Check what a tax code means
Every UK tax code explained, with exact tax-free pay for each pay schedule.
HMRC reports that 5.6 million taxpayers checked their pay in the HMRC app in the 2025 to 2026 year, a total of 100 million times, and that 20 million people used their Personal Tax Account over the same period [1]. Once a change is reported, HMRC tells the employee and the employer the new tax code within 15 working days [2].
Those two figures frame the whole process. A tax code is not something an employee or an employer can simply edit in payroll. Only HMRC can change it, and the route to a new code always runs through HMRC's records of income, benefits and allowances [5]. What employees control is the information HMRC holds. What employers control is how quickly and accurately a new code reaches the payslip.
This guide sets out both halves. It covers the reasons a code changes, the channels an employee uses to correct HMRC's records, the timeline from report to payslip, the notices employers receive, how a new code is applied on a cumulative or week 1 and month 1 basis, and worked examples showing how much a code change moves take-home pay for the 2026-27 tax year.
Key takeaways
- Only HMRC can change a tax code. Employers must operate the code HMRC sends and must not use another code unless HMRC says so [5].
- Employees change a code by updating the details HMRC holds, through HMRC's online Income Tax checking service, the HMRC app or HMRC's income tax enquiries channels [2] [4].
- HMRC notifies both parties within 15 working days, and the new code usually appears on the next or following monthly payslip, or by the third weekly payslip [2].
- Employers update the payroll record as soon as possible and before the next payday, adding any previous pay and tax included with the notice [3].
- No single deduction of tax can exceed 50% of gross pay for the period, whatever the code [7].
Who can change a tax code
The legal starting point is that HMRC determines the code. The PAYE regulations require HMRC, when it sets a code, to take account of the employee's reliefs, other PAYE income and any tax underpaid or overpaid in earlier years [11]. The code is the output of that calculation, not an input an employer chooses.
The employee's role
An employee cannot pick a code, but can change the facts the code is built on. HMRC's guidance directs employees to check the employment, pension, estimated taxable income, company benefit and expenses details HMRC holds, and to update anything wrong or missing [2]. When those details change, HMRC recalculates the code and issues a new one if the result differs [4].
Not every update produces a new code. HMRC states that not all changes will affect a tax code, and that it will tell the employer or pension provider when one does [4]. A pay rise that leaves the employee in the same income tax band, for example, may change nothing on the payslip beyond the higher gross pay [22].
The employer's role
The employer's duty is narrower and stricter. The Low Incomes Tax Reform Group summarises it plainly: employers operate the code HMRC gives them, change it when HMRC says so, and must not use another code under any circumstances unless HMRC advises it [5]. A code quoted by an employee, a code from an old payslip or a code an employee believes is correct carries no authority until HMRC issues it.
When an employee disputes a code, the employer's job is to pass the employee to HMRC. HMRC does not explain on the employer's notice how the code was calculated, so the employer usually cannot answer the question in any case [5]. The employee can see the calculation in HMRC's online Income Tax checking service [16].
Employers who apply the wrong code risk an underpayment that HMRC will first seek from the employer, not the employee [5]. That is why disciplined code handling is a core feature of any HMRC-recognised payroll software for SMEs.
Why a tax code needs changing
HMRC lists the common triggers for a new code, and most fall into a handful of families [6]. The table groups them by the action that usually follows.
| Trigger | Typical effect on the code | Who usually tells HMRC |
|---|---|---|
| New job or second job | Allowance split, or BR, D0 or D1 on the second job | Employer through the FPS, employee online |
| Company benefit starts or stops | Allowance reduced or restored, sometimes a K code | Employer, employee online |
| Marriage Allowance claimed | M or N suffix | Employee |
| Work expenses claimed | Allowance increased | Employee |
| Tax owed from an earlier year | Allowance reduced to collect it | HMRC |
| Savings interest or other untaxed income | Allowance reduced | Employee, HMRC data |
| State Pension changes | Allowance reduced on another source | HMRC |
The triggers are drawn from HMRC's list of reasons a code changes [6], and the letter meanings from HMRC's employer guidance [14].
Changes in income and benefits
Income changes are the most frequent trigger. HMRC asks employees to update their estimated income when they start earning regular overtime, receive a pay rise or bonus, see their hours change on a zero hours contract, or partially retire [4]. The estimate matters because HMRC uses it to decide whether allowances need restricting across all employments [7].
