How Do I Pro-Rata an Employee's Pay for a Part Month?

To calculate a part-month salary in the UK, take the employee’s monthly gross pay, divide it by the number of working days in that month, then multiply by the number of working days they actually worked. So for someone on £2,500 a month who worked 10 of the month’s 22 working days, that’s £2,500 ÷ 22 × 10 = £1,136.36. There’s no single method the law forces you to use, but you must pick one and apply it consistently to every starter and leaver.

“Pro rata” is Latin for “in proportion”. A pro rata salary is simply a full salary scaled down to the portion an employee is actually entitled to. When someone joins or leaves partway through a month, they haven’t worked the whole month, so you pay them proportionally for the days they did work rather than a full month’s pay.

You’ll meet the same idea when someone works part-time (a share of a full-time salary) or when you work out holiday entitlement. Here we’re focused on the most common trigger for a payroll query: an employee starting or leaving mid-month.

The most widely used approach in UK payroll is the working-day method. Here’s the step by step:

1. Confirm the exact start or leaving date. Everything hinges on this, so check the contract or resignation letter.
2. Count the working days in that month. For a standard Monday-to-Friday role, count every weekday in the month (adjust for anyone whose contracted pattern differs).
3. Count the working days the employee actually worked. From their start date to month-end for a starter; from the first of the month to their last day for a leaver.
4. Divide monthly gross pay by the total working days, then multiply by the days worked.

That gives you the gross figure to run through payroll. PAYE tax, National Insurance and any pension are then deducted as normal, and the person is paid on your usual pay date.

Say an employee earns £30,000 a year, or £2,500 gross a month, and their last day is Friday 13 March 2026.

• Working days in March 2026: 22
• Working days they actually worked (Mon 2 to Fri 13 March): 10
• Final pay: £2,500 ÷ 22 × 10 = £1,136.36 gross

That’s how you work out final pay when someone leaves a job mid-month, before tax and NI.

Now a new starter on £36,000 a year (£3,000 gross a month) who begins on Monday 16 March 2026.

• Working days in March 2026: 22
• Working days they worked (16 to 31 March): 12
• First-month pay: £3,000 ÷ 22 × 12 = £1,636.36 gross

So if you start a job mid-month, your first payslip is proportionate to the days you worked, not a full month.

There is no legally mandated way to pro-rata part-month pay in the UK, so a few methods are all valid:

• Calendar-day method: monthly pay ÷ number of calendar days in the month × calendar days employed. Our leaver above would get £2,500 ÷ 31 × 13 = £1,048.39, a different figure to the working-day result.
• Annual method: annual salary ÷ 260.71 working days (or ÷ 365 calendar days) × days worked.
• 1/12 method: used when whole months are involved rather than part months.

Each method can produce a slightly different amount, so you can’t cherry-pick the cheapest one each time. Choose a single method, apply it to everyone, and set it out in your contracts or staff handbook so it’s fair and defensible.

If someone joins close to your usual payroll cut-off, they may have to wait until the following month’s run to be paid. It’s worth flagging this early so they can budget, or you can agree a one-off payment outside the normal run by BACS.

If you’d rather hand the whole thing off, our outsourced finance department runs payroll accurately and on time so part-month calculations, starters and leavers are all handled for you.

If part-month pay, holiday and final-pay sums are eating into your time, FD Works can take payroll off your plate entirely. Call us on 01454 300 999 or email [email protected].