How PAYE Works Behind the Scenes: UK 2026/27 Guide

Most employees know PAYE as the system that takes income tax and National Insurance from their wages before they receive them. What fewer people understand is how that actually happens: what your employer does each pay period, how HMRC receives the information, how your tax code gets applied in a calculation, and why the deductions on a bonus month look so different from a normal one.

Understanding the mechanics is not just interesting. It explains why emergency tax happens, why income tax corrects itself after a bonus but National Insurance does not, why your payslip might change without warning, and why some errors in the system persist for months before anyone notices. Once you see how the pieces fit together, the payslip makes considerably more sense.

This guide walks through the full PAYE process from the moment HMRC issues a tax code to the point a year-end P60 lands in your hands, explaining each step in plain language.

Quick Answer: PAYE works by having your employer calculate your income tax and National Insurance each pay period using a tax code issued by HMRC, deduct those amounts from your gross pay, and report everything to HMRC electronically on or before your payday. HMRC then reconciles your tax position over the year and corrects any errors or over- or underpayments through your tax code, payroll, or a direct adjustment.

The Starting Point: The Tax Code

Before your employer can calculate a single penny of income tax, they need to know how much of your income is tax-free in each pay period. That information comes from your tax code, which HMRC issues and updates. Your employer cannot invent or choose your tax code. They must apply whatever HMRC sends them.

A standard tax code such as 1257L translates to a tax-free personal allowance of £12,570 per year. HMRC reaches that figure by starting with the standard personal allowance and then adjusting it for:

• Taxable benefits in kind, such as a company car or private medical insurance, which reduce the allowance to collect the benefit tax through PAYE
• Underpaid tax from previous years being coded out, which further reduces the allowance
• Marriage Allowance received from a partner, which increases the allowance
• Other income sources, such as the State Pension or rental income, that HMRC wants to tax through the employment source

The number in the code (1257 in the standard code) is the tax-free allowance divided by ten. HMRC communicates tax codes to employers electronically through the PAYE system. Employers receive a coding notice called a P9T (or P9X for bulk annual updates) which instructs them on the code to apply for each employee. Your employer does not decide your code and cannot change it independently.

Our complete guide to UK tax codes explains every code type and suffix in detail.

How HMRC Issues and Updates Tax Codes

HMRC maintains a record for every UK taxpayer. When your circumstances change, such as starting a new job, receiving a benefit in kind, or having a previous year's underpayment identified, HMRC updates your record and issues a new code to your employer electronically. You should also receive a P2 Notice of Coding by post or through your HMRC personal tax account explaining what your code is and why it has changed.

Codes are also updated in bulk each April at the start of a new tax year to reflect changes in the personal allowance, benefit values, and other standard adjustments. Most employed workers receive an updated code in March or April each year even if nothing significant has changed in their circumstances.

HMRC can update a code mid-year at any point. If you start receiving a taxable benefit, if HMRC updates their income estimate for you, or if a previous year's underpayment is identified, the code can change without any action on your part. The change flows directly to your employer and affects your very next payslip.

The Monthly Payroll Calculation

On each payday, your employer's payroll software runs through a calculation for every employee. Here is what happens for a monthly-paid employee with a standard cumulative tax code.

Step 1: Establish Gross Pay

The payroll system records the employee's earnings for the period: basic salary, overtime, bonus, commission, shift allowances, and any other taxable payments. This sum is the gross pay for the period and the starting point for all deductions.

Step 2: Calculate the Tax-Free Amount for the Period

The tax code is divided into a per-period allowance. For a monthly-paid employee on 1257L, the annual allowance of £12,570 is divided by 12, giving a monthly tax-free amount of £1,047.50. This is the employee's free pay for the month.

Step 3: Calculate Cumulative Taxable Pay

Here is where cumulative PAYE works differently from most people's intuition. The payroll system does not just look at this month in isolation. It looks at total year-to-date earnings and subtracts total year-to-date free pay. This gives the cumulative taxable pay from April to the current month.

For example: an employee on £36,000 per year who has been paid for seven months (April to October) has year-to-date gross pay of £21,000. Their year-to-date free pay is 7 × £1,047.50 = £7,332.50. Cumulative taxable pay is £21,000 minus £7,332.50 = £13,667.50.

Step 4: Calculate Cumulative Income Tax Due

The payroll applies the income tax rates to the cumulative taxable pay to calculate the total income tax that should have been paid from April to now. On £13,667.50, all within the basic rate band, the income tax due is 20% × £13,667.50 = £2,733.50.