Benefits in kind work the same way. When an employer reports that an employee has started or stopped receiving a benefit such as a company car, HMRC reduces or restores the tax-free allowance in the code [3] [6]. Where deductions exceed allowances, HMRC's system issues a K code, which adds an amount to taxable pay instead of giving an allowance [8]. The mechanics of K codes are covered in the guide to the K tax code.
Allowances, reliefs and old debts
Claims that increase the allowance include Marriage Allowance and tax relief on work expenses [6]. Marriage Allowance lets one partner transfer part of their Personal Allowance to the other, and the code shows the transfer through an M suffix for the recipient and an N suffix for the transferor [20] [14].
Codes also change to collect tax. An employee who owes less than £3,000 on a Self Assessment bill, and who filed a paper return by 31 October or an online return by 30 December, can have the debt collected through the code [25]. An employee who was on the wrong code in an earlier year and owes tax will see a reduced allowance until the debt is cleared [6] [15].
How an employee changes a tax code
HMRC offers three routes, and the online route is the quickest [2]. The table compares them.
| Route | How it works | Best for |
|---|---|---|
| HMRC's online Income Tax checking service | Sign in, review employment, pension, income and benefit details, update anything wrong | Most changes, including expenses claims |
| HMRC app | PAYE section, select 'update your estimated income', or 'add missing income' | Pay rises, bonuses, missing employments |
| HMRC income tax enquiries | Contact HMRC directly when the online service cannot be used | Complex cases, digital exclusion |
| Agent (P2 query form) | The agent reports the client's income change on the PAYE Coding Notice query form | Clients with an accountant |
The routes are set out on HMRC's tax code update page [2] and its guidance on reporting a change in employment income [4].
Online and in the HMRC app
In the Personal Tax Account, the employee opens the PAYE section, selects 'check current tax year', then 'view or update employment details' and 'update your estimated taxable income' [4]. In the app, the same section offers 'update your estimated income', and missing income is added through the 'add missing income' option [4]. HMRC reports that app users rose by 28% in a year, reaching 7.6 million unique users in the 2025 to 2026 year [1].
The app also shows the current code, the National Insurance number and income and employment history for the past five years [28]. For an employee who suspects a wrong code, checking the app before calling HMRC usually resolves the question of which detail is out of date [16].
Second jobs and pensions
Employees with more than one job usually have a different code on each, with the second job on BR, D0 or D1 [23]. HMRC can split the Personal Allowance between jobs on request, but warns that the split may produce the wrong amount of tax where income varies [23]. The guide to the BR tax code explains how a second job is taxed when no allowance is given.
Pension income follows a parallel route. HMRC publishes separate guidance for telling it about a change to pension income, which then feeds through to the code operated by the pension provider [24] [6].
New starters and emergency codes
A new starter's code is not changed by the employee at all in the first weeks. HMRC advises waiting 35 days after starting a job for HMRC to receive the new income details before contacting it [2]. Emergency codes end in W1, M1 or X, and tax is worked out on the pay for that week or month only [12].
Giving the new employer a P45 helps remove an emergency code sooner, and the employee should ask a former employer for a missing P45 [12] [2]. Where the employee has another job or a pension, the starter declaration is C and the correct code is BR, and an employer who cannot complete the starter checklist uses declaration C with code 0T [19]. The full process sits in the guide to emergency tax codes.
The timeline from report to payslip
The table sets out how long each stage takes once an employee updates HMRC's records.
| Stage | Timing | Source |
|---|---|---|
| New starter waits before contacting HMRC | 35 days from the start date | HMRC tax code guidance |
| HMRC notifies employee and employer | Within 15 working days of the change | HMRC tax code guidance |
| New code appears, monthly pay | Next or following payslip | HMRC tax code guidance |
| New code appears, weekly pay | By the third payslip | HMRC tax code guidance |
| Employer updates the record | As soon as possible, before the next payday | HMRC employer guidance |
Each stage is taken from HMRC's update guidance [2] and its employer guidance on code changes [3].
When the new code does not appear
If the new code is not on the payslip within those windows, HMRC tells the employee to speak to the employer to check the notice has arrived [2]. The most common causes are an employer that has not downloaded the notice, payroll software that does not pull notices automatically, or a notice that arrived after the payroll cut-off [3] [27].
