By the end of September you have had six payslips this tax year. A quick halfway tax check now catches a wrong tax code before it costs you at Christmas.
Nobody puts a tax check in the calendar. It is the kind of ten-minute job that quietly saves people hundreds of pounds, and almost everybody skips it until something goes wrong. The end of September is a good moment to do it, because you now have six monthly payslips for the 2026/27 tax year sitting in your email, and six is enough to tell whether things are on track.
Think of it as a halfway reading. You are not filing anything. You are just checking that the numbers add up, so that if a tax code is wrong or a deduction has crept in, you catch it with half a year still to run rather than discovering it in April when it is harder to fix.
PAYE, the system that takes tax from your wages, works cumulatively. Each payday it looks at your total pay and total tax for the year so far and works out what you should have paid by that point. That is usually a good thing, because it self-corrects. But it also means a wrong tax code can build up a growing error month after month, and the longer it runs the bigger the correction when it finally lands.
By the end of September you are at month six, exactly halfway through the tax year that started on 6 April. Your year-to-date figures are now big enough to be meaningful. A code that is quietly wrong has had six months to do damage, but there are still six months left in which a fix can put it right smoothly through your payslips. Leave it until February and the correction has to happen in a rush.
You do not need a spreadsheet. Pull up your most recent payslip and check these five things.
Find the tax code on your payslip. For most people with one job and the full personal allowance it will be 1257L. If yours is different, that is not automatically wrong, but it is the thing most worth understanding. An emergency code, a code with a benefit adjustment, or a K code all change what you pay. Open the HMRC app and check that the code HMRC holds matches the one your employer is using. If they differ, your payroll team is working from old information.
Your payslip shows a year-to-date, or YTD, figure for gross pay. At month six this should be roughly six times your normal monthly gross, allowing for any overtime, bonuses, or quiet months. If you work variable shifts it will not be a clean multiple, but it should be in the right ballpark. A figure that looks far too high or too low is worth questioning.
With a 1257L code, your £12,570 personal allowance is spread evenly across the year, so by month six you should have had about half of it, roughly £6,285, protected from tax. You will not see that exact phrase on the payslip, but if your taxable pay looks like almost all of your gross with very little tax-free, something is off, often an emergency code applying the allowance on a non-cumulative basis.
National Insurance is worked out each pay period rather than cumulatively, so it does not build up errors the same way. Still, check it is there and looks sensible. For 2026/27 you pay 8 per cent on earnings between £12,570 and £50,270 a year, and 2 per cent above that. If you had one huge month, you may have paid more National Insurance than feels fair, because it is not smoothed across the year like income tax.
Check your pension contribution is going out at the rate you expect, especially if you had a pay rise in April that should have changed it. If you repay a student loan, check the plan type looks right. Plan 1, Plan 2, Plan 4, Plan 5, and postgraduate loans all have different thresholds, and being on the wrong one means the wrong repayment.
Say you earn £30,000 a year on code 1257L, paid monthly. By the end of September you would expect your YTD gross to be around £15,000. Your tax so far should be roughly £1,743, which is six months of tax on income above the monthly slice of your allowance. If your payslip shows YTD tax of, say, £2,600 on that salary, you have very likely been on the wrong code for part of the year and are owed a refund once it is corrected.
You do not have to get this to the penny. The point is to spot a figure that is clearly in the wrong zone, then investigate it rather than shrug and move on.
If your tax code is the problem, fix the code and the rest usually follows. Because PAYE is cumulative, correcting the code part way through the year prompts an automatic recalculation, and any overpaid tax comes back through your next payslip rather than needing a separate claim. You can report a wrong code through the HMRC app or by calling HMRC.
If the issue is a deduction you do not recognise, a pension rate that looks wrong, or a student loan plan that does not match, raise it with your payroll or HR team first, because those are set by your employer rather than HMRC.
There is a practical reason to do this now rather than in December. If you are owed money and you fix the code in October, the refund lands in your next pay run, which is a genuinely useful thing to have before the expensive end of the year. If you owe a little because a benefit was not coded in, finding out now means the catch-up is spread over several months instead of hitting one December payslip.
If you work regular hours on a fixed salary, your year-to-date figures march up in a straight line and problems are easy to see. If you work variable shifts, with busy weeks, quiet weeks, overtime, and the odd big month, the numbers are lumpier and it is tempting to assume you cannot check anything meaningful. The opposite is true. Variable pay is exactly where tax codes go wrong and where National Insurance can bite, so the halfway read matters more.
Two things to watch if your hours move around. First, National Insurance is calculated on each pay period in isolation, not smoothed across the year. So a single very large month can cost you more National Insurance than the same money spread evenly would have, and there is no end-of-year rebate for that the way there is for income tax. It is not an error, but it is worth understanding before you plan a run of long weeks. Second, if your hours dropped for a stretch, you may have built up unused personal allowance that a later busy month will soak up, so your tax in a big month can look heavier than you expect while PAYE catches up.
The practical move is to look at the year-to-date totals rather than a single payslip. One month in isolation tells you very little when your hours swing. Six months of cumulative figures tell you whether the overall picture is right.
If you picked up a second job or started a side hustle this year, the halfway point is a good moment to check how it is being taxed. A second employed job is usually taxed on a BR code, meaning the whole amount is taxed at 20 per cent because your personal allowance is already used by your main job. That is often correct, but if your main job does not use all your allowance, you could be overpaying and may be able to split the allowance across both jobs. Side income from self-employment is not taxed through PAYE at all, so set money aside for it rather than expecting your code to handle it.
If you want to sanity-check what your take-home should be on your salary and tax code, the NetPay app works out income tax, National Insurance, pension, and student loan from your real numbers so you can compare it against your payslip in a couple of minutes. Free to download.
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By the end of September you have had six monthly payslips in the 2026/27 tax year, which is enough data to spot a wrong tax code, an unexpected deduction, or a drifting year-to-date figure. Catching it now means any refund or correction comes through before Christmas rather than after the tax year ends.
On monthly pay, the end of September is month six of the tax year, so your year-to-date gross should be roughly six months of your normal pay, and your tax-free allowance used should be about half of £12,570, so around £6,285. If the figures are wildly different, it is worth finding out why.
Open the HMRC app or your personal tax account, which show your current code and what it is based on. Compare it with the code on your latest payslip. If they disagree, your employer is using stale data. If both agree but the code looks wrong for your situation, contact HMRC.
If you are on the wrong code and have overpaid, correcting the code usually triggers an automatic refund through your next payslip, because PAYE recalculates on a cumulative basis. You do not normally need to claim separately for the current tax year once the code is fixed.
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NetPay UK works out your real net pay after tax, NI, pension and salary sacrifice, for hourly, shift and variable-income workers. Free to download.
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A note on financial advice: NetPay UK calculates take-home pay based on official HMRC tax rules. This article reflects rules in force at the time of publication (30 September 2026). Tax rules change. For complex situations, consult a qualified UK accountant or visit gov.uk/income-tax.
