How much back pay are you owed?
If a pay award takes effect on 1 July but your payroll pays the new rate later, the difference is owed as arrears. Choose the award, your role and the months paid at the old rate to see gross back pay, month by month, with overtime and an estimated take-home figure.
Your back pay
The new rate is first paid in Oct 2026 pay. Basic arrears = (new annual − old annual) ÷ 12 × 3.
Month by month
Gross| Period | Old rate | New rate | Basic arrears | Overtime arrears | Running total |
|---|---|---|---|---|---|
| Jul 26 | £3,240.08 | £3,363.17 | £123.08 | — | £123.08 |
| Aug 26 | £3,240.08 | £3,363.17 | £123.09 | — | £246.17 |
| Sep 26 | £3,240.08 | £3,363.17 | £123.08 | — | £369.25 |
| Total | £369.25 | — | £369.25 | ||
From gross to what you may receive
2026/27 tax rules, estimateTax code and one-off payment effects
- One payslip, one month. Arrears are paid as pay in the month they arrive. PAYE works on cumulative pay, so the tax is calculated as if you had earned it that month. A lump can look heavily taxed on the payslip even though it is the same tax you would have paid had the rise arrived on time.
- Higher-rate spill. If your annual pay plus arrears crosses the higher-rate threshold, the part above it is taxed at 40% (Scotland uses its own bands). That is a real cost of the timing, not an error.
- Wrong or emergency code. If your code changes (for example a BR or 0T code on a second job), the lump is taxed on that code. Check the code on the payslip where the arrears appear and correct it with HMRC if it is wrong.
- Overtime and pension. Overtime arrears are normally not pensionable; basic-pay arrears usually are. Whether your administrator collects contributions on arrears depends on the scheme, which is why the pension deduction has a switch.
- Student loan and other deductions are not included here. Employees paid four-weekly are approximated by converting periods to months for the tax estimate.
- Periods that straddle 1 July. If a pay period contains days before and after 1 July, payroll may split it by days, so the first period can differ by a few pounds.
Illustrative planning estimate based on published rules, not financial, legal, pension or tax advice. Your payroll record, contract and scheme administrator control your actual position.
When back pay arises
Firefighter basic pay is agreed by the National Joint Council for Local Authority Fire and Rescue Services and published in numbered circulars that give a new pay table and an effective date. In every award from 2012 to 2026 that date is 1 July. Payroll can only apply a new rate once the circular has been agreed and processed, and that can happen after 1 July. When it does, the extra pay for the months that were paid at the old rate is paid later, usually as a separate arrears amount on a payslip. That amount is back pay.
The effective date, not the payment date, decides what you are owed. If the award applies from 1 July and your service first pays the new rate in October, you have three monthly payslips (July, August and September) paid at the old rate, and you are owed the difference for each of them. The calculator asks how many pay periods were paid at the old rate and does the arithmetic for you.
The calculation
For basic pay: arrears = (new annual pay − old annual pay) ÷ pay periods a year × pay periods paid at the old rate. Monthly pay uses 12 periods a year and four-weekly pay uses 13. For a competent firefighter and the 2026 award the annual difference is £40,358 − £38,881 = £1,477. That is £123.08 a month, or £113.62 for each four-week period. Three months is £369.25 gross, three four-week periods is about £340.85, and six months is £738.50.
For overtime the calculator multiplies the overtime hours you work in a month by the increase in the published overtime rate. A competent firefighter's rate rose from £26.63 to £27.65, which is £1.02 an hour, so ten hours of overtime a month for three months adds £30.60 and takes the three-month total to £399.85. The overtime rate is time and a half of the hourly rate. Group and Area Managers do not have a nationally published overtime rate, so the tool does not calculate overtime arrears for those roles unless you enter your own rates using the other pay scale option.
| Role and stage | Old annual (2025) | New annual (2026) | Per month | Three months |
|---|---|---|---|---|
| Firefighter, Development | £30,384 | £31,539 | £96.25 | £288.75 |
| Firefighter, Competent | £38,881 | £40,358 | £123.08 | £369.25 |
| Crew Manager, Competent | £43,104 | £44,742 | £136.50 | £409.50 |
| Watch Manager, Competent B | £48,202 | £50,034 | £152.67 | £458.00 |
| Station Manager, Competent B | £55,301 | £57,402 | £175.08 | £525.25 |
The awards before 2026 work in the same way, with the old and new rates read from the NJC tables for that year. The calculator covers every award from 2012 to 2026.
