Why is my tax code wrong? What it means and how to fix it

What does 1257L mean?

1257L means you get £12,570 of tax-free pay from that job in the tax year, which is the standard Personal Allowance for 2026/27. According to GOV.UK, 1257L is "the tax code currently used for most people who have one job or pension". The numbers show your tax-free pay with the last digit removed, and the L means you get the standard allowance.

Your employer spreads the allowance across the year. On monthly pay, 1257L gives £1,047.50 of tax-free pay each month. On a £30,000 salary paid monthly, the engine behind our tax code checker gives Income Tax of £290.50 a month, or £3,486 for the year.

According to GOV.UK, HMRC works out your number by starting with your Personal Allowance, taking off untaxed income and deductions such as company benefits or the High Income Child Benefit Charge, and replacing the final digit with a letter. You get a separate tax code for each job or pension.

Why HMRC changes your tax code

HMRC changes your tax code when it needs you to pay a different amount of tax, usually because your income or circumstances have changed. According to GOV.UK, the common reasons are:

• you start a new job, or start getting income from an additional job or pension
• your employer tells HMRC you have started or stopped getting benefits from your job, such as a company car or medical insurance
• you get taxable state benefits, or your weekly State Pension changes
• the interest on your savings is more than your Personal Savings Allowance
• you claim Marriage Allowance or tax relief on work expenses
• you pay the High Income Child Benefit Charge through your wages or pension
• you need to pay back a Winter Fuel Payment or Pension Age Winter Heating Payment
• you have been on the wrong tax code and owe tax

Worked example: a benefit in kind

GOV.UK gives the example of medical insurance from your employer worth £1,570. HMRC takes it off the £12,570 allowance, leaving £11,000, so your code becomes 1100L. On a £30,000 salary, the tax code checker shows £3,800 of Income Tax under 1100L, which is £314 more than under 1257L. That £314 is 20% of the £1,570 benefit.

Underpayments from an earlier year

If you paid too little tax, according to GOV.UK, HMRC usually collects it by changing your tax code, in equal instalments over 12 months from the start of the following tax year. Your code number falls, or becomes a K code if the amount is larger than your allowance.

Emergency tax codes: 1257L W1, 1257L M1, X and NONCUM

An emergency tax code is a code ending in W1 (paid weekly), M1 (paid monthly) or X (pay dates vary), and some payroll software shows NONCUM instead. According to GOV.UK, your tax is then "worked out based on what you're paid in that week or month only", as if you were paid that amount every week or month of the year, which can mean you pay the wrong amount.

You usually get one when you start a new job and your employer does not have your previous pay and tax details, for example because you had no P45. According to GOV.UK, HMRC usually updates your code once it has details from your new and previous employers, which can take up to 35 days from your start date. You can also be put on one when you start getting company benefits or the State Pension, and then it lasts until the end of the tax year.

Worked example: starting a new job in October on 1257L M1

Say you start a job on £30,000 a year in October, the seventh tax month, with no other pay since 6 April 2026. On 1257L M1 you get only one month's allowance each month, so over the 6 months to April you pay £1,743 in tax. On the normal cumulative basis you would get the allowance for the whole year so far against your £15,000 of pay, and the tax would be £486. You overpay £1,257, which HMRC refunds through your pay once it has your details.

The tax code checker shows this comparison when you enter an emergency code with a start month. According to GOV.UK, if you have not paid enough tax on an emergency code, you stay on it until you have paid the correct tax for the year.

0T and BR on your only job mean you are probably paying too much

A 0T or BR code on your only or main job gives you no tax-free pay, so you pay tax on every pound. According to GOV.UK, 0T means your allowance has been used up, or you have started a new job and your employer does not have the details it needs. BR taxes all pay from the job at 20% and is usually for a second job or pension.

These figures come from the tax code checker's engine with annual pay £30,000, paid monthly, for the whole of 2026/27, in England. On 0T or BR you pay £209.50 a month more than on 1257L. If this is your only job, check your code straight away.

What a K code means and how much it takes

A K code means your untaxed income or deductions are bigger than your Personal Allowance, so an amount is added to your pay before tax. According to GOV.UK, this can happen when you pay tax you owe from a previous year, get the State Pension or taxable state benefits, get company benefits such as a company car, or earn savings interest above your Personal Savings Allowance.

