M and N: the Marriage Allowance codes
M and N look like a matched pair — one code goes up by the same amount the other goes down. The codes are symmetric. The law behind them is not, and the difference matters if you are trying to work out whether the transfer is worth making.
This independent tax-code explainer site outlines how the system operates. We cannot see your records.
The Marriage Allowance allows a taxpayer to transfer a portion of their personal allowance to their partner. This results in the payroll software applying an M code for the recipient and an N code for the transferor. While the administrative letters look symmetric, the underlying legal mechanism is fundamentally asymmetric. They do not operate in the same way.
The transferable amount is defined strictly in law. Under ITA 2007 s.55B(4)(b), the Marriage Allowance transferable amount is a set proportion of the s.35(1) personal allowance. Furthermore, s.55B(5) mandates that the resulting figure rounds UP to the nearest ten pounds. The legislation explicitly requires this rounding up to determine the exact transferable allowance. For 2026/27 that produces a transferable amount of £1,260: the M code adds it to the recipient’s free pay, and the N code takes the same amount off the transferor’s.
This transferable amount represents a rigid statutory mechanism. The law does not permit you to choose an arbitrary proportion to transfer. The legislation fixes the proportion precisely, and the mandatory rounding ensures that the payroll software deals with standard intervals. The employer simply receives the instruction to apply the calculated allowance adjustment.
The recipient experiences a distinct process. Under s.55B(1), the recipient gets “a tax reduction ... of the appropriate percentage of the transferable amount”. According to s.55B(3), that percentage is the basic rate. Crucially, the recipient is receiving a tax reduction, not an outright statutory increase in their core personal allowance.
This tax reduction is an end-of-year calculation concept that HMRC accelerates into your regular pay. By granting the reduction through the payroll, you experience the benefit gradually over the periods, rather than waiting for a rebate. However, the legal reality remains that it is a reduction of the final tax liability, translated into an allowance for administrative convenience.
HMRC delivers this reduction mechanically through the tax code by increasing the recipient’s tax-free pay. They can only achieve this mechanical shortcut safely because s.55B(2)(b) stipulates that the recipient must not be liable above the basic rate. If the recipient were a higher-rate taxpayer, giving them extra free pay would incorrectly reduce their tax by the higher rate. The restriction in s.55B(2)(b) makes the payroll shortcut mathematically sound for the basic rate.
The transferor undergoes a completely different mechanism. Under s.55B(6), the transferor’s personal allowance “is reduced ... by the transferable amount”. This is a direct reduction of the personal allowance itself. The transferor simply has less free pay available before tax is calculated.
When the transferor’s personal allowance is reduced, their free pay drops. This means a larger portion of their gross wage is exposed to taxation. The employer processes this without knowing that the allowance was surrendered to a partner. The payroll software simply sees a smaller allowance and calculates the deduction accordingly.
Therefore, M and N are NOT the same kind of relief. One is a straightforward reduction of allowance. The other is a statutory tax reduction delivered mechanically via an allowance increase. The code letters act as a flag to the payroll software to apply the correct tables.
It is important to remember that the payroll software operates automatically based on these instructions. The software does not know your personal circumstances; it only knows the data HMRC has transmitted. If the data transmitted is outdated, the deduction will be mathematically accurate according to the instruction, but factually wrong for your life. This is why maintaining accurate records with the authorities is the only way to ensure the system operates as intended.
The only place a person’s actual code can be viewed is HMRC’s authenticated Personal Tax Account. This site cannot know it, and does not try to. You must log in to your account to establish which code applies to your record and whether the transferred amount is currently active.
Independent tool not affiliated with HMRC. For your official code, log in to your Personal Tax Account. Check your tax code on GOV.UK.
Tax codes are not the same across the UK. A code starting with S uses the Scottish rates, which have more bands than the rest of the UK; a code starting with C uses the Welsh rates, which are built by a different rule even in a year when they come to the same figures. Check the first letter of your code before you read any rate on this site.
Letters and law are not the same thing. The PAYE Regulations create these codes by describing what they DO — “the basic rate code”, “the nil tax code” — and, with one exception, never write down the letters HMRC prints on your payslip. The letters are HMRC’s naming for those functions. This site labels which is which on every page, and the sources page shows the search behind that statement.
The figures and rules on this site are taken from the Income Tax (Pay As You Earn) Regulations 2003, the Income Tax Act 2007 and HMRC’s published guidance, and are shown as they were found. They have not been signed off by a reviewer. Where the law and HMRC’s naming differ, the page says which is which rather than blending them. For anything that affects a decision, the cited document governs and this site does not.