From gross to net
Back pay is taxable pay, so what reaches your bank account is lower than the gross figure. The tool estimates the difference by running the arrears through a month-by-month PAYE model on 2026/27 rules: cumulative income tax on your tax code, employee National Insurance, and pension contributions on basic-pay arrears at the percentage you enter. It calculates the payslip in the month the arrears arrive with and without the lump and reports the difference. For a competent firefighter on the standard code with a 12.59% pension contribution, £369.25 of gross arrears comes out at about £228.67 net, made up of £64.55 income tax, £29.54 National Insurance and £46.49 pension contributions.
Those figures are an estimate. Pension bands are worked out by the scheme, your tax code may not be the standard one, and student loan deductions are not included. Wales and Northern Ireland use different pension contribution rules, so the percentage is editable. It is best treated as a sense check on the size of the amount, not a prediction of the payslip to the penny.
Tax code and one-off payment effects
PAYE income tax is worked out on your pay so far in the tax year, not month by month in isolation. When arrears arrive in a single payslip, that month's taxable pay jumps, and the tax due to date rises with it. Over the year you pay the same tax as if the rise had been paid on time, unless the lump pushes some income across the higher-rate threshold. The extra tax on a large lump can therefore look like an overpayment on the payslip even when it is not.
Your tax code matters more than most people expect. On the standard 1257L code the lump is taxed using your personal allowance across the year. If a payslip shows an emergency or basic-rate code such as BR, the lump is taxed on that code until HMRC corrects it, and any overpayment is reconciled afterwards. Check the code on the payslip where the arrears appear. If arrears are paid in a later tax year than the months they relate to, they are taxed on the rules and code of the year they are paid in.
How late have awards been?
The NJC circular for the 2022 award, NJC/1/23, is dated 6 March 2023, eight months after its 1 July 2022 effective date, so that award was backdated. For a competent firefighter, the difference between £32,244 and £34,501 is £188.08 a month, and eight months is £1,504.67 gross. On the tool's estimate about £943 of that is take-home. That is an illustration of the arithmetic, not a record of what any service paid or when, because implementation dates differ by service. More recently the circulars were issued close to or before the effective date: NJC/3/24 on 20 May 2024, NJC/3/25 on 30 May 2025 and NJC/2/26 on 3 July 2026. London Fire Brigade's policy records implementation of the July 2026 rates in July 2026 salaries. The premium pack's scenarios tab shows these dates alongside what a payroll run in the circular's month would have meant.
Checking your payslip
- Find your role and stage and the old and new annual rates in the NJC circular for the award year.
- Check that the basic pay on the payslip from the implementation month equals the new annual figure divided by 12 (or 13).
- Look for an arrears line, which may be called arrears, backdated pay or pay award adjustment. If there is none, ask whether it is included in another line.
- Compare the amount with this calculator using the exact number of periods paid at the old rate. Allow a few pence for rounding.
- Check overtime, on-call activity pay, retainers and CPD lines too. Anything based on the pay rate should also have moved from 1 July.
- Keep every payslip and your service's implementation notice.
What the calculator does not do
It does not know your service's payroll calendar, so you supply the number of periods. It does not include on-call retainers and activity payments, CPD, allowances, flexible duty supplements, holiday pay averaging or student loan repayments. It treats each pay period as a whole month or a whole four-week block, and a period that straddles 1 July may be split by days by payroll. It uses the national NJC scale unless you use the other pay scale option. If your figures disagree with your payslip, your payslip and your payroll department are the authority.
