How is National Insurance Calculated?

National Insurance is a key part of the UK tax system. If you are employed or self-employed and earn above a certain threshold, you will usually need to pay National Insurance. The money you pay contributes towards specific public benefits, but the way it is calculated and charged is separate from income tax.

This guide explains how National Insurance works, who pays it, how the amounts are calculated, and where to check your contributions.

What is National Insurance?

National Insurance is separate from income tax. While income tax goes into general government spending, National Insurance contributions are used to fund specific benefits such as the State Pension, statutory maternity pay, unemployment support and a portion of NHS funding.

Many people find National Insurance confusing because it looks and feels like a tax but is calculated and charged differently. It has separate thresholds, separate rates and applies differently depending on how you earn. It’s also not based on total income in the same way as income tax.

National Insurance exists as a separate system because it was originally designed to act more like a social insurance scheme. Rather than simply collecting tax, it links your contributions to your entitlement to certain benefits. For example, your eligibility for the State Pension depends on how many qualifying years of National Insurance contributions you have made.

Although today the lines between NI and income tax have blurred, this connection between paying in and getting something back is still a core part of the system. Most employees pay National Insurance automatically through their wages. Self-employed people report and pay it through their Self-Assessment tax return. Contributions help build your record for future benefits and should not be overlooked.

Types of National Insurance

There are several classes of National Insurance. The one you pay depends on your employment status and income:

•  Class 1 is paid by employees and is deducted from your payslip. Employers also make a contribution on top of your salary.
• Class 2 is paid by self-employed people at a flat weekly rate.
• Class 3 is voluntary and is used to fill gaps in your National Insurance record if, for example, you were not working or earning enough to contribute.
• Class 4 is also paid by self-employed people, based on their annual profits.

Understanding which class applies to you is the first step in working out what you owe.

How National Insurance is calculated for the self-employed (Class 2 and Class 4)

If you are self-employed, you may need to pay both Class 2 and Class 4 contributions. Class 2 is a flat weekly rate of £3.50 for the 2025 to 2026 tax year. You only pay this if your annual profits are over £6,845. The fact that this is a flat rate can catch people out who are not used to being self-employed, so make sure you are aware of this when calculating your budgets and income.

Class 4 is based on your annual profits. You pay 6 percent on profits between £12,570 and £50,270, and 2 percent on profits above £50,270. For example, if you make £30,000 profit in a year, you will pay around £1,045 in Class 4 and £182 in Class 2.

These amounts are calculated and paid when you complete your Self-Assessment tax return.

Changes and thresholds to watch

National Insurance thresholds and rates can change from year to year. These changes are usually announced in the Spring Statement or Autumn Budget. If you are close to a threshold or your work status changes, it is worth checking the latest figures on HMRC’s website or speaking with an accountant.

Where to find your National Insurance details

If you are employed, your payslip should show how much National Insurance you have paid in that period. You can also check your full contribution record by logging into your Personal Tax Account on HMRC’s website.

If you are self-employed, your contributions will be shown in your Self-Assessment summary after you submit your return. If anything looks incorrect or you think you may have overpaid or underpaid, you can contact HMRC to check and request an adjustment if necessary.

Need help with your National Insurance?

Whether you’re self-employed or work through PAYE, understanding how National Insurance is calculated helps you stay in control of your finances and plan ahead.

If you’re unsure whether you’re paying the right amount or want to check your record, we’re here to help. Get in touch with our team by calling 0330 058 6559 or email hello@scruttonbland.co.uk to speak with one of the team.