The underlying mechanism — cumulative PAYE
The single most important thing to understand about UK monthly take-home: PAYE is cumulative. Each pay period doesn't calculate tax in isolation. Instead, your employer:
- Adds up your taxable pay for the year to date
- Works out your tax-free pay for the year to date: your code's allowance spread evenly over the year (about £1,048 a month on 1257L in 2026/27, so about £5,240 by Month 5)
- Works out the tax due on the difference, using the tax bands spread over the same months
- Subtracts the tax you've already paid in previous months
- Deducts the difference from this month's pay
For a steady salary with no other changes, this produces an identical deduction every month. But when anything changes — your salary, your tax code, your benefits — the cumulative recalculation rebalances against the new position. That's what creates "unexpected" pay changes.
National Insurance works differently — it's non-cumulative, calculated standalone on each pay period using monthly thresholds. So NI variations don't carry over month-to-month the way Income Tax does.
Why does my PAYE (Income Tax) change every month?
If your salary and tax code are the same every month, your PAYE should be the same every month, give or take a few pence. If it moves, one of the inputs to the year-to-date calculation has changed. That could be your pay (overtime, a bonus, unpaid leave, statutory pay), your tax code, or a taxable benefit. Because PAYE looks at the whole year so far, the month the change lands often shows a bigger catch-up than later months.
A worked example for 2026/27. You earn £3,000 a month on 1257L, so you get about £1,048 a month tax-free. In Month 6 (September) HMRC sends your employer a new code of 1100L because you now have a taxable benefit worth £1,570 a year:
- Your tax-free pay drops by about £131 a month (£1,570 ÷ 12)
- At 20%, that is about £26 more tax a month from now on
- In Month 6 the cumulative calculation also collects the extra tax for Months 1 to 5, so that payslip shows about £157 more tax (6 × £26)
- From Month 7 the extra is back to about £26 a month
It works the other way too. A code that gives you more allowance produces a refund through your next payslip. You can decode any new code with the Tax Code Checker.
Knowing which calculation is which explains most variation patterns:
- Income Tax went down? Probably a cumulative correction (you'd over-paid earlier in the year)
- Big NI in a bonus month? Per-period calculation hit a single high gross-pay month
- Both went up sharply? Tax code change applied retrospectively
The eight common causes
Most monthly variations trace back to one of these. The pattern lets you diagnose quickly.
1. Tax code change
HMRC issues a new tax code whenever your circumstances change — and many changes happen without you actively reporting them: you started receiving a taxable benefit, claimed Marriage Allowance, owe tax from a previous year, or left a second job that HMRC needed to reallocate your allowance away from.
Because PAYE is cumulative, a code change part-way through the year reaches back and recalculates everything from 6 April. A code that gives you more allowance produces an immediate refund through the next payslip. A code that gives you less causes an extra deduction. Either way, the change shows up as one big variance.
2. The 6 April tax-year reset
The UK tax year runs from 6 April to 5 April. On the changeover:
- HMRC may have updated allowances, NI thresholds, or student loan thresholds
- Year-to-date counters reset to zero for the new year
- Any mid-year corrections from the previous year stop carrying forward
- New tax codes for the new year take effect
The result: your first April or May payslip often differs from March's even with no underlying change. It's usually the cleanest, simplest payslip of the year because nothing is being carried forward.
3. Statutory pay replacing normal earnings
If you've had time off for sickness or parental reasons, statutory pay rates apply for those periods:
- Statutory Sick Pay (SSP): £123.25/week in 2026/27, or 80% of your average weekly earnings if that is lower. From 6 April 2026 it is paid from the first day of sickness, for up to 28 weeks
- Statutory Maternity Pay (SMP): 90% of average weekly earnings for 6 weeks, then £194.32/week (or 90% of average, whichever is lower) for the next 33 weeks
- Statutory Paternity Pay (SPP): 2 weeks at £194.32/week (or 90% of average earnings if lower)
- Shared Parental Pay (ShPP): broadly the same structure as SMP
Statutory pay is significantly lower than most salaries. A partial month of sick leave or the start of maternity leave produces a sharp gross-pay drop. Many employers top up statutory pay to full salary ("enhanced maternity pay" is the most common) — others don't.
4. Bonuses and one-off payments
A bonus paid in a single month pushes your year-to-date earnings well above the steady-state. Cumulative PAYE notices and deducts more tax that month to balance the books.
The headline impact is alarming — a £10,000 bonus in December for a £40,000 earner can look like the bonus has been taxed at 50%. It hasn't. The actual tax on the bonus across the full year is the same 28% basic-rate or 42% higher-rate combination as any other income. The big single-month deduction is cumulative catch-up, not an extra rate.
The Bonus Tax Calculator shows both the year-end true rate and the month-of-payment feels-like rate side by side. The difference between them is the source of the perception.
