You get one Personal Allowance between all your jobs, and it is normally given in full at the main one, so the second job is usually taxed from its first pound on a BR code at 20%. National Insurance works the other way round: it is charged on each job separately, in each pay period, so two jobs each below the threshold for the period pay none at all.
One allowance, split between employers
Your Personal Allowance is £12,570 for 2026/27, and it belongs to you rather than to a job. Your tax codes are the mechanism that divides it between employers, and giving the whole of it to one job is the simplest way to make sure it is used once and only once.
That is why a second job usually arrives on a BR code. BR gives no allowance and charges 20% on every pound from that job. It is the ordinary answer rather than a sign of a problem.
Where your income from both jobs together is high enough, HMRC may use D0 on the second job instead, which charges 40% on all of it. That is the same logic applied one band higher: your allowance and your basic-rate band are already used at the first job.
What the second job pays
Take a main job of £30,000 and a second job of £8,000 on a BR code. The second job is charged £1,600.00 of income tax and leaves £6,400.00.
The pair together come to £38,000 of gross pay and £31,520.08 of take-home pay for the year. Comparing that total with what one job on the same money would produce is the check worth doing, because it shows whether the codes between the two jobs are dividing the allowance correctly.
National Insurance does not work the same way
This is the difference that catches people out, and it runs the opposite way to income tax.
National Insurance is charged on the earnings paid in respect of any one employment, in each pay period, without regard to any other payment of earnings in respect of any other employment. There is no annual total for an employee and no end-of-year reconciliation.
So the threshold is a per-period one and each job gets its own: £1,048 a month, or £242 a week. Two jobs each paying under the monthly threshold attract no employee National Insurance on either, even when the two together come to well over £12,570 a year. Above the threshold, the main rate of 8% applies to each job in its own right.
Two situations undo this. Earnings from more than one job with the same employer in the same period are added together and treated as a single payment. So are earnings from two employers who carry on business in association with each other, which covers two companies in the same group. Both rules give way where adding the earnings up is not reasonably practicable. Two genuinely unconnected employers fall into neither.
When the codes take the wrong amount
Too much tax happens when the main job pays less than your Personal Allowance. Part of the allowance sits unused while the second job is taxed on every pound. The fix is to have HMRC split the allowance between the two codes, which it can do once it knows what each job pays.
Too little tax happens when the two jobs together pass £50,270. A BR code charges 20% and ignores the higher rate, so the shortfall builds up quietly and HMRC collects it later, usually by moving one job to D0 or by lowering the number at the main job.
Either way the correction comes from HMRC rather than from an employer. Employers apply the code they are sent and cannot change it.
Which job should hold the allowance?
Whichever job uses it first, the total tax across both is usually the same by the end of the tax year, because PAYE is settling one annual liability through two codes.
Putting the allowance against the job that reliably pays more than it only avoids leaving part of it unused month to month. Where the jobs are similar in size, or where one is irregular, splitting the allowance between the two codes can produce a steadier result across the year.
We can show you the figures for any combination. The decision, and the conversation with HMRC, are yours.
Student loans and pensions across two jobs
A student loan repayment is worked out on each job separately, against the threshold for the pay period, in the same way as National Insurance. Two jobs each below the threshold produce no repayment, and neither employer knows about the other.
Automatic enrolment also runs per employer, against the earnings from that employment alone, so a second job may or may not trigger it depending on what it pays.
Both of those can leave an annual position that differs from what a single job on the same total would have produced. Neither is an error, and neither is reconciled through payroll.
What if the second income is self-employed?
Then none of the above applies to it. Self-employed profit is reported through Self Assessment rather than PAYE, it is charged to income tax on top of your employment income, and it carries its own class of National Insurance with its own thresholds.
HMRC can collect the tax on a small amount of self-employed profit by changing your PAYE code rather than asking for a payment, which is why a second income can show up as a lower code number at your job.
What to tell HMRC, and when
Give each new employer a P45 or a completed starter checklist, and answer the starter statements accurately: the statement that says you have another job is what produces the BR code in the first place.
Then check the estimated pay HMRC holds for each job in the check your Income Tax service, because that is what both codes are built from. An out-of-date estimate for one job is the most common reason a pair of codes takes the wrong amount.