, first identify whether the calculation uses a working-pattern ratio, 12 monthly pay periods or calendar days for an incomplete month. Then confirm whether the final employment date is included. The relevant calculator shows the figures it used, so you can compare them with your contract or payroll policy.
For a specific UK employment question, the focused tools go further: use the UK part-time salary calculator to convert a full-time salary, the UK pro rata holiday calculator for a part-time weekly pattern, and the pro rata annual leave calculator for joining or leaving during the leave year. All three rest on the proportional formula above; the difference is which unit the situation needs.
▲ Check your own figure
Plug your numbers into the UK pro rata salary calculator — it shows the full working, so you can see exactly which days were counted.