Understanding Tax Codes: A Plain-English Guide for Employers

Every employee has a tax code, and that code determines how much income tax you deduct from their pay. If you get it wrong, your employee either takes home too little or too much — and either way, it creates problems down the line.

Despite how important tax codes are, they remain one of the least understood parts of payroll. This guide explains what tax codes mean, how to read them, what to do when they change, and how to avoid the most common mistakes.

What is a tax code?

A tax code is an alphanumeric identifier assigned to each employee by the tax authority. It tells you, as the employer, how much of the employee's earnings are tax-free and at what rate to deduct tax on the remainder.

The code is typically made up of a number and one or more letters. The number represents the employee's tax-free allowance (the amount they can earn before paying tax), while the letters indicate special circumstances that affect how the code should be applied.

For example, a common tax code might look like 1257L. The number 1257 means the employee has a tax-free allowance of 12,570 in their local currency. The letter L means it is a standard code with no special conditions.

How tax codes are assigned

Tax codes are issued by the tax authority, not by the employer. When a new employee starts work, they should provide documentation showing their current tax code. If they cannot provide this, you may need to apply a default or emergency code until the tax authority issues the correct one.

The tax authority calculates each person's code based on:

• Their personal tax-free allowance
• Any additional allowances (such as marriage allowances or blind person's allowance)
• Any reductions to their allowance (such as owing tax from a previous year, or receiving untaxed income like rental income)
• Whether they have multiple jobs

If an employee's circumstances change — they get married, start a second job, or begin receiving a new type of income — the tax authority will issue an updated code. You must apply the new code from the date specified in the notification.

Reading the numbers

The number in a tax code represents the employee's annual tax-free allowance, but with the last digit removed. So a code of 1257 means a tax-free allowance of 12,570.

To calculate the monthly tax-free amount, divide the annual allowance by 12 (or by the number of pay periods if you pay weekly). Any earnings above this amount are taxed at the applicable rate.

Some codes use the prefix K instead of a suffix letter. A K code means the employee's deductions and allowances result in a negative tax-free amount — in other words, they owe more tax than a standard code would collect. K codes are relatively rare but important to handle correctly.

Reading the letters

The letter suffix tells you the type of code and how to apply it. Here are the most common ones:

• L — Standard personal allowance. The most common code.
• M — The employee has received a transferred marriage allowance from their partner.
• N — The employee has transferred part of their personal allowance to their partner.
• T — The tax authority needs to review the employee's code. Apply it as instructed until a new code is issued.
• BR — All income taxed at the basic rate. No tax-free allowance. Usually used for a second job.
• D0 — All income taxed at the higher rate. Also common for second jobs.
• NT — No tax to deduct. Rare, and usually applies to specific types of income.
• 0T — No personal allowance. The employee's full earnings are taxed.

Understanding these letters is important because they change how you calculate deductions. A BR code, for example, means you apply a flat rate to all earnings with no tax-free portion.

Emergency tax codes

If you hire someone who cannot provide their tax code — perhaps they have lost their documentation or they are starting their first job — you may need to put them on an emergency tax code.

Emergency codes are typically based on the standard personal allowance but applied on a non-cumulative basis. This means each pay period is treated in isolation, without considering what has been earned or deducted earlier in the tax year.

The practical effect is that emergency codes often result in too much tax being deducted, especially if the employee starts partway through the tax year. The overpayment is corrected once the tax authority issues the correct code and you switch to cumulative calculation.

If an employee complains about high deductions and they are on an emergency code, the answer is usually to wait for the correct code to be issued. You cannot change the code yourself — only the tax authority can do that.

Cumulative versus non-cumulative codes

Most tax codes are cumulative, meaning each pay period takes into account all earnings and deductions since the start of the tax year. This smooths out the tax burden and ensures the correct total is deducted by year-end.

Non-cumulative codes (sometimes shown with a W1 or M1 suffix) treat each pay period independently. They are typically used for emergency codes or when the tax authority wants to ensure a specific amount is deducted each period regardless of prior earnings.

If you see W1 (week 1) or M1 (month 1) appended to a code, apply the code on a non-cumulative basis. Once the tax authority removes the suffix, switch back to cumulative.

What to do when a tax code changes

Tax code changes happen throughout the year. The tax authority may issue a new code because:

• The employee's personal circumstances have changed
• They have started or stopped a second job
• They have underpaid or overpaid tax in a previous year
• A benefit in kind has been added or removed

When you receive a new code, apply it from the date specified. If the code is cumulative, your payroll software will automatically recalculate the year-to-date position and adjust the next payment accordingly. This might mean the employee receives more or less than usual in the next pay period as the correction works through.

Always inform the employee when their tax code changes, especially if it will noticeably affect their take-home pay. They have the right to query the code directly with the tax authority if they believe it is incorrect.

Multiple employment

Employees with more than one job will have different tax codes for each employment. Typically, their personal allowance is allocated to their main job, and any secondary employment uses a BR or D0 code (taxing all income at a flat rate).

If an employee tells you they have another job, do not change their code yourself. The tax authority manages the allocation of allowances across employments. Your responsibility is to apply the code you have been given for your payroll.

Common mistakes employers make

Applying the wrong code

This is the most frequent payroll error related to tax codes. It usually happens when a new code notification is missed or applied to the wrong employee. The result is incorrect deductions that may take months to notice and require year-end corrections.

Not applying codes promptly

When you receive a new code, it needs to be applied from the specified date — not from the next convenient pay period. Delayed application leads to incorrect cumulative calculations.

Ignoring employee queries

If an employee questions their tax code, take it seriously. While you cannot change the code yourself, you can help by confirming what code you are using and directing them to the tax authority to investigate. An incorrect code can cost an employee significant money over the course of a year.

Manual errors

If you manage tax codes in a spreadsheet, the risk of transposition errors, missed updates, or incorrect calculations is high. Payroll software that receives code changes electronically and applies them automatically is significantly more reliable.

How payroll software helps

Modern payroll software handles tax codes in several ways:

• Automatic updates: Many systems receive tax code notifications electronically and apply them without manual intervention.
• Cumulative calculations: The software maintains year-to-date figures and recalculates automatically when a code changes mid-year.
• Validation: Good software flags unusual codes or changes that might indicate an error.
• Audit trails: Every code change is logged, making it easy to review the history during audits or employee queries.

When your payroll is integrated with your wider business systems — accounting, HR, timesheets — the data flows consistently and reduces the risk of things falling through the cracks. Platforms like Relentify are built around this principle, bringing payroll into a single workspace alongside the tools you already use.

Key takeaways

Tax codes are not as complex as they first appear. The number tells you the tax-free allowance, the letters tell you how to apply it, and the tax authority tells you when to change it. Your job as an employer is to apply the code you are given, apply it from the correct date, and keep accurate records.

If you are ever unsure about a code, do not guess. Check with the tax authority or consult your accountant. Getting tax codes right is one of the simplest ways to keep your payroll accurate and your employees happy.