When the code arrives late in the tax year
A code that arrives too late to use in the current tax year is applied in the new tax year [3]. At the turn of the year, the P9X instructions tell employers to carry forward any authorised code that has not changed, to keep only the P9(T) or other notice with the most recent date, and never to carry over a week 1 or month 1 marking [17] [26]. For the 2026-27 tax year the emergency code is 1257L for all employees [17].
How an employer applies a new code
When a code changes, HMRC emails the employer an alert. The new code can then be retrieved from PAYE Online under tax code notices, from the PAYE Desktop Viewer, or directly by payroll software that supports it [3] [27]. The notice is sometimes called a P6.
The three steps on receipt of a P6
HMRC's employer guidance reduces the process to three steps [3]:
- Retrieve the new code from PAYE Online, the Desktop Viewer or the payroll software.
- Confirm whether the notice includes the employee's previous pay and tax, and note the figures if it does.
- Update the payroll record as soon as possible and before the next payday, adding any previous pay and tax.
The Low Incomes Tax Reform Group confirms the P6 should be used on the first payday after it is received unless HMRC says otherwise [5]. Codes longer than seven characters cannot be issued automatically, so HMRC issues those P6 notices by hand [7]. Bureaux processing notices for dozens of clients usually rely on a multi-client payroll dashboard that flags new notices against each employer reference.
Cumulative and week 1 or month 1 operation
A code is operated either cumulatively or on a week 1 or month 1 basis, and the notice states which [10]. The table compares the two.
| Basis | How tax is worked out | Effect of a code change |
|---|---|---|
| Cumulative (default) | Pay and tax to date across the tax year | Over-deducted tax can be refunded, under-deducted tax caught up |
| Week 1 or month 1 | Each period's pay on its own, earlier pay ignored | No refund and no catch-up in payroll |
The definitions come from HMRC's PAYE manual [10] and the Low Incomes Tax Reform Group's employer guide [5].
Cumulative operation is the usual method, and it means a new code with a higher allowance can produce a refund through payroll [10]. The week 1 or month 1 basis exists to prevent the employer making heavy deductions or giving a large refund, and an NT code issued cumulatively requires the employer to refund tax already deducted [10].
In-year adjustments and the 50% cap
When a code change shows tax underpaid so far in the year, HMRC's system calculates an in-year adjustment and builds it into the code, applied on a week 1 or month 1 basis, so the shortfall is collected over the remaining paydays [9]. In most cases the whole adjustment is collected in the current year, with any excess carried to the next year where it would breach the deduction cap [9].
That cap is the overriding regulatory limit. Tax deducted on any one day cannot exceed 50% of the gross pay for that period, and the rule applies to all tax codes [7] [8]. Payroll software must apply the cap automatically when a K code or an in-year adjustment would otherwise take more.
How much a code change moves take-home pay
The examples below use an employee in England paid £3,000 a month, with National Insurance category A, for the 2026-27 tax year. The figures come from the Moonworkers calculator engine, which applies the 20% basic rate and the £12,570 Personal Allowance [22] [13].
| Tax code | Income tax a month | Employee NI a month | Take-home a month |
|---|---|---|---|
| 1257L | £390.20 | £156.16 | £2,453.64 |
| 1100L | £416.40 | £156.16 | £2,427.44 |
| BR | £600.00 | £156.16 | £2,243.84 |
| K154 | £625.80 | £156.16 | £2,218.04 |
| S1257L (Scotland) | £392.27 | £156.16 | £2,451.57 |
Moving from 1257L to 1100L, the typical result of a new taxable benefit, costs the employee £26.20 a month in extra income tax. Moving from 1257L to BR costs £209.80 a month, because BR gives no allowance at all [14]. National Insurance does not change in any row, because the tax code has no effect on National Insurance [13].
The Scottish code shows the effect of the S prefix. On the same £3,000 a month, an employee with S1257L pays £392.27 in income tax, £2.07 more than under 1257L, because Scottish bands apply [21]. Welsh codes carry a C prefix and use the same rates as England for the 2026-27 tax year [31].
The annual view
Over a full year the gap widens into a figure employees notice on a P60.
On a salary of £36,000, income tax is £4,684.20 under 1257L and £4,998.20 under 1100L, a difference of £314 over the tax year [13] [22]. Take-home pay falls from £29,441.40 to £29,127.40. An employee who believes the benefit behind the reduced code has ended should update HMRC rather than ask the employer to restore 1257L [2] [5].
