Back Pay Calculator UK

Back pay is money you earned but weren't paid at the time, whether through a payroll error, a pay rise applied late, missed overtime, or an employer paying below the legal minimum. It sounds simple: they owe you, they pay you. In practice, the questions of how much, how it's taxed when it finally arrives, and how long you have to claim it all have specific answers worth knowing before you raise it.

Common situations that create back pay

• A pay rise applied late — agreed from January, not processed until April; you're owed the difference for those months
• Payroll errors — wrong hourly rate, missed shifts, incorrect deductions
• Unpaid or underpaid overtime and commission — hours worked or sales made but never paid
• National Minimum Wage underpayment — including subtle forms like unpaid time spent on mandatory training, security checks, or travel between assignments
• Back-dated outcomes — a grievance, equal pay claim, regrading exercise, or tribunal award applied retrospectively
• Statutory pay miscalculated — holiday pay that ignored regular overtime is a classic source of arrears

Calculating what you're owed

The core arithmetic is: (what you should have been paid − what you were paid) x the affected periods. The traps are at the edges:

• A missed pay rise doesn't just affect basic pay, it should ripple through anything calculated from it: overtime rates, holiday pay, pension contributions, and sometimes bonuses
• For minimum wage claims, arrears are calculated at the current minimum wage rate, not the (lower) rate that applied at the time of the underpayment, a detail that works in your favour
• Employer pension contributions on the missing pay may also be owed into your pension scheme

How back pay is taxed

Back pay is taxed in the year it's actually paid, not the year it was earned. If arrears from last year land as a lump sum this year, they count towards this year's income. Two consequences:

• A large lump sum can be taxed heavily in the month it arrives, since PAYE may treat it as your new monthly norm, this usually self-corrects across the year
• Less commonly, a big arrears payment can genuinely push your total income for this year into a higher band, tax you'd have avoided had you been paid correctly on time. In some cases involving multiple years of arrears, HMRC can apportion the payment back to the years it related to on request, worth raising with HMRC for substantial multi-year sums

National Insurance, by contrast, is charged on the lump sum in the period it's paid, using that period's thresholds.

Worked example

Sam's pay rise from £30,000 to £32,400 was agreed effective 1 January but not applied until 1 July, six months late:

• Monthly difference: £200 gross
• Basic arrears: 6 x £200 = £1,200 gross
• Plus: any overtime worked in that window was paid at the old rate, and pension contributions were calculated on the lower salary, both should be corrected too
• The £1,200 is taxed as July income; for a basic rate taxpayer, roughly £240 tax and £96 NI come off, leaving about £864

Time limits: the part people get wrong

Unpaid wages can be claimed as an unlawful deduction from wages in an employment tribunal, but the time limit is tight: three months less one day from the deduction (or from the last in a series of deductions). Two further constraints:

• Tribunal claims for a series of deductions can generally reach back a maximum of two years
• A gap between deductions does not automatically break the series after the 2023 Agnew Supreme Court ruling; the key question is whether the underpayments are linked by a common cause
• In Great Britain, the main practical limit is usually the two-year backstop, although that backstop is currently being challenged in the courts

Alternatively, breach of contract claims in the civil courts have a six-year limit, a route worth discussing with an adviser for older or larger sums. Either way: don't sit on it.

How to raise it

Start informal: most back pay issues are genuine errors, and a clear email to payroll setting out the calculation resolves many of them within a pay cycle. If that fails, raise a formal grievance. If that fails, Acas early conciliation is the required first step before any tribunal claim, and it's free. For minimum wage underpayments specifically, you can also report the employer to HMRC, which investigates and can compel payment of arrears plus penalties, without you needing to bring a claim yourself.

This is general guidance, not legal advice. For a specific back pay dispute, contact Acas, and act quickly given the three-month tribunal window.