How does Class 4 National Insurance actually work on self-employed profits?

Yes — you've got the rates exactly right. For both 2025/26 and 2026/27, Class 4 is 6% on profits between £12,570 and £50,270, and 2% on profits above £50,270.

How and when it's paid: exactly as you guessed. It's calculated automatically on your Self Assessment tax return and added to the same bill as your income tax, payable by 31 January. If you make payments on account, part of it is collected in the July payment too. There's nothing separate to arrange or pay — one return, one bill.

Your £30k example: 6% on (£30,000 − £12,570) = 6% × £17,430 = £1,045.80 for the year. That's the whole Class 4 charge.

Below £12,570: you pay no Class 4 at all. It's charged on your profits — that's income minus your allowable expenses, not your turnover — so the expenses you claim reduce it.

Class 2: you're right to query it. Class 2 was abolished from 6 April 2024, so it doesn't apply for 2025/26 or 2026/27. One thing worth knowing though: Class 4 doesn't count towards your State Pension or contribution-based benefits. If your profits are low in any year, you can pay voluntary Class 2 contributions to keep your State Pension record intact — the details are on GOV.UK.

Anna