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Free UK calculator

Back Pay Calculator UK

Check gross back pay after a salary or hourly-rate increase. For employees checking their pay and payroll teams checking an adjustment.

Your pay change

Choose how you are paid, then enter your old and new rates.

When did the increase apply?

Include only the dates you were still paid at the old rate.

DD/MM/YYYY, for example 31/10/2026.

Defaults to today. If your new salary started earlier, choose the last day covered by your old salary. Enter DD/MM/YYYY.

Use your actual annual salary for part-time work. Partial months are estimated using calendar days; your employer’s method may differ.

Payroll controls and adjustments

Leave this at £0 if you have not received any back pay. Use the gross amount before deductions.

Salary adjustments

Calculate first, then replace an individual month where the contract or payroll method produces a different gross amount. An explanation is required.

Salary month adjustments will appear here after the first calculation.

How the estimate works

Calculate the gross difference before payroll deductions

Back pay is normally the difference between what was paid and what should have been paid after a rate change. This tool deliberately stops at gross pay so it does not imitate maintained PAYE software or make assumptions about an employee’s deductions.

Salary example

A salary increases from £30,000 to £33,000, effective for three complete months. The annual difference is £3,000, or £250 a month, giving estimated gross arrears of £750.

Hourly example

A rate increases from £12 to £13 across 160 ordinary hours, producing £160. Ten overtime hours at 1.5× add £15, for estimated gross arrears of £175.

Checks payroll teams should make separately

  • Deductions: PAYE, National Insurance, pensions, student loans and attachment orders should be calculated through maintained payroll software.
  • Partial periods: confirm the contractual or established payroll proration method before replacing the calendar-day estimate.
  • Statutory pay: review whether a backdated increase changes average weekly earnings or an earlier statutory payment.
  • Leavers: check payment-after-leaving reporting and use the original leaving information where HMRC requires it.

Backdated pay-rise questions

Back Pay Calculator — FAQ

Gross back-pay estimates for salary and hourly-rate changes.

What is back pay?

Back pay, sometimes called salary arrears, is gross pay owed for an earlier period. It can arise when a pay rise takes effect before payroll applies the new salary or hourly rate.

Does this calculator include tax and National Insurance?

This calculator estimates gross back pay only. Your employer's maintained payroll software should calculate Income Tax, National Insurance, pension deductions and the final net payment when the arrears are paid.

How are partial salary months calculated?

The salary estimate divides the annual increase by 12 for complete calendar months. Partial months use inclusive calendar days. Employment contracts and payroll systems may instead use working days, annual days or another documented method.

Can I calculate hourly back pay and overtime?

Yes. Use the hourly-pay tab and enter the actual affected hours. Payroll teams can instead enter dated periods with separate ordinary and overtime hours and an explicit overtime multiplier.

Can a leaver receive back pay?

A former employee may still be due arrears if a backdated change covers employment before they left. The employer should also review HMRC's payment-after-leaving rules and any effect on final pay or statutory payments.