Compare line by line — the fastest diagnosis
If your UK take-home dropped (or rose) unexpectedly this month and your salary hasn't changed, the cause is almost always identifiable in 60 seconds with a side-by-side comparison.
Open this month's payslip alongside last month's. Look down each row:
- Gross pay different? → look for bonus, overtime, statutory pay, holiday pay buyout, backdated pay
- Tax code different? → HMRC has issued a P6 to your employer; check Personal Tax Account for why
- NI letter different? → likely a State Pension age transition or NI category change
- Income Tax different (gross unchanged)? → tax code change or cumulative PAYE catch-up
- National Insurance different (gross unchanged)? → NI letter change, rate threshold update on 6 April
- Pension different? → contribution rate change or scheme migration
- Student loan different? → plan threshold update on 6 April, or you've crossed a threshold
- Salary sacrifice items different? → sacrifice rate changed (annual review, mid-year change)
The line that changed identifies the cause. The eight sections below walk through each scenario.
1. Tax code change
The single most common cause of an unexpected take-home change. HMRC issues a new code whenever your circumstances change — and many changes happen without you actively reporting them:
- You started receiving a taxable benefit (company car, private medical) that's been added to your payroll
- You've claimed or been credited with Marriage Allowance
- HMRC is recovering tax from a previous year through PAYE
- You've left a second job and HMRC has reallocated your full personal allowance to your main job
Because PAYE is cumulative, a code change reaches back and recalculates everything from 6 April. A code that gives you more allowance produces an immediate one-off refund. A code that gives you less causes an immediate one-off extra deduction. Either way, the change shows up as a single big variance on the first payslip after the new code takes effect.
Diagnostic: compare the "Tax code" line on this month's payslip to last month's. If different, decode the new one with the Tax Code Checker to see what allowance it represents.
Fix if wrong: log into the Personal Tax Account to see why HMRC issued the new code. Most issues correct themselves there.
2. New or changed benefit in kind
Benefits in kind (BiK) — company cars, private medical insurance, gym memberships, etc. — are usually taxed through your tax code rather than through direct cash deductions. When the benefit starts, changes value, or ends, your tax code adjusts accordingly.
The cash impact depends on the benefit's value:
- A new EV company car at 3% BiK has tiny impact (~£15/month change for a £30,000 EV)
- A petrol BMW at 37% BiK can shave £150–£250/month off take-home
- Private medical for a family of four can cost £40–£70/month in tax
Diagnostic: did your tax code change, and is there a benefit-in-kind entry in your Personal Tax Account that's new or different?
Fix if wrong: if the BiK value is wrong (e.g., HMRC has the wrong car), correct it in the Personal Tax Account.
3. Bonus or one-off payment
A bonus paid in a single month pushes your gross up substantially, which triggers cumulative PAYE to deduct more tax to catch up to where your year-to-date position should be if the bonus pattern continued.
The headline impact looks alarming — bonuses often appear to be "taxed at 50%" — but it's actually cumulative PAYE rebalancing. Your full-year position is correct; only the bonus month looks unusual.
The Bonus Tax Calculator shows both the year-end true rate (the actual tax on the bonus) and the bonus-month feels-like rate (what the payslip shows). The difference between these is the source of the "tax theft" perception.
Diagnostic: did gross pay this month include a bonus, commission, sales incentive or one-off payment?
No fix needed: cumulative PAYE evens out over the rest of the tax year.
4. Statutory pay (sick, maternity, paternity)
If you took time off for sickness or parental reasons during this pay period, you're being paid Statutory Sick Pay (SSP), Statutory Maternity Pay (SMP), Statutory Paternity Pay (SPP) or Shared Parental Pay (ShPP) instead of (or alongside) your normal salary:
- SSP 2026/27: ~£118.75/week, payable for up to 28 weeks after a 4-day waiting period
- SMP: 90% of average weekly earnings for 6 weeks, then £187.18/week (or 90% of average, whichever is lower) for 33 weeks
- SPP: 2 weeks at £187.18/week (or 90% of average earnings)
Statutory pay is significantly lower than most salaries — so a partial month of sick leave or the start of maternity leave can produce a sharp gross-pay drop.
