Why is my take-home pay lower this month?

A smaller payslip almost always comes from one of five places: the tax code changed, an emergency W1, M1 or X marker appeared, gross pay moved, a deduction line started or jumped, or two employers paid you in the same month. The payslip in front of you identifies which. Some of those causes correct themselves, and some never will.

Work through the questions below with this month’s payslip beside last month’s. A payslip carries the tax code, gross pay, deductions - tax, National Insurance, pension, student loan - and net pay^[1]: everything the diagnosis needs.

Has the tax code itself changed?

Read the code off both payslips. If the characters before any suffix have moved - 1257L to 1185L, or an L code becoming a K code - HMRC has revised the tax-free amount it expects to give you over the rest of the year, and the deduction follows.

Check that against HMRC rather than the employer’s word: compare the code on the payslip with the one HMRC holds in the check your Income Tax service or the HMRC app, and speak to the employer if the two differ^[1]. HMRC’s guidance is blunt that making sure the right amount of tax is paid is the employee’s responsibility^[1].

If the code moved, that is the diagnosis, and what a tax code means decodes the new one. If not, carry on down the tree.

Does the code end in W1, M1 or X?

A code ending W1, M1 or X is an emergency code, which payroll software may print as NONCUM^[2]. Tax is then worked out on what is paid in that week or month only, as though that amount were paid every week or month of the year, instead of on total income so far in the tax year^[2]. Months with no earnings behind them release no allowance, so the deduction runs high.

How long it lasts depends on the trigger. After a new job starts, HMRC usually issues the right code once it has details from the new and previous employer - up to 35 days from the job start; a P45 given to the new employer speeds that up^[2]. Where too much tax has been paid a refund is due; where too little has, the code stays until the correct tax for the year has been paid^[2]. An emergency code triggered by company benefits or the State Pension instead runs to the end of the tax year, with a non-emergency code for the new one^[2].

Did gross pay change?

Compare the gross figures, not the net ones. A bonus, overtime, commission, unpaid leave, dropped hours or a mid-year pay rise all move gross - and tax moves faster than gross.

The bands explain why. The standard Personal Allowance is £12,570^[3], basic rate is 20% on taxable income from £12,571 to £50,270 and higher rate is 40% from £50,271 to £125,140^[3]. Cumulative PAYE releases the allowance and the basic-rate band in equal monthly slices, so a month much larger than its slice spills into 40% there and then, even when the year’s total lands inside basic rate.

Did a deduction line change?

If the code and the gross are unchanged, the difference sits below them.

Pension. Automatic enrolment starting, or a contribution rate changing, lands as a new deduction. In most automatic enrolment schemes contributions are based on total earnings between £6,240 and £50,270 a year, including bonuses, commission and overtime^[4] - so a bonus lifts the pension line too. Salary sacrifice works differently again, and salary sacrifice explained covers it.

Student loan. The Plan 2 threshold is £29,385 a year, or £2,448 a month^[5], and repayments are 9% of income over the threshold on Plans 1, 2, 4 and 5, or 6% on a Postgraduate Loan^[5]. A repayment is taken in any month income goes over the monthly threshold, a bonus included, and a refund can be requested at the end of the tax year only if annual income came in below the yearly threshold^[5].

National Insurance. Employee Class 1 on category A is 0% on monthly earnings up to £1,048^[6], 8% between £1,048.01 and £4,189 and 2% above £4,189^[6]. It is assessed on an earnings period - for someone paid once a month, the earnings period is a month^[7] - rather than cumulatively across the year, so a spike is charged in isolation and no later month gives it back.

Were you paid by two employers in the same month?

Finishing one job, starting another and being paid by both in the same month is one of HMRC’s listed causes of paying the wrong tax for the year, alongside being on the wrong code^[8]. Two pay runs share one month’s allowance and band, and the split rarely comes out right on its own. A shifted pay date does the same thing, pulling two pay dates into one tax month.

Where the year still ends out of line, HMRC reconciles it after 5 April and sends a tax calculation letter (P800) or a Simple Assessment letter^[8], sent between June and March of the following tax year^[8].

Which of these fix themselves?

Cause	Corrects itself?
Emergency code after a job start	Yes - within up to 35 days, refund through payroll
Bonus month at 40% under a cumulative code	Yes - later months release the unused band
Student loan repayment in an over-threshold month	Not in-year - refund at year end only if annual income was under the threshold
Pension enrolment, a new student loan, higher National Insurance	No - permanent at the new pay level
Emergency code from company benefits or the State Pension	Only at the year end - it runs to 5 April
A genuinely wrong code	No - it needs HMRC, and a P800 or Simple Assessment if it ran all year

How do people normally work this out?

Some hold last month’s payslip against this one and hunt for the line that moved - which works when one line moved, and fails when the code, the gross and a deduction all shifted at once. Some search the symptom and land on a list of possible reasons with no way to tell which applies. Some open a take home calculator, which assumes a flat annual salary spread evenly and cannot model the month that broke the pattern. And plenty wait to see whether it comes back.

How you’d do this in Surplus

Surplus reads the payslip rather than the code alone. The payslip analyser takes the figures the payslip itself reports - gross pay, Income Tax, National Insurance, pension and student loan - and models what PAYE would produce from them, so a month that moved shows up as a measured difference against the modelled figure rather than a hunch. Pay is tracked month to month, so the change is anchored to the cycle it started in rather than a vague sense that things feel tighter. The bank side stays separate: read-only Open Banking provided by Yapily means the take-home figure the app works from is the amount that actually landed, not a projection. Surplus cannot change a tax code - only HMRC does that. What it can do is name the month the number moved, and by how much.

A worked example: the bonus month

Take someone on £3,000 a month gross, tax code 1257L cumulative, category A National Insurance and a Plan 2 student loan. A £6,000 bonus lands in May, month 2 of the tax year, making May’s gross £9,000. Figures are illustrative; pension is left out to keep the comparison clean.

Payslip line	A normal month	May (bonus month)
Gross pay	£3,000.00	£9,000.00
Income Tax	£390.50	£2,314.83
National Insurance	£156.16	£347.50
Student loan (Plan 2)	£49.68	£589.68

By May, pay to date is £12,000 against free pay of £2,095 (two months of £1,047.50), leaving £9,905 taxable. Only £6,283.33 of basic-rate band has been released by month 2, so £3,621.67 is taxed at 40% - even though the year’s total of £42,000 is entirely basic rate. At a flat 20% the month would have cost £1,590.50, so roughly £724 of May’s deduction is timing, not liability.

It unwinds: June, July and August each show £152.67 of Income Tax instead of £390.50, September £379.67, and from October the normal £390.50 returns. The year’s tax lands at £5,886.00 - exactly 20% of the £29,430 left after the £12,570 Personal Allowance. No claim, no letter.

The other lines do not. May’s National Insurance of £347.50 is proportionally lighter than a normal month, because everything above £4,189 attracts only 2%, but the charge is per pay period and never returns. The £589.68 student loan repayment stays too: £42,000 is above the £29,385 Plan 2 threshold, so no year-end refund arises. Knowing which lines reverse is the difference between waiting and acting. For what is spendable while a month like this settles, what’s safe to spend before payday picks up the thread.

Every figure above is cited to its source where it appears and re-verified against that source whenever UK rates move.