Step 5: Subtract Tax Already Paid

The payroll system knows how much income tax has already been deducted in previous months of the tax year. It subtracts that year-to-date figure from the cumulative tax due. The difference is the income tax deduction for this month.

If the employee has been paid correctly throughout the year, this month's deduction should be close to one-twelfth of their annual tax liability. But if something changed, such as a bonus earlier in the year or a tax code update, the calculation automatically corrects the year-to-date position.

Step 6: Calculate National Insurance

Unlike income tax, National Insurance is not cumulative. It is calculated on this pay period's gross earnings independently, with no reference to year-to-date totals. The NI-liable earnings threshold for the period is applied (£1,047.50 per month), and 8% is charged on gross earnings between that threshold and the Upper Earnings Limit (£4,189.17 per month), with 2% above that. The result is the NI deduction for this period only.

This non-cumulative nature of NI means it does not self-correct in the way income tax does. If you receive a bonus in one month, the NI deduction in that month is higher than normal and stays higher permanently for that period. Income tax in the bonus month may be higher due to the PAYE projection, but subsequent months will see lower income tax deductions as the cumulative calculation balances out.

Step 7: Apply Other Deductions

After income tax and NI, the payroll system processes other deductions in the correct order. These typically include student loan repayments (calculated on the period's gross pay above the plan threshold, non-cumulative), pension contributions (either deducted from gross pay for salary sacrifice or from net for other methods), and any Attachment of Earnings orders or court-ordered deductions.

Step 8: Calculate Net Pay

Gross pay minus all deductions gives net pay, which is what is transferred to the employee's bank account.

Real Time Information: How the Data Reaches HMRC

Before 2013, employers submitted PAYE information to HMRC annually. The system was slow, produced reconciliation errors, and allowed significant delays before HMRC discovered discrepancies. In 2013, HMRC introduced Real Time Information (RTI), which transformed the process.

Under RTI, employers must submit payroll data to HMRC on or before the day they pay their employees. This is not optional and not a formality. The submission is a legal requirement, and late or incorrect submissions can attract penalties.

The Full Payment Submission (FPS)

The core RTI document is the Full Payment Submission (FPS). Every time an employer runs a payroll and pays employees, they send an FPS to HMRC. This file contains, for every employee paid in that run:

• The employee's name, National Insurance number, and tax code
• Gross earnings for the period
• Income tax deducted for the period
• Employee National Insurance contributions
• Employer National Insurance contributions
• Year-to-date totals for all of the above
• Student loan deductions
• Starter and leaver information where applicable

HMRC receives this data on or before payday, updates every employee's record in real time, and uses it to calculate what the employer owes in PAYE for that payroll period. According to the Low Incomes Tax Reform Group's guide to reporting PAYE in real time, the FPS is the main submission that must be made to HMRC under RTI and is usually sent every time employees are paid.

The Employer Payment Summary (EPS)

The Employer Payment Summary is a supplementary RTI document sent separately from the FPS. It covers information that does not fit into the standard payroll run, such as:

• Statutory payment reclaims (Statutory Maternity Pay, Statutory Sick Pay, etc.) where the employer is eligible to recover some or all of the cost from HMRC
• Employment Allowance claims (which reduce the employer's NI bill)
• Months in which no employees have been paid
• Notification that a PAYE scheme has ceased

The EPS is typically due by the 19th of the month following the payroll period it relates to.

What Happens After HMRC Receives the FPS

HMRC's systems process each FPS against the employee records they hold. They update year-to-date earnings and deductions, check for discrepancies between what was submitted and what their records expected, and use the information to maintain the employee's tax account. This real-time picture is what allows HMRC to identify over- and underpayments during the year rather than only at year end, and to issue corrected tax codes when something changes.

The Cumulative Calculation: Why It Matters

The cumulative nature of income tax under PAYE is one of the most misunderstood aspects of the system. It is also the mechanism that makes the UK system largely self-correcting for income tax across the tax year.

Because each month's income tax deduction is based on the difference between cumulative tax due and tax already paid, any over- or underestimation in one month is automatically corrected in subsequent months. If PAYE applies temporarily higher tax in a bonus month because it projected elevated annual income, the following months' deductions will be lower to compensate once the year-to-date position makes clear that the annual income is not as high as projected.

This is why many workers notice their income tax deduction is unusually low in the month or two after receiving a large bonus. It is not an error. The payroll system is correcting the cumulative year-to-date position.

