Welcome back to The Personal Ledger.

Before assuming the number that landed in your account this month is correct because it came from a computer, here's something worth checking: HMRC reckons a meaningful share of UK employees are on the wrong tax code right now, and almost nobody checks.

Here's what we're covering:

HMRC estimates a meaningful share of employees are on the wrong tax code at any given time. That's not a rare glitch, it's the baseline, and almost nobody checks their own deductions against it.

Five things are worth thirty seconds each on your next payslip: your tax code (ending in L is standard), your National Insurance category (Category A for most employees), your student loan plan if you have one, whether your pension contribution matches what you agreed with HR, and whether the gross to net arithmetic actually adds up.

A concrete anchor: on a £35,000 salary with a standard tax code and 5% pension contribution, expect around £4,198 in income tax, £1,794 in National Insurance, and take home close to £27,065 a year, roughly £2,255 a month. If your numbers are meaningfully off that and you can't explain why, that's worth ten minutes of investigating, not a shrug.

The gap between trusting a number and checking it is about four minutes. Payslips are built to be filed, not read.

Takeaway: Check five lines on your next payslip: tax code, NI category, student loan plan, pension percentage, and the maths. Four minutes, once a month.

The Pay Slip Decoder (https://personalledger.gumroad.com/l/payslip-decoder) runs this exact check for you, line by line, against your real numbers.

1257L is the tax code most people have and most people have never actually decoded. Multiply the number by ten and you get £12,570, your Personal Allowance, the amount you earn before paying any income tax at all. The L means standard allowance, nothing added or removed. That's the whole code.

The other letters are where it gets useful. BR means everything from that job is taxed at basic rate, usually a second job. A K code means you owe tax on income that wasn't taxed elsewhere, such as company benefits, and it reduces your allowance rather than adding to it. 0T means no personal allowance at all, often because HMRC is missing information about you. Emergency codes ending in W1, M1 or X mean you're being taxed on a this pay period only basis rather than your full year picture. It usually self corrects, but not always fast enough to avoid a few months of overpaying.

If your code doesn't end in L and you can't say why, five minutes on HMRC's website is worth it.

Takeaway: Don't just glance at your tax code. Know what the letter after the number is actually telling you.

No NI on earnings up to £12,570. 8% between £12,570 and £50,270. 2% above that. Three numbers, and almost nobody knows them off the top of their head, which is exactly why NI is one of the least checked lines on a payslip despite being one of the easiest to verify.

The real world error usually isn't the percentage, it's the category. Most employees sit in Category A, but directors, certain apprentices, and anyone over State Pension age fall into different categories with different rates, and payroll systems occasionally apply the wrong one after a job change or a birthday.

Quick sense check: on a £35,000 salary, Category A National Insurance should land around £1,794 a year, about £150 a month. Meaningfully different, and it's worth asking payroll which category you're assigned and why.

Takeaway: NI looks like the most official, least negotiable line on your payslip. It's just arithmetic, and arithmetic can be wrong.