Some employers top up statutory pay to full salary ("enhanced maternity pay" being the most common) — others don't. Your contract or employee handbook tells you which.
Diagnostic: did the period covered by this payslip include any sick days or parental leave?
No fix needed if intentional: the gross-pay change reflects actual hours/days at statutory rates.
5. Pension contribution rate change
Pension contributions are usually a percentage of gross pay. Several things can change the rate without you actively requesting it:
- Annual scheme review at the salary review point
- Auto-enrolment age-based step-ups (some schemes increase your % on milestone birthdays)
- A scheme migration where the employer changed pension provider and reset everyone to the new scheme's default
- You opted in or out of an enhanced contribution arrangement
A 2-3% change to your pension contribution is a meaningful take-home change. On a £50,000 salary, moving from 5% to 8% pension contribution reduces take-home by about £150/month.
Diagnostic: did the pension line on your payslip change as a percentage (calculate pension ÷ gross)?
Fix if wrong: contact HR or your pension provider — they can usually revert to your previous rate.
6. NI category letter change
Most people stay on NI category A their entire working life. But changes do happen, most commonly:
- Turning State Pension age — category should change to C (no employee NI). If your employer misses this, you're paying NI you don't owe
- Apprenticeship ending — category H (apprentice rate) reverts to A
- Turning 21 — category M (under 21) becomes A
The first one matters most because it's a silent overpayment that can continue for months if nobody notices.
Diagnostic: compare the NI category letter on this month's payslip to last month's. If you've turned State Pension age and your letter isn't C, that's a fix to make.
Fix if wrong: contact HMRC. Your employer applies whatever NI category HMRC assigns.
7. Student loan threshold update on 6 April
Student loan repayments are 9% of income above the plan threshold (6% for postgraduate). The 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)
- Plan 4 (Scotland): £33,795 (up from £32,745)
- Plan 5 (2023+ starters): £25,000 (frozen)
- Postgraduate: £21,000 (frozen)
If you're earning 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 small (a percentage point of the threshold change).
Diagnostic: was the change first visible on your April or May payslip? If yes, threshold update is likely the cause.
No fix needed: this is automatic and correct.
8. Salary sacrifice change taking effect
Salary sacrifice arrangements (pension, cycle-to-work, EV lease) typically allow changes at one or two windows per year — often April (tax year start) and an annual scheme review.
When a salary sacrifice rate changes, both your gross pay and your tax/NI deductions change simultaneously. The headline effect on take-home depends on the direction:
- Sacrificing more → gross drops, tax and NI drop proportionally, but take-home drops too (just by less than the sacrificed amount)
- Sacrificing less → gross rises, tax and NI rise, take-home rises
Diagnostic: did the gross pay or the "pension SS" / "salary sacrifice" line on your payslip change?
Fix if wrong: contact HR — most schemes allow corrections to recently-set rates if you flag promptly.
When the change is positive: refunds via PAYE
Most of the scenarios above can also produce a positive take-home change. The main causes of an unexpected upward swing:
- Tax code increased (benefit removed, Marriage Allowance received, prior-year over-payment refunded)
- HMRC switched you from emergency basis to cumulative — your year-to-date position recalculates, and any over-paid tax comes back as a single-month refund
- End of student loan repayments — once your balance is cleared (or you finish a course and the plan ends), the deduction stops
- Reduced pension contribution — voluntary or scheme-driven
None of these need fixing — they're working as intended.
The 5-minute check
If your pay changed unexpectedly:
- Identify the line that changed — gross pay, deductions, or both
- Check the Personal Tax Account if a tax code change is involved
- Compare YTD columns to spot cumulative-PAYE adjustments
- Use the Take-Home Pay Calculator to verify the new figure matches the rules
- Contact HR for payroll questions, HMRC for tax-code questions — they're the right addresses for different problems
The change is almost always explainable. The few cases where it isn't are usually genuine payroll errors that HR can fix quickly once flagged.