5. Pension contribution rate changes
Pension contributions are usually a percentage of gross pay. The rate can change for several reasons:
- Annual scheme review at salary review time
- Auto-enrolment age-based step-ups (some schemes increase your % on milestone birthdays)
- Scheme migration when the employer changed pension provider and reset defaults
- You opted into or out of an enhanced contribution arrangement
A 2-3% change to your pension contribution is meaningful. On a £50,000 salary, moving from 5% to 8% reduces take-home by about £150/month.
6. National Insurance category letter changes
Most people stay on NI category A their whole working life. But changes do happen:
- Turning State Pension age → category should change to C (no employee NI). The most common silent overpayment when employers miss this.
- Apprenticeship ending → category H reverts to A
- Turning 21 → category M (under 21) becomes A
The State Pension transition matters most because it can quietly cost you 8% of pay above the threshold for months if nobody notices.
7. Student loan threshold updates
Student loan repayments are 9% of income above the plan threshold (6% for postgraduate). Thresholds update on 6 April each year. For 2026/27:
- Plan 1: £26,900 (up from £26,065 in 2025/26)
- Plan 2: £29,385 (up from £28,470 in 2025/26)
- Plan 4 (Scotland): £33,795 (up from £32,745 in 2025/26)
- Plan 5: £25,000 (frozen)
- Postgraduate: £21,000 (frozen)
If you earn around the threshold, the increase reduces your monthly student loan deduction slightly from April. If your salary is well above the threshold, the impact is modest.
8. Salary sacrifice rate changes
Salary sacrifice arrangements (pension, cycle-to-work, EV lease) typically allow changes at one or two windows per year — often April and an annual scheme review. When the rate changes, both gross pay and the related tax/NI deductions change simultaneously.
Sacrificing more reduces gross and reduces tax/NI proportionally; net take-home drops but by less than the sacrificed amount (the difference being the tax saving). Sacrificing less does the opposite.
When the change isn't worth worrying about
Some monthly variations are noise:
- A few pence between months for an otherwise-identical pay scenario: rounding inside cumulative PAYE. Larger swings usually come from overtime, hours or a code update
- Small NI variation when month length affects pay-period boundaries
- Tax code suffix appearing or disappearing (e.g., L → L X → L): emergency basis being applied or lifted briefly
These usually balance out within 1-2 months. A small unexplained variance you can't trace to one of the eight causes above is normally cumulative PAYE doing its job.
When the change is worth investigating
Some signals warrant a closer look:
- Take-home moved by more than £100 with no obvious bonus, leave or scheme change
- Tax code changed and you don't recognise the new code
- NI letter changed without an obvious reason (especially if you're approaching State Pension age)
- Year-to-date figures don't reconcile when checked against the Take-Home Pay Calculator at your current salary and tax code
In these cases, the HMRC Personal Tax Account at gov.uk/personal-tax-account is the first stop — it shows your current code, the reasoning behind it, and the year-to-date position HMRC has on file.
If the issue is with specific deductions (pension rate, salary sacrifice setup) rather than tax, contact HR. If with the tax code or NI category, contact HMRC directly.
Work out what changed, step by step
If you can't see the cause straight away, check things in this order. Each step rules out the most common causes first. To do it in one go, enter this month's payslip figures in the free payslip checker: it compares your tax, NI, student loan and net pay with the expected 2026/27 figures and flags any that don't match.
- Tax code. Compare this month's code with last month's. If it changed, decode both in the Tax Code Checker, which also shows what the code means for your take-home. For codes that look wrong, see why is my tax code wrong? and emergency tax codes.
- Gross pay and year-to-date figures. Use how to read a payslip to check gross pay, taxable pay and the YTD columns line by line.
- National Insurance. Check the NI category letter (usually A). NI is worked out separately for each pay period, so a high-paid month raises NI that month only. See National Insurance.
- Pension and salary sacrifice. A rate change or a scheme switch moves both gross and net pay. See how pension contributions affect take-home pay.
- Student loan. Repayments are 9% (6% for postgraduate) of pay above the plan threshold for that pay period. The thresholds changed on 6 April 2026. See student loan thresholds, or work out the exact monthly deduction in the student loan calculator.
- Expected figure. Put your salary, code, pension and student loan into the Take-Home Pay Calculator to see what a normal month should look like.
The 5-minute monthly habit
A useful payday routine: open this month's payslip alongside last month's and run down the lines: tax code, NI letter, gross pay, deductions, year-to-date. The change is almost always identifiable from line-by-line comparison. Anything you can't account for in five minutes is worth investigating.
For an in-the-moment troubleshooting walkthrough (rather than this general explainer), see Why has my pay changed this month? — same underlying causes, framed as a step-by-step diagnostic.