How to Calculate Back Pay for Unpaid Wages or Salary Errors?

Last reviewed: 19 October 2026

Payroll errors are more common than most businesses expect — a pay rise applied late, hours missed from the timesheet, or a minimum-wage rate that wasn’t updated in April. When that happens, the shortfall doesn’t just disappear: the employer has to make it up through back pay.

This guide explains how to calculate back pay for hourly and salaried employees in the UK, with worked examples using current rates, plus the tax treatment, claim time limits, and the mistakes to avoid. If you’re new to the term, start with our guide on what is back pay.

What Does Back Pay Mean on a Payslip?

Back pay — sometimes called “arrears of pay” — is the difference between what an employee was paid and what they were entitled to receive for work already done. On a payslip, it usually appears as a separate line under “Earnings”, labelled “Back Pay” or “Arrears”, showing the gross amount before deductions. It’s kept apart from the current period’s pay so the record stays clear for both the employee and any future audit.

Why Does Back Pay Arise? Common Causes

• Delayed or backdated pay rises — the increase was agreed but only applied months later.
• Payroll errors — a wrong rate, missed hours, or a dropped pay run.
• Unpaid overtime — extra hours worked but never recorded or paid.
• National Minimum Wage underpayments — the rate in the payroll system wasn’t updated. From 1 April 2026, the National Living Wage is £12.71 an hour for workers aged 21 and over (see the current National Minimum Wage rates on GOV.UK).
• Missed holiday pay — statutory or contractual holiday pay not paid, or not recalculated after a pay rise.
• Equal-pay or tribunal outcomes — compensation ordered after an employment dispute.

How to Calculate Back Pay: Step by Step

Step 1 — Identify the back-pay period

Pin down the exact start and end dates the underpayment covers. If a pay rise took effect on 1 July but was only applied in October, the period is 1 July to 30 September.

Step 2 — Find the difference in pay rate

For hourly staff: correct hourly rate − rate actually paid. For salaried staff: (new annual salary − old annual salary) ÷ pay periods per year (12 for monthly pay, 52 for weekly pay).

Step 3 — Multiply by the time owed

Hourly: rate difference × hours worked in the period. Salaried: monthly or weekly difference × the number of months or weeks owed.

Step 4 — Add affected extras

Include overtime at the correct overtime rate (not the standard rate), bonuses or commission earned in the period, and holiday pay recalculated at the higher rate. Check whether pension contributions need adjusting too, since they’re based on qualifying earnings.

Step 5 — Apply tax and National Insurance

Back pay is processed through PAYE like normal earnings — it is not tax-free. See the tax section below for the exact treatment.

Step 6 — Report it correctly

The adjustment is paid in the next payroll run and reported to HMRC through the Full Payment Submission, allocated to the correct earnings period.

Worked Example: Salaried Employee

An employee’s annual salary increases from £25,000 to £30,000, backdated for 3 months:

• Old monthly pay: £25,000 ÷ 12 = £2,083.33
• New monthly pay: £30,000 ÷ 12 = £2,500.00
• Monthly difference: £2,500.00 − £2,083.33 = £416.67
• Back pay owed: £416.67 × 3 months = £1,250

Worked Example: Hourly Employee (Minimum Wage)

A 24-year-old employee worked 100 hours in April 2026 but was paid the 2025/26 National Living Wage of £12.21 instead of the current £12.71:

• Rate difference: £12.71 − £12.21 = £0.50 per hour
• Back pay owed: £0.50 × 100 hours = £50

Important: minimum-wage arrears are calculated at the rate that applied when the work was done, not today’s rate — so always check the rate in force for the period concerned.

When Can Employees Claim Back Pay?

UK employment law sets strict time limits (per current ACAS guidance):

• Single underpayment: 3 months minus one day from the date the wages should have been paid to bring a claim to an Employment Tribunal.
• A series of linked underpayments (the same error, with less than 3 months between deductions): 3 months minus one day from the most recent deduction — and you can claim up to 2 years back.
• National Minimum Wage underpayments: you can complain to HMRC instead of going to a tribunal — HMRC can issue a notice of underpayment going back up to 6 years. Note you must choose one route; you can’t run the same issue through both processes.
• Employees who have already left can still claim, within the same time limits.

See the ACAS guidance on claiming unpaid wages for the full details.

Do Employers Have to Pay Back Pay?

