Your tax code determines how much tax is deducted from your pay. While 1257L is the most common, different letters and numbers can affect how much you owe. From marriage allowance to emergency codes, here’s how to decode what HMRC assigns you.

Your tax code is basically a set of letters and numbers that show whether you are entitled to the annual tax-free personal allowance (the amount you can earn without paying tax). These codes are updated each year and help your employer figure out how much tax to take off your pay.

For the current and next tax years, the standard personal allowance is £12,570, and if you are entitled to this, your tax code will likely be 1257L. This is the most common code and applies to people with one job, no untaxed income, and no taxable benefits like a company car.

But tax codes are not always that straightforward. There are all sorts of other letters and numbers that might pop up. For example, if you are claiming the marriage allowance, your code might have an "M" in it. If you are paying tax at the Scottish rates, your code will start with an "S." And if your personal allowance gets reduced for some reason, like unpaid tax or income adjustments, your code will change accordingly.

Then there are the emergency tax codes—W1 or M1—which are used when someone starts a new job and does not have a P45 yet. These codes mean your tax will be calculated based on just that specific pay period, rather than your full income.

If you spot a 'K' at the start of your tax code, it means deductions (for things like company benefits, state pension, or previous tax owed) are greater than your personal allowance. Your tax deduction won’t be more than half of your pay or pension.