A 120 tax code means HMRC has set your tax-free income for the year at just £1,200, well below the standard Personal Allowance of £12,570. The number in a PAYE code is your annual tax-free allowance with the last digit dropped, so 120 works out to £1,200 spread across the year, or about £100 a month before tax starts. Every pound above that monthly slice gets taxed at the applicable rate, which is why a 120 code makes a visible dent in your take-home pay.
If your code should really be the standard 1257L, you’re missing out on more than £11,000 of tax-free income. That gap of £11,370 between your allowance and the full one usually points to a specific reason: taxable benefits from your employer, HMRC clawing back a past underpayment, or your income crossing the £100,000 tapering threshold. Sometimes it’s simply wrong, and you can get it corrected.
How HMRC Builds the Number
HMRC starts with your Personal Allowance and subtracts anything that reduces it, such as untaxed income or the taxable value of workplace perks. Whatever’s left has its last digit dropped and becomes the number in your code.1GOV.UK. Tax Codes: What Your Tax Code Means Your employer or pension provider then uses that number to work out how much of each payment is tax-free before applying income tax to the rest.2GOV.UK. Tax Codes
The Personal Allowance for the 2026/27 tax year is £12,570 and has been frozen at that level since April 2022.3GOV.UK. Income Tax in Wales For a 120 code to appear on your payslip, HMRC must have subtracted £11,370 from that figure.
Why HMRC Might Have Assigned a 120 Code
Benefits in Kind
Company cars, private medical insurance, and other workplace perks carry a taxable value that HMRC deducts from your allowance. A car valued at £8,000 for tax purposes plus private health cover worth £3,370 would, on their own, take your allowance from £12,570 down to £1,200 and produce a 120 code. The precise value HMRC uses for each benefit comes from the P11D form your employer submits at the end of the tax year.4GOV.UK. Expenses and Benefits for Employers: Reporting and Paying
Recovering a Previous Underpayment
If you underpaid tax in an earlier year, HMRC will often collect it back by shrinking your current allowance rather than sending you a bill. A large enough underpayment can knock thousands off your Personal Allowance and pull the code down to 120 or lower. You should see the underpayment listed as a separate line on your coding notice.
High-Income Tapering
Once your adjusted net income passes £100,000, your Personal Allowance drops by £1 for every £2 above that threshold and reaches zero at £125,140.5GOV.UK. Income Tax Rates and Personal Allowances Someone earning around £122,740 would see their allowance tapered to roughly £1,200, which produces a 120 code. This is worth double-checking if your pay has fluctuated, because HMRC bases the code on estimated earnings and can get it wrong if your income has changed since their last calculation.
Is It a Temporary Emergency Code?
A 120 code sometimes shows up on a temporary basis when you start a new job and HMRC hasn’t yet confirmed your details with your new employer. In that situation, you may see W1 (for weekly pay) or M1 (for monthly pay) after the number, or “NONCUM” printed on your payslip.6GOV.UK. Emergency Tax Codes
A normal cumulative code calculates tax based on everything you’ve earned so far in the tax year and adjusts each pay period so the annual total works out right.7HM Revenue & Customs. PAYE Manual – Coding: Ways an Employer Can Operate a Code A Week 1 or Month 1 code ignores that history and taxes each pay period in isolation. It often results in overpayment once your correct cumulative code is applied.
If you’ve just started a new job, HMRC asks you to wait 35 days for your income details to update before contacting them about the code.8GOV.UK. If You Think Your Tax Code Is Wrong Beyond that, if you’re still on an emergency version, it’s time to get in touch.
What the Letter After 120 Tells You
A 120L code is the most common form. The L simply confirms you’re entitled to the standard Personal Allowance and that yours has been reduced to £1,200 but is otherwise handled in the normal way.1GOV.UK. Tax Codes: What Your Tax Code Means A 120T code carries the same £1,200 allowance but tells your employer that HMRC needs to review the code manually before automatic adjustments (such as those following a Budget) can apply.9HM Revenue & Customs. PAYE Manual – Suffix Codes: The Suffix
If you live in Scotland, your code will start with an S (for example, S120L), and Scottish rates apply, including a starter rate of 19%, an intermediate rate of 21%, and higher bands reaching 48% on income above £125,140.10GOV.UK. Income Tax in Scotland Welsh residents get a C prefix, though Welsh rates currently match those in England and Northern Ireland.3GOV.UK. Income Tax in Wales A missing or wrong prefix will leave you taxed at the wrong regional rate, so flag it with HMRC if you spot the mismatch.
Check the Working Before You Challenge It
Before contacting HMRC, gather these records so you can see where the numbers diverge:
- Recent payslips, which show your current code, year-to-date pay, and tax deducted so far.
- Your coding notice (P2), HMRC’s letter setting out each item that went into your code.
- Your P11D, if you receive taxable benefits, which lists each benefit and the value HMRC will use.4GOV.UK. Expenses and Benefits for Employers: Reporting and Paying
- Your P60, which summarises total pay and tax for the previous tax year.11GOV.UK. Your P45, P60 and P11D Form
Comparing the P2 against your P11D and payslips is where most people find the problem. A benefit you no longer receive, an underpayment already repaid, or an income estimate that’s too high will all show up as a mismatch between HMRC’s figures and your own.
How to Get the Code Changed
The quickest route is the Check your Income Tax service on GOV.UK. You can see your current code, view exactly how HMRC calculated it, and report changes to your income or benefits.12GOV.UK. Check Your Income Tax for the Current Year Your Personal Tax Account provides the same access and lets you handle other tax matters at the same time.13GOV.UK. Personal Tax Account: Sign In or Set Up The HMRC app offers a mobile version. If none of that works, phone HMRC and ask for a manual review.
Once HMRC agrees a change is needed, they’ll update the code and notify you and your employer within 15 working days.8GOV.UK. If You Think Your Tax Code Is Wrong The new code should appear on your next payslip. Check that pay cycle to confirm it went through.
Reclaiming Overpaid Tax
If your 120 code turns out to be wrong, you’ve probably paid too much tax. How you get it back depends on timing. When the code is corrected during the tax year, PAYE handles it automatically: because the system works cumulatively, the pay period after the correction usually includes a larger-than-usual payment that returns the overpayment.
If the overpayment isn’t picked up until after the tax year ends, HMRC will send a tax calculation letter called a P800. These are issued between June and March following the end of the tax year.14GOV.UK. Tax Overpayments and Underpayments If your P800 confirms a refund, you can claim it online through your Personal Tax Account and receive the money within five working days, or request a cheque, which takes up to six weeks.15GOV.UK. If Your Tax Calculation Letter (P800) Says You’re Due a Refund
Self Assessment taxpayers don’t get a P800; any adjustment is made through the Self Assessment process itself.14GOV.UK. Tax Overpayments and Underpayments If you haven’t received a P800 by early the following year but you believe your tax was wrong, contact HMRC rather than waiting.
If HMRC Won’t Change It
Most 120 code disputes get resolved by the online service or a phone call. If HMRC issues a coding notice you believe is wrong, you also have a formal right of appeal in writing within 30 days of the date on the notice. HMRC will either amend the code or confirm their position. If they confirm and you still disagree, you have another 30 days to respond, and HMRC may offer an internal review by an officer who wasn’t involved in the original decision. If that doesn’t resolve things, the First-tier Tribunal (Tax Chamber) sits outside HMRC and can hear the case. In practice, the informal route settles almost all 120 code queries provided your paperwork supports what you’re saying.