The formula
How to calculate part-time equivalent
A part-time equivalent expresses reduced hours as a fraction of a full-time role. It is the basis for pro-rata pay, holiday, pension contributions and most other entitlements.
Under the Part-time Workers Regulations, part-time staff must not be treated less favourably than comparable full-timers. In practice that means benefits are scaled by the FTE fraction rather than withheld.
Fill in the following:
- Part-time hours per week (hours)
- Full-time hours per week (hours)
- Full-time salary
The result updates on every keystroke. The URL updates too, which makes the filled-in version easy to bookmark or send to someone else.
Some of the fields above will accept figures that seem unusual for your own situation, and that is deliberate: the formula behind part-time equivalent works the same way regardless of scale, so the calculator does not stop you testing a hypothetical scenario a long way from your actual numbers — often the fastest way to see which input the result is most sensitive to.
Why part-time equivalent matters
A calculation like this usually gets used at a decision point rather than out of curiosity — comparing two real options, checking a number a lender or adviser has quoted, or working out whether a plan that sounded fine in conversation still holds up once it is written down with actual figures. The maths itself is rarely complicated; what is hard is remembering which figures to use and in what order, which is exactly what a dedicated calculator is for.
It is also useful as a sense check before signing anything. A quote, an offer letter or a spreadsheet from someone else can contain an error, an optimistic assumption, or simply a different convention for rounding — running the same inputs through an independent calculator is a quick way to confirm a number before relying on it.
It is also worth being clear about what a single figure like this can and cannot settle on its own. It answers the specific question the formula was built to answer, and nothing more — a favourable part-time equivalent result does not automatically mean a decision is a good one overall, since plenty of other factors that a formula cannot capture, from personal circumstances to how comfortable a commitment feels, usually matter just as much as the arithmetic. Use the number as one solid input among several rather than the whole of the decision.
A calculator like this one is often bookmarked and returned to repeatedly over months rather than used once, particularly for anything tied to an ongoing plan such as a mortgage, a savings goal or an investment being tracked. Because the figures live in the web address rather than only in memory, coming back to the same page with updated numbers is quicker than starting from a blank spreadsheet each time.
Worked example
A concrete run-through, using the values already in the fields:
- Part-time hours per week: 22.5 hours
- Full-time hours per week: 37.5 hours
- Full-time salary: 42,000
That gives:
- FTE fraction: 0.6 ×
- Pro-rata salary: 25,200
- Statutory holiday entitlement: 16.8 days
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
Reading the result
The holiday figure is the statutory minimum of 5.6 weeks, pro-rated. For someone working three days a week, that is 16.8 days — the same 5.6 weeks, counted in their own working days.
Where this goes wrong. Bank holidays are the usual source of dispute. Part-timers who never work Mondays lose out unless entitlement is calculated in hours, which is why many employers add bank holidays into a single pro-rated pot.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
Multiply weekly working days by 5.6, capped at 28 days. Three days a week gives 16.8 days; four days gives 22.4. Calculating in hours avoids the bank holiday problem entirely.
Contributions scale with actual pay, so the percentage stays the same and the amount falls. Auto-enrolment has an earnings trigger, so very low part-time earnings may fall below it.
The headline figure is fTE fraction. With 22.5 hours part-time hours per week, 37.5 hours full-time hours per week and 42,000 full-time salary, that comes to 0.6 ×. Change any field and the figure moves with it.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.