Your payslip, your tax code, and where the money went

Pull up your last payslip. Not the bank notification, the actual document. It's probably a PDF sitting in a portal you log into twice a year.

Now find three things: your tax code, your year-to-date gross, and your year-to-date tax. If you can't find them in ninety seconds, the document is doing a bad job and you're about to get better at reading it.

This is general education, not personal advice. Your situation has details mine doesn't. For anything specific, talk to HMRC directly or to an accountant.

Gross pay isn't the number tax is calculated on

Two figures matter at the top of the payslip, and people conflate them constantly.

Gross pay is everything your employer paid you this period. Salary, overtime, bonus, commission, statutory sick pay, the lot.

Taxable pay is gross pay after anything that comes off before tax. Most commonly that's a workplace pension under a salary sacrifice or net pay arrangement, plus things like cycle-to-work or a holiday purchase scheme.

So if you earn £3,000 a month gross and £150 goes into your pension before tax, income tax is worked out on £2,850, not £3,000. That's why a pension contribution never costs you its full face value out of take-home pay.

If your payslip only shows one figure labelled "gross", look for a "taxable gross" or "taxable pay to date" column instead. It's almost always there somewhere.

Decoding the tax code

Your tax code tells your employer how much you can earn before tax starts. The number is your tax-free allowance with the last digit removed. The standard code in recent years has been 1257L, which corresponds to a personal allowance of £12,570.

Spread over twelve months, that's roughly £1,047 tax-free each month. Earnings above that get taxed at the basic rate, then the higher rate once you cross the threshold, and so on. Check GOV.UK for the current year's bands, because they move.

The letters matter more than most people realise:

• L — the standard allowance. Nothing unusual going on.
• M or N — you've received or given away part of your allowance under the marriage allowance.
• BR — every pound taxed at the basic rate, no allowance applied. Normal for a second job. A problem if it's your only job.
• D0 and D1 — everything at higher or additional rate. Again, usually a second income.
• 0T — no allowance at all. Often a sign payroll doesn't have a starter declaration from you.
• K — your taxable benefits or untaxed income exceed your allowance, so an amount gets added to your taxable pay instead of subtracted.
• W1, M1 or X on the end — an emergency, non-cumulative code.

That last one causes more confusion than all the rest combined.

Cumulative versus emergency, and why it matters

Normally PAYE is cumulative. Each payday, payroll looks at your total earnings for the year so far, works out the tax due on the whole lot, subtracts what you've already paid, and takes the difference. That's why a quiet month automatically claws back tax you overpaid in a busy one.

An emergency code switches that off. Each month is treated in isolation, as if it's the first month of the year. If you've had a gap between jobs, or your new employer didn't get your P45, you'll often sit on an emergency code and overpay.

It usually corrects itself once HMRC catches up, and the correction arrives as a chunky refund in one payslip. But "usually" isn't "always", and it can drag for months. If you've changed jobs this year and your code has a W1 or M1 on it, chase it.

National Insurance follows different rules

Income tax and National Insurance look similar on the payslip. They behave nothing alike.

NI is charged per pay period, not cumulatively. There's a threshold you earn below tax-free, a main rate on earnings above it, and a lower rate above an upper limit. So a one-off bonus can attract a lot of NI in the month it's paid, and unlike income tax, that doesn't average out later in the year.

It also stops entirely once you reach State Pension age, even if you keep working. And it's the contribution that builds your qualifying years toward the State Pension, which is why your NI record is worth checking on your personal tax account every few years.

Rates change often. Look them up rather than trusting a number you remember from a few years ago.

Pension, student loan, and the rest of the deductions

Your pension line is a transfer, not a loss. Under auto-enrolment there's a minimum total contribution, a slice of which has to come from your employer. If you opt out to boost your take-home, you're turning down the employer portion. That's the expensive decision, and people make it in their twenties when it costs the most.

Check how your scheme handles tax relief. Under net pay or salary sacrifice, the contribution comes off before tax and you get full relief automatically. Under relief at source, the provider claims basic rate relief for you, and higher-rate taxpayers have to claim the rest through a tax return. That extra relief isn't automatic, and plenty of people never claim it.

Student loan deductions run as a percentage of earnings above a plan-specific threshold. The plan number matters. Being on the wrong one means the wrong deduction every month, and it's a common error after a job change. Postgraduate loans deduct separately and stack on top.

Then the miscellaneous lines: season ticket loans, share schemes, union subs, charitable giving through payroll, court-ordered attachment of earnings. Read them. Twice a year is enough.

Ten minutes of arithmetic

Take your year-to-date taxable pay. Subtract the portion of your personal allowance used so far — roughly £1,047 per month elapsed on a standard code. Multiply what's left by the basic rate, adding higher rate on anything above the threshold.

Compare that to your year-to-date tax. If the two are within a few pounds, your code is doing its job. If they're hundreds apart, something's wrong.

Common causes: a company car or private medical cover being valued incorrectly in your code, an old employment still sitting on your HMRC record, or an underpayment from a previous year being collected quietly through a reduced allowance.

All of that is visible in your personal tax account on GOV.UK, and most of it you can correct yourself in a few minutes. Payroll can't change your code. Only HMRC can, and then they tell your employer.

Keep your P60 each May. Keep your P45 when you leave a job. Keep your P11D if you get benefits. Three PDFs a year, in one folder.

Your employer's payroll team processes hundreds of these every month. You process one. Work out which of you is more likely to notice the mistake.