Worked example: K154

HMRC's PAYE Manual gives the example of a £12,570 allowance and a company car benefit of £14,120. The allowance minus the benefit is minus £1,550, which becomes K154. On a £30,000 salary, K154 takes £6,310 of Income Tax a year, £2,824 more than 1257L, which is 20% of the £14,120 car benefit.

There is a safety limit. According to GOV.UK, "employers and pension providers cannot take more than half of your pre-tax wages or pension when using a K tax code". A K code is not a mistake in itself, but check the benefits and income HMRC has listed, because a company car you have handed back or a benefit that has ended should come off.

Why a second job has a different tax code

A second job usually has a BR, D0 or D1 code because you get only one Personal Allowance each tax year, and it is normally given to the job that pays the most. According to HMRC's guidance on more than one job (updated 26 May 2026), BR taxes all pay at 20%, D0 at 40% and D1 at 45%, depending on your total income from all sources.

Worked example: £30,000 main job and £8,000 second job

With 1257L on a £30,000 main job and BR on an £8,000 second job, both paid monthly, the second job tax calculator gives £3,486 of tax on the main job and £1,600 on the second, £5,086 in total. That matches the correct tax on £38,000, so nothing is owed at the end of the year.

If both jobs used 1257L, you would get the allowance twice and pay only £3,486 during the year, leaving £1,600 to pay later. That is one of the most common reasons for an underpayment letter.

National Insurance works differently. It is worked out separately for each job, so the £8,000 job, at £666.67 a month, is below the £1,048 monthly threshold and pays none. Total National Insurance is £1,393.92, compared with £2,034.40 if all £38,000 came from one job.

When you start a second job you will not get a P45, because you still work for your first employer. According to HMRC, you fill in a starter checklist and say you already have another job, so the new employer can use the right code from your first pay. When you leave a job, tell HMRC through your Personal Tax Account or the HMRC app so your other job's code is updated.

How to check and fix your tax code with HMRC

The quickest way to fix a wrong tax code is to correct the details HMRC holds about you in the Check your Income Tax online service. According to GOV.UK, a wrong code is usually caused by incorrect or missing information, not by a mistake in the code itself.

Where to find your current code

• On your payslip.
• On a Tax Code Notice letter from HMRC, if you get one.
• In the Check your Income Tax online service, where you can also see codes for previous tax years and sign up for emails when your code changes.
• In the HMRC app, which according to GOV.UK (updated 7 September 2026) lets you check your tax code and report income from a second job.

Steps to correct it

1. Sign in to the Check your Income Tax online service.
2. Check your employment, pension, estimated taxable income, company benefit and expenses details.
3. Update anything that is wrong or missing, such as a job you have left or a benefit that has ended.
4. Claim tax relief on your employment expenses, if you have any.
5. If you left a job and did not get a P45, ask your old employer for one.

According to GOV.UK, if your code needs to change HMRC tells you and your employer the new code within 15 working days. It should then appear on your next or following payslip if you are paid monthly, or on your third payslip if you are paid weekly. If it does not, ask your payroll team to check they have received it.

If you have just started a new job, wait 35 days before contacting HMRC. If you cannot use the online service, the Income Tax helpline is 0300 200 3300, open Monday to Friday from 8am to 6pm, according to GOV.UK. Friends or family can help you as a trusted helper.

Before you contact HMRC, enter your code and salary in the UK tax code checker to see what each part means and how your tax compares with 1257L.

How you get overpaid tax back, or pay what you owe

If you overpaid because of a wrong code, your employer usually refunds the difference through your pay once HMRC issues the corrected code. According to GOV.UK, HMRC checks whether it has all your income details for the year, works out the difference and asks your employer or pension provider to refund it.

If HMRC does not yet have your details, the refund waits until after the tax year ends. HMRC then sends a tax calculation letter, also called a P800, between June and March of the following tax year, telling you how to get a refund or pay what you owe. You might get a Simple Assessment letter instead if you owe HMRC more than £3,000. If you are registered for Self Assessment, your bill is adjusted instead.

If you underpaid, HMRC usually collects it through your tax code in equal instalments over 12 months from the start of the next tax year, according to GOV.UK.