Yes. If an employee hasn’t been paid what they’re contractually or legally entitled to, the employer must make up the difference — failing to do so can be a breach of contract, and unauthorised deductions from wages are unlawful under the Employment Rights Act 1996. In practice: calculate the arrears, explain the error to the employee in writing, and pay the difference in the next payroll cycle.

How Is Back Pay Taxed in the UK?

• Income Tax: for arrears of pay, tax generally relates to the tax year in which the employee was originally entitled to the earnings. For a backdated pay rise, it’s usually taxable when the employee becomes entitled to the additional amount.
• National Insurance: NICs are normally calculated on the pay period in which the arrears are actually paid.
• Tax-band effect: a large lump sum can push a single pay period into a higher tax band — this usually evens out over the tax year.
• A significant payment that crosses tax years may affect a Self Assessment tax return.

How to Recover Back Pay

1. Gather evidence — payslips, your employment contract, timesheets or rotas, and any written pay-rise offer.
2. Raise it in writing with your employer — most errors are corrected in the next payroll cycle once flagged.
3. Escalate if needed — raise a formal grievance, then contact ACAS for Early Conciliation before the tribunal time limit expires.

Back Pay After Resignation

Leaving the job doesn’t cancel the debt. Identify the start and end dates of the underpayment, calculate the difference for the hours or weeks worked, and include final entitlements such as accrued holiday pay. Back pay remains taxable and subject to Income Tax and NI under PAYE, and the same claim time limits apply.

Common Mistakes to Avoid

• Treating back pay as tax-free — it must go through payroll (PAYE).
• Forgetting holiday pay — it must be recalculated at the higher rate; it’s a legal requirement.
• Miscalculating overtime — use the correct overtime rate, not the standard hourly difference.
• Ignoring pensions — contributions may need adjusting on the new, higher earnings.
• Relying on assumptions — check the actual contract and timesheets rather than guessing.

Best Practices for Organisations

• Communicate promptly in writing: the pay rise, its effective date, and when the back pay will arrive.
• Give each affected employee a breakdown of how their figure was calculated.
• Update payroll records immediately so the error doesn’t compound.
• Use reliable payroll software to automate calculations and tax deductions.
• Show back pay separately on the payslip to avoid confusion.
• Review contract terms to confirm when the entitlement legally begins.

Can Payroll Software Simplify Back Pay?

Yes — modern payroll software can calculate retroactive pay for hourly and salaried staff (including tax and NI), generate corrected payslips showing the back pay separately, and keep a clear audit trail. If payroll is handled externally, a payroll provider can process the correction and handle the HMRC reporting. For the broader picture, see our guides on how outsourced payroll works and payroll for small businesses.

Get Payroll Right the First Time

Managing payroll corrections alongside micro entity accounts can be time-consuming — and getting it wrong is expensive. If you’d like support with payroll calculations, back-pay adjustments and compliant financial reporting, call us on 0203 441 1258 or email info@microentityaccounts.co.uk.

Frequently Asked Questions

How do you calculate back pay for hourly employees?

Multiply the hours worked by the difference between the correct hourly rate and the rate actually paid, then add any affected overtime, bonuses, commission and holiday pay.

Is back pay taxed in the UK?

Yes — it’s employment income processed through PAYE. Income Tax generally relates to the tax year the employee was entitled to the earnings, while National Insurance is calculated when the arrears are paid.

How far back can I claim unpaid wages?

Through an Employment Tribunal, up to 2 years back for a linked series of deductions, with the claim made within 3 months minus one day of the last deduction. HMRC can pursue minimum-wage arrears going back up to 6 years.

Does back pay appear on my payslip?

Usually yes — as a separate “Back Pay” or “Arrears” line under Earnings, shown gross before deductions and kept apart from the current period’s pay.

Do employers have to pay back pay?

Yes, where the employee was legally or contractually entitled to it. Unpaid amounts can amount to a breach of contract and an unlawful deduction from wages.

The Bottom Line

Back pay is simply the difference between what was paid and what was owed — but calculating it correctly means pinning down the exact period, using the rates that applied at the time, adjusting for overtime, holiday pay and pensions, and processing it properly through PAYE. Done right, it keeps employees whole and your business compliant with HMRC and UK employment law.

Disclaimer: The content provided on Micro-Entity Accounts, including our blog and articles, is for general informational purposes only and does not constitute financial, accounting, or legal advice.