Why your second payslip looks more heavily taxed
HMRC needs somewhere to apply your tax-free Personal Allowance, and by default it gives all of it, £12,570 for 2026/27, to whichever job it treats as your main one, usually whichever you started first or told HMRC pays the most. Your second job then starts being taxed from the very first pound, with no allowance of its own, which is why the same hourly rate can leave a much smaller amount in your pocket on the second payslip than the first.
The tax codes that show up on a second job
| Code | What it means | When it's typically used |
|---|---|---|
| BR | All pay taxed at the basic rate (20%), no Personal Allowance | Most common default for a second job |
| D0 | All pay taxed at the higher rate (40%) | Your first job's pay already uses up your basic rate band |
| D1 | All pay taxed at the additional rate (45%) | Your first job's pay already uses up your higher rate band |
| 0T | No Personal Allowance, taxed through all the normal bands | HMRC doesn't yet have enough information to issue a different code |
Why the total tax you owe doesn't actually change
The reassuring part: splitting a given amount of income across two jobs doesn't change how much Income Tax you owe overall, only how it's collected. Take someone earning £30,000 from a main job and £10,000 from a BR-coded second job. Their main job's Income Tax, after the Personal Allowance, comes to £3,486; the second job's BR tax on the full amount comes to £2,000. Together that's £5,486 - exactly what someone earning the same £40,000 from a single job would pay (£5,486). If your tax code is right, splitting the income changes nothing about your final tax bill.
National Insurance is the part that genuinely is different
Unlike Income Tax, employee National Insurance is calculated separately for each employer, using its own threshold each time (unless HMRC treats your jobs as "associated employments", broadly meaning the same employer or connected companies). In the example above, the main job's National Insurance comes to £1,394, but the second job earns £10,000, below the £12,570 threshold on its own, so it attracts £0 of National Insurance. Combined, that's £1,394 in total - compared with £2,194 if the same combined income had come from one job instead. Two separate jobs can genuinely mean less total National Insurance than one job paying the same overall amount, purely because of how the threshold is applied.
If your second income is self-employed rather than a second job
Side income from freelancing, selling, or other self-employed work isn't taxed through a tax code at all. The first £1,000 of trading income each tax year is tax-free under the Trading Allowance, with nothing to report. Above that, you need to register for Self Assessment and pay Income Tax and Class 4 National Insurance (currently 6% on profit between £12,570 and £50,270, and 2% above that) on your profit, on top of whatever's already being collected through your employed job's payroll. The self-employed income tax calculator works out that side separately.
Checking your tax code is right
Sign in to your Personal Tax Account online to see exactly how HMRC has split your Personal Allowance across your jobs, and to request a change if it's given your allowance to the wrong one (for example, if your "second" job now actually pays more). If a code turns out to have been wrong for part of the year, HMRC usually corrects it automatically at year end through a P800 or Simple Assessment, refunding any overpaid tax without you needing to do anything.
This is a general explainer of how second-job tax codes work, not personalised advice. Run your own numbers through the take-home pay calculator for each job to see what you'd expect to be left with, and compare that against your actual payslips.