Correcting the tax already paid
A new code fixes future deductions. It does not always settle the past. On a cumulative code, a corrected allowance flows back through pay to date, so an overpayment is usually refunded through payroll on the next payday [10] [32].
Overpayments and underpayments after the year ends
Where the code is on a week 1 or month 1 basis, or the year has ended, the difference is settled outside payroll. HMRC reconciles PAYE records after the end of the tax year and contacts the employee if too much or too little tax was paid [15] [9]. Underpayments are commonly collected by reducing the allowance in a later code, which is why an old debt can reappear as a lower code months after the original error [6].
Reading the code history
HMRC staff can view every code calculation for an individual in date order, with the most recent at the top, including the basis of operation and the status of each P2 and P6 notice [30]. Employees see a simplified version through HMRC's online Income Tax checking service, which explains why a code changed [16] [6]. The guide to why a tax code has changed walks through the most common explanations.
What good tax code handling looks like in payroll
The process above is simple on paper and error-prone in practice. Most problems come from timing: a notice downloaded after the payroll run, a week 1 marking carried into a new year, or a starter left on an emergency code after HMRC has sent a replacement [3] [17].
Automating notice retrieval
Payroll software that retrieves P6 and P9 notices directly removes the manual download step HMRC describes [3]. For platforms that embed payroll into an HR or accounting product, the same retrieval can be exposed through an HMRC-recognised payroll API, so the code on the employee record updates before the next pay run without anyone opening PAYE Online [27] [29].
Validating codes before each pay run
A pre-run check catches the errors HMRC's guidance warns about. It confirms the code came from an HMRC notice or the starter checklist, that W1 and M1 markings were removed at the turn of the year, that the 50% cap is applied, and that the S or C prefix matches the notice [17] [7]. The standard 1257L tax code is the baseline against which most of these checks run.
Decode a tax code before the next pay run
Before applying a new code, an employer or employee can decode it with the Moonworkers tax code checker, which explains the letters, the allowance and the basis of operation under the 2026-27 rules.
On your payslip, P45 or P60. Suffixes W1, M1 or X welcome.
What you'll get
Enter any UK tax code to see:
- · what each letter and number means
- · which nation's rates apply (S and C prefixes)
- · cumulative vs emergency W1/M1/X basis
- · the exact tax-free pay for every pay schedule, to the penny
Try 1257L, a K code, or an emergency W1 code.
Payroll that applies every tax code correctly
Moonworkers runs the full HMRC exact percentage method on every payslip, including K codes, emergency codes and in-year code changes, and files RTI automatically.
Conclusion
Changing a tax code is less a request than a correction of data. The employee changes what HMRC knows, HMRC recalculates and issues a notice, and the employer applies it before the next payday. Every delay in that chain shows up as a wrong deduction, and the examples show how quickly that adds up: a move from 1257L to BR on a £3,000 monthly salary changes take-home pay by £209.80 a month.
As HMRC moves more of its letters into the app and the Personal Tax Account, the employee side of the process is becoming faster. The employer side now carries most of the risk of delay, which is why the gap between a notice arriving and a code reaching the payslip is shrinking towards a single automated step inside payroll software.
Frequently asked questions
Can an employer change an employee's tax code if the employee asks?
No. An employer must operate the code HMRC issues and must not use any other code unless HMRC instructs it [5]. An employee who believes a code is wrong should update the details HMRC holds through HMRC's online Income Tax checking service or the HMRC app, or contact HMRC. HMRC then sends the employer a new code if one is due [3].
How long does it take for a new tax code to show on a payslip?
HMRC tells the employee and the employer about a new code within 15 working days of the change being reported [2]. For monthly pay, the new code should appear on the next or the following payslip. For weekly pay, it should appear by the third payslip. If it does not, the employee should check with the employer that the notice was received, and employers should update the record before the next payday [3].
Will a new tax code give a refund of tax already overpaid?
On a cumulative code, a higher allowance is applied to pay to date, so overpaid tax is usually refunded through payroll on the next payday [10]. On a week 1 or month 1 code, earlier pay is ignored and no refund is given in payroll. In that case HMRC settles the difference after the end of the tax year [15].
How does someone change an emergency tax code after starting a new job?
The quickest fix is to give the new employer a P45 from the previous job, or to complete the starter checklist accurately. HMRC usually replaces the emergency code once it receives details from the old and new employers, which can take up to 35 days [12]. If the code is still wrong after that, the employee can update it online or contact HMRC [2].
