Class 2 and Class 4 National Insurance for the Self-Employed, Explained

Self-employed National Insurance rarely gets its own line in anyone's mental budget, because HMRC calculates it alongside Income Tax on the same Self Assessment bill rather than deducting it from every payment you receive. Two classes apply to most sole traders — Class 2 and Class 4 — and they do different jobs.

Class 2: protecting your state pension

Class 2 is a small flat weekly amount whose main purpose is to build your entitlement to the State Pension and other contributory benefits, rather than to raise significant revenue. If your profits are above the Small Profits Threshold, Class 2 is now typically credited to your record automatically as part of your Self Assessment, without a separate payment being taken. Below that threshold, you can choose to pay voluntarily to keep your NI record unbroken.

Class 4: the one that moves your tax bill

Class 4 is calculated as a percentage of your taxable profits and is the element that actually changes how much you owe. A main rate applies to profits between the Lower Profits Limit and the Upper Profits Limit, and a lower, reduced rate applies to profits above the Upper Profits Limit — broadly mirroring the way employee National Insurance steps down at higher earnings.

• No Class 4 is due on profits below the Lower Profits Limit.
• The main Class 4 rate applies between the Lower and Upper Profits Limits.
• A reduced rate applies to profits above the Upper Profits Limit.
• Class 4 is worked out and collected through your Self Assessment return alongside Income Tax.

Why it catches people out

Because Class 4 only appears on the Self Assessment calculation, sole traders who set aside money only for Income Tax are often surprised when their actual January bill is higher than expected. When budgeting for tax as a percentage of profit, treat Income Tax and Class 4 National Insurance as a combined figure, not two separate, smaller numbers.

How this interacts with employment

If you are both employed and self-employed in the same tax year, you pay Class 1 National Insurance on your employment income and Class 4 on your self-employed profits. There is an annual maximum amount of combined National Insurance you can be asked to pay — if your circumstances mean you are close to or over it, it is worth checking whether you have overpaid, since HMRC does not always adjust this automatically.