What a date-based pro rata calculation is for
Most pro rata questions are really date questions. A job starts on the 8th of the month, not the 1st. A tenancy ends two weeks into a rental period. A premium is cancelled with four months of the year left to run. In each case a full amount — a month's salary, a month's rent, a year's premium — covers a period that was only partly used, and the fair amount is the same fraction of the money as the fraction of the time.
That is all this calculator does, but it does it carefully: the dates are counted inclusively, so both the first and the last day count, and your actual dates are clipped to the period, so days outside the period add nothing. Those two conventions are where most disagreements about part-period charges actually live — not in the multiplication.
The three day-count methods
| Method | How days are counted | Where it is used |
|---|---|---|
| Actual days | Real calendar days between the dates, both ends included. | Payroll part-months, rent part-months, most everyday charges. The default, and the method to use unless an agreement says otherwise. |
| 30-day month basis | Every month counts as 30 days; a date on the 31st counts as the 30th. A full month is always 30, a full year always 360. | Finance, bonds and some commercial leases, where tidy equal months make interest and charges predictable. |
| 365-day year basis | Active days divided by 365, applied to an annual amount. | Annual salaries, annual insurance premiums and subscriptions — anywhere the full amount is a yearly figure. |
For most everyday amounts the methods differ by pennies or a few pounds. They diverge most in February, and where a period crosses the 31st of a month. If someone else's figure disagrees with yours, matching their method usually explains the whole difference — which is why this calculator states its method and shows its fraction, rather than just printing an amount.
Two worked examples
Example 1 — moving in mid-month (rent)
Rent is £2,000 a month and the tenancy starts on 8 June. The full period is June: 30 days. The active part is 8 to 30 June inclusive: 23 days. On actual days the fraction is 23 ÷ 30 = 0.7667, and the rent for June is £2,000 × 0.7667 = £1,533.33. The calculator above is pre-filled with exactly this example. If the question is only about rent, the dedicated pro rata rent calculator shows both rent methods side by side.
Example 2 — leaving a job part-way through the year (salary)
An annual salary of £36,500 is agreed, and the employment runs from the start of the year to 23 September — or, the way payroll counts it for someone leaving on 23 September, 100 days of the year remain from 23 September to 31 December. On the 365-day basis, 100 days is 100 ÷ 365 of the year, and 100 ÷ 365 × £36,500 = £10,000.00 exactly — which is why annual salaries divide so cleanly on this method: £36,500 a year is £100 a day. For part-time patterns rather than part-periods, use the pro rata salary calculator.
Cancelling insurance or a subscription mid-term
Insurers and subscription services run this same calculation backwards when you cancel mid-term: the unearned part of what you paid is the fraction of the period still to run — days remaining ÷ total days × the premium. On a pure pro rata basis, a £365 policy cancelled with 100 days left returns £100. You can model your own case with the calculator above: set the period to the policy term, the actual dates to the days remaining, and the amount to the premium, and the result is the pure pro-rata refund.
The warning is that many insurers do not refund on a pure pro rata basis. Under a short-rate cancellation, the insurer keeps a penalty — a common form keeps 10% of the unearned premium, so you receive 90% of the pro rata refund — and a cancellation or administration fee may come off as well. The actual refund is therefore often noticeably less than the figure above. Short-rate terms are set out in the policy's cancellation section; check it before you cancel, and treat the difference between the two figures as the real cost of cancelling. This is general information only, not financial advice.
Frequently asked questions
Calendar days, for almost everything. Rent, subscriptions, insurance premiums and monthly salaries are all built on calendar periods, so a part-period is counted in calendar days — weekends included. Working days appear in a different place: some employers count a part-month of salary by the working days in the month instead. If a figure matters, ask which count was used; the difference is the weekends.
Your first month's pay is the fraction of the month from your start date to the month end, counted inclusively. Start on 8 June in a 30-day month and you are paid for 23 of the 30 days: 23 ÷ 30 of the monthly salary. Payroll software usually uses actual calendar days for this; a few employers use the 30-day basis, which is why first payslips occasionally differ from your own sum by a few pounds.
Most UK payroll software prorates a part-month of salary on actual days in the month, or on the annual 365-day basis for salaried staff — both are offered above. The 30-day basis is more common in finance and commercial agreements than in payroll. If your employer's figure differs from yours, the method is the first thing to check, before the arithmetic.
The calculator clips them: only the part of your actual dates that falls inside the period is counted. If you enter a leaving date after the period ends, the days beyond the period end add nothing — which is how a part-period charge should behave.
Because many insurers do not refund on a pure pro rata basis. Under a short-rate cancellation the insurer keeps a penalty — commonly 10% of the unearned premium, so you receive 90% of the pro rata refund — and may add a cancellation or administration fee on top. The policy document sets out which basis applies; the cancellation section above explains how to model both figures with this calculator.