Cumulative vs Non-Cumulative Codes

Emergency codes with a W1 (Week 1) or M1 (Month 1) suffix, or the X suffix, break the cumulative system. Under these codes, each pay period is treated as if it were the first period of the tax year. The payroll applies that period's allowance only, with no reference to the year-to-date history. This means the self-correcting mechanism does not operate.

Emergency codes are used when HMRC cannot confirm the employee's full tax position, typically when they start a new job without a P45. Once HMRC issues the correct cumulative code, the payroll system immediately switches to the cumulative basis and any overpaid tax from the emergency code period is recovered automatically in the first payroll run under the new code.

The PAYE Payment Calendar

Employers do not send HMRC the deducted tax on the same day they pay employees. There is a payment schedule.

The UK tax month runs from the 6th of one month to the 5th of the next, following the traditional tax year boundaries. An employee paid on 28 April is in tax month 1 (6 April to 5 May). Their employer must pay the PAYE liability for that run to HMRC by 22 May.

New Starters and Leavers

When You Start a New Job

When you join a new employer, one of two things happens. If you provide a P45 from your previous employer, your new employer uses the information on it to enter your year-to-date pay and tax, and HMRC issues the correct code for your new employment. If you do not have a P45, your employer asks you to complete a starter checklist. Your answer determines which temporary code they use while waiting for HMRC to confirm the correct one. Answering the checklist incorrectly can result in emergency tax being applied or too little tax being deducted. The starter checklist has three options: this is your only job in the current year, this is a second job, or you have had benefits or are starting your first ever job.

When You Leave a Job

When you leave, your employer runs a final payroll for you and marks you as a leaver in the FPS they submit for that period. They must issue a P45 to you within a reasonable time. The P45 contains your tax code, year-to-date earnings, and tax paid, and is the document you carry to your new employer to ensure PAYE continues correctly.

End of Year: P60 and the Reconciliation Process

At the end of the tax year on 5 April, the PAYE system carries out a full reconciliation. Your employer must issue a P60 to every employee still on the payroll as at 5 April by 31 May. The P60 summarises your total pay and total tax paid for the year and is the definitive record of your earnings and deductions under that employment.

HMRC also runs its own reconciliation using the RTI data submitted throughout the year. Where discrepancies exist between what was deducted and what should have been deducted, HMRC issues a P800 tax calculation. A P800 showing an overpayment means a refund is due, which can be claimed through the HMRC personal tax account or arrives as a cheque. A P800 showing an underpayment typically means HMRC will collect the outstanding tax by adjusting the following year's tax code rather than sending an immediate bill.

Why PAYE Sometimes Gets It Wrong

PAYE is largely automatic, but errors do occur. Understanding why helps identify what to check.

HMRC Has Outdated Information

HMRC's calculation of your tax code is based on their records of your income and circumstances. If those records are wrong, the code will be wrong. A benefit in kind you no longer receive, an old employer still showing as active, an incorrect income estimate, or an error in a previous self assessment return can all produce the wrong code and therefore the wrong deductions for months or years.

The Starter Checklist Was Completed Incorrectly

New starters who select the wrong option on the starter checklist (for example, ticking that it is a second job when it is actually their only employment) will have a BR code applied, which removes the personal allowance and taxes all earnings at 20%. HMRC will correct this once they receive the FPS and update the record, but the first one or two payslips may show higher deductions than necessary.

A P45 Was Not Processed Correctly

If a P45 is handed over but not entered into the new employer's payroll software correctly, the year-to-date figures used for the cumulative calculation may be wrong. This can produce either over- or under-deductions for several months until HMRC's RTI reconciliation identifies the discrepancy.

Payroll Software Errors

Payroll software is generally very reliable but is not infallible. Errors in rate tables, incorrect threshold applications, or data entry mistakes can all cause deductions to be wrong for a period. Most are caught quickly through the employee noticing their payslip or through HMRC's RTI reconciliation, but some persist longer.

How PAYE Handles Multiple Employments

Each employment has its own PAYE record and its own tax code. The personal allowance can only be applied in full to one employment at a time. HMRC allocates it to whichever employment is treated as the main one, usually the higher-paying or first registered. Secondary employments receive a BR code (all earnings taxed at 20% with no personal allowance) or D0 (all at 40%) depending on the expected rate applicable.