For background on the mechanics read the guide to firefighter pay rise back pay, the 2026 pay rise guide and every pay point compared. To see what awards were worth in real terms, use the pay vs inflation calculator. To check whether your payslip has the new rates, use the payslip checker. The paid Pay History & Back-Pay Pack adds delay scenarios by role, overtime scenarios, a printable briefing and AI answers grounded in your numbers.
Frequently asked questions
How do I calculate firefighter back pay?
Take the difference between the old and new annual pay for your role, divide by 12 if you are paid monthly (13 if paid every four weeks) and multiply by the number of pay periods paid at the old rate. For a competent firefighter and the 2026 award that is (£40,358 − £38,881) ÷ 12 = £123.08 a month, so three months is £369.25 gross. Add overtime hours multiplied by the rise in the overtime rate (£1.02 an hour for a competent firefighter in 2026).
Is the 2026 firefighter pay rise backdated?
The NJC circular NJC/2/26 states the 3.8% increase takes effect from 1 July 2026, so if your service pays the new rate in a later month the difference for earlier months is owed as arrears. The circular is dated 3 July 2026, two days after the effective date. Whether you are owed anything depends on when your payroll first applied the new rate.
How is back pay taxed?
Back pay is pay, so it is subject to income tax, National Insurance and, on the pensionable part, pension contributions through payroll in the month it arrives. PAYE works on cumulative pay, so the lump does not change the tax you owe for the year in principle, but it can make that payslip look heavily taxed and can push part of it into a higher band. The calculator gives an estimate on 2026/27 rules.
Does back pay include overtime?
It should include anything that is calculated from the pay rate and rose on the effective date, which includes overtime worked between 1 July and the date the new rate was applied. Enter your typical overtime hours a month and the tool multiplies them by the increase in the published overtime rate. Group and Area Manager roles have no nationally published overtime rate, so overtime shows as zero for them.
Is back pay pensionable?
Basic pay is normally pensionable and overtime normally is not, so the tool deducts a pension contribution only on the basic-pay arrears, and you can switch it off. How your administrator records contributions on arrears depends on the scheme and your service, so treat this as an estimate and check your payslip.
What if I am paid every four weeks?
Choose 'Every 4 weeks'. The tool divides the annual increase by 13, lists each four-weekly period from 1 July and adds them up. For the net estimate it converts periods into months, which is an approximation, and a period that straddles 1 July may be split by days by payroll.
Can I use it for London Fire Brigade or another local scale?
Yes. Turn on 'Other pay scale' and enter your old and new annual pay from your service's pay policy. Overtime rates can also be entered by hand. London Fire Brigade's Policy 558 lists its own scale, and it is not the same as the NJC table.
What should I do if my back pay is missing or looks wrong?
Write down your role, stage, old and new annual pay, the periods paid at the old rate and your expected gross figure, then ask payroll in writing for the implementation date and the calculation, quoting the circular reference. If you are an FBU member your local representative can help. The tool is a check on the arithmetic, not a ruling on what you are owed.
Sources and assumptions
- NJC Circular NJC/2/26 (3.8% from 1 July 2026, dated 3 July 2026) and NJC/3/25 (3.2% from 1 July 2025): annual pay, hourly and overtime rates.
- NJC/1/23 (2022 award, dated 6 March 2023), NJC/3/23 and NJC/3/24; FBU pay settlement pages (example: 2018) for 2012 to 2021.
- FBU circular 2026HOC093SW: applying the award as a matter of urgency; LFB Policy 558: London's own scale and July 2026 implementation.
- GOV.UK: tax codes. Tax and National Insurance use the 2026/27 rules held in
tax2026.ts; pension percentage is your input, defaulted from the FPS 2015 England and Scotland bands. - Assumptions: the new rate applies from 1 July; periods are whole months (or four-week blocks); overtime uses the published rate; net uses cumulative PAYE on the payslip in which arrears arrive; local implementation, deductions not listed and pension treatment of arrears may differ. Illustrative, not payroll, tax or legal advice.