What Is a Tax Code and How Do I Know If Mine Is Right?

A tax code is a short instruction from HMRC to your employer, telling them how much of your pay to treat as tax-free. It is one of the few numbers on a payslip that can quietly cost hundreds of pounds a year, and one of the few that nobody checks on your behalf.

How the code works

Under PAYE, income tax is deducted every time you are paid rather than settled in a lump at year end. For that to work, your employer needs to know how much tax-free income you are entitled to.

The code encodes exactly that. Take the number, multiply by ten, and you have your annual tax-free allowance. Code 1257L means 12,570 a year, which is 1,047.50 a month or 241.73 a week. Earnings above that are taxed at the appropriate rate.

The calculation is cumulative. Each pay period, your employer works out your total pay so far this tax year, your total allowance so far, and therefore the total tax due so far, then deducts whatever you have already paid. This self-corrects: a month where too much was taken is followed by a month where less is.

What the letters mean

Code	Meaning
L	Standard personal allowance
M	You have received 10 percent of a partner’s allowance through Marriage Allowance
N	You have transferred 10 percent of your allowance to a partner
T	The code includes other calculations, often where income is high enough for the allowance to taper
BR	All income taxed at basic rate, no allowance
D0	All income taxed at higher rate
D1	All income taxed at additional rate
NT	No tax deducted
K	Untaxed income exceeds the allowance, so the code adds to taxable pay rather than subtracting from it
S or C prefix	Scottish or Welsh rates apply
W1, M1 or X suffix	Emergency code, each pay period taxed in isolation

Codes that mean something is worth checking

BR on your main job. BR is correct for a second job or a pension, where the personal allowance is already being used elsewhere. On a single main job it means you are receiving no tax-free allowance at all, and at basic rate that is roughly 2,500 a year of tax you should not be paying.

An emergency code that persists. W1, M1 or X suffixes are entirely normal for a month or two after starting a job, particularly if you began without a P45. If one is still in place after three months, your record has not updated and it is worth a call, because emergency codes ignore your earnings history and frequently overtax.

A number well below 1257. Something is reducing your allowance. The usual causes are a company car or private medical insurance being taxed as a benefit in kind, an underpayment from a previous year being collected through the code, or untaxed income HMRC has been notified about. All of these are legitimate mechanisms. What matters is whether the figures behind them are correct.

A K code. Unusual, and it means your taxable benefits or tax owed exceed your allowance, so the code adds to your taxable pay instead of reducing it. Worth understanding rather than ignoring, since it usually signals a substantial benefit or a sizeable arrears collection.

Two jobs, two full allowances. If both employers are applying 1257L, you are underpaying tax and HMRC will reclaim it, usually through a lower code the following year. Better to fix it now than receive an unexpected bill.

Where to find your code

On any payslip, on the P60 issued after the tax year ends, on a P45 when you leave a job, and in your HMRC personal tax account online.

The online account is the most useful of these, because it shows not just the code but how it was calculated, itemising anything that has reduced your allowance. That is the quickest way to answer the question of why your code is not the standard one.

Getting it corrected

Your employer cannot change your tax code. They are legally required to apply whatever HMRC issues, so raising it with payroll achieves sympathy and nothing else.

Contact HMRC through the personal tax account or by phone, with your National Insurance number and recent payslips available. If the correction means you have overpaid, the refund normally comes through your payroll in the next pay run rather than as a separate payment, and it appears as a negative tax figure on the payslip.

For a previous tax year you can generally claim back up to four years, which is worth knowing if you discover a long-standing error rather than a recent one.

Why codes change

Codes are reissued when circumstances change: a new job, a company benefit starting or ending, a change in state benefits, a Marriage Allowance transfer, or an adjustment for an earlier year.

HMRC sends a coding notice explaining the change. These are worth reading rather than filing unopened, because they are the only explanation you will receive and they state precisely which figures were used.

The one-minute check

Once a year, ideally in April when new codes are issued, look at your code and ask two questions. Does it start with 1257, or is there an explanation you understand for why it does not? And does it carry a W1, M1 or X suffix it should no longer have?

Those two checks catch most of what goes wrong, and they take less time than reading the coding notice that prompted them.