Because each employer calculates deductions independently and only for their employment, the total deductions across multiple jobs may not exactly reflect the correct annual liability. HMRC reconciles the combined position at year end through the P800 process, issuing refunds or requesting additional tax as needed. If you have unused personal allowance in a main job because that income is low, you can request HMRC to split the allowance across two employments to reduce the deduction burden on the secondary income.

PAYE and Self Assessment

Some employed workers also need to complete a self assessment tax return. Common triggers include having self-employment income, rental income above £1,000 per year, savings or dividend income above the relevant tax-free allowances, total income exceeding £100,000, or having claimed Child Benefit while the High Income Child Benefit Charge applies.

When a self assessment return is filed, HMRC reconciles the employment income reported (which they already have from RTI) with the additional income declared. Any tax owed on the total position is either collected through the tax return payment or, for smaller amounts, coded into the following year's PAYE code. Overpaid tax is refunded directly. This is the mechanism through which higher rate taxpayers on relief at source pension schemes claim their additional pension tax relief: the self assessment return reports their pension contributions and HMRC refunds the extra relief.

Common PAYE Misunderstandings

Thinking Employers Decide How Much Tax You Pay

Employers apply the code HMRC gives them and calculate deductions accordingly. They have no discretion over your tax code and cannot reduce or increase your tax independently. If your deductions look wrong, the source of the issue is almost always HMRC's records rather than your employer's payroll.

Thinking HMRC Receives Your Tax in Real Time

HMRC receives the payroll data in real time via RTI, but the actual money moves on the PAYE payment schedule: typically the 22nd of the following month for most employers. The RTI submission tells HMRC what has been deducted and what is owed; the physical transfer of the funds happens later.

Thinking National Insurance Works Like Income Tax

Income tax is cumulative and self-corrects across the year. National Insurance is per-period and does not correct retrospectively. This is one of the most practically significant differences in how the two deductions behave on payslips, particularly in months where earnings are unusual.

Thinking the P60 Is Just a Summary Document

The P60 is the definitive record of your employment income and tax for the year. Lenders, HMRC, and benefits agencies all treat it as authoritative. Keep your P60s for at least four years, as they are often needed for mortgage applications, tax credit claims, pension records, and self assessment queries.

How to Check Your Own PAYE Position

1. Check your tax code on your payslip. Verify it at gov.uk/personal-tax-account against what HMRC has on file. If the codes differ, something has not updated correctly. Our guide to checking your tax code walks through the process.
2. Compare year-to-date income tax with what it should be. Take your year-to-date gross pay from your payslip, subtract the year-to-date free pay (number of months so far multiplied by £1,047.50 for a standard 1257L code), and apply 20% (or the split rate if above £50,270 annualised). Compare with the year-to-date tax on your payslip.
3. Check for W1/M1 codes. If your tax code has either suffix, you are on a non-cumulative emergency basis. The self-correcting mechanism will not activate until the code is updated. Contact HMRC if the emergency code has been in place for more than two months.
4. Verify NI deductions separately. NI is per-period. Our National Insurance calculator gives the expected deduction for any gross monthly earnings figure.
5. Use our income tax calculator to cross-check. Our income tax calculator shows what total income tax and NI should be for a given annual salary. Comparing with your year-to-date payslip figures shows whether you are broadly on track.
6. Review your P60 when it arrives. Check that the totals match your understanding of the year. If they do not, contact HMRC before the deadline for reclaiming overpaid tax for that year passes.

Frequently Asked Questions

Conclusion

PAYE is a sophisticated, largely automated system that handles the income tax and National Insurance for the majority of UK workers without requiring any active involvement on their part. The cumulative calculation method for income tax makes it broadly self-correcting across the year. RTI ensures HMRC has an accurate, real-time picture of pay and deductions. And the end-of-year reconciliation through P800 and P60 catches most remaining discrepancies.

The places where PAYE can go wrong are almost always connected to the data inputs: an outdated HMRC record, an incorrectly completed starter checklist, a P45 not processed in time, or a benefit in kind that has changed without the code being updated. Checking your tax code, reviewing your year-to-date deductions occasionally, and keeping your P60s when they arrive are the three habits that keep your PAYE position under control.

For a cross-check of what your deductions should be at your salary level, our income tax calculator and National Insurance calculator give the expected figures for any gross salary. For a line-by-line explanation of every item on your payslip, our PAYE payslip guide covers every field from gross pay to net. And if your tax code looks wrong, our guide to checking and correcting your tax code walks through how to identify the issue and get it fixed.