What Is Class 4 National Insurance? Definition

At 6% on profits between £12,570 and £50,270, and 2% above that, Class 4 National Insurance can add well over £2,000 to a sole trader's annual tax bill before they've even thought about income tax. It is one of two National Insurance charges the self-employed face, yet it is the one most people underestimate when they first set their day-rate or invoice total.

How Class 4 NI Is Actually Calculated

Class 4 NI runs on a two-band structure tied to the same thresholds as income tax. Your taxable profits land in one of three zones.

Worked example: a freelance graphic designer earning £38,000

Say your net taxable profit after allowable expenses is £38,000.

That £1,525.80 sits on top of your income tax liability and any Class 2 NI, all due on 31 January after the tax year ends. If you want to run your own numbers in seconds, the TapTax sole trader tax calculator handles the full picture automatically.

Class 4 vs Class 2: What Is the Difference?

For years, the self-employed faced both a flat-rate Class 2 charge and the profit-linked Class 4 charge. From April 2024, Class 2 was abolished for most self-employed people earning above the Lower Profits Limit, meaning Class 4 is now the dominant NI charge you will see on your Self Assessment calculation. If your profits fall below the Small Profits Threshold (£6,725 for 2025/26), you can make voluntary Class 2 contributions to protect your State Pension record, but the compulsory Class 4 charge simply does not apply.

You can read a fuller breakdown of how both classes fit the broader system on our national insurance glossary page.

The Mistake That Catches New Sole Traders Out

The single most expensive misconception is treating Class 4 NI as something separate you register for later. It is not. The moment you file a Self Assessment return with self-employment profits above £12,570, HMRC calculates it automatically. New freelancers who only budget for income tax find themselves facing a combined bill on 31 January that is hundreds of pounds larger than expected.

A practical fix: set aside roughly 25-30% of every payment you receive into a separate pot. That buffer covers both income tax and Class 4 NI for most sole traders earning between £20,000 and £50,000.

If your profits exceed £50,270

Once your self-employment profits pass the Upper Profits Limit, every additional pound of profit attracts only 2% Class 4 NI, not 6%. So a contractor earning £65,000 in profit pays 6% on the £37,700 between the two thresholds, and 2% on the £14,730 above £50,270. The marginal rate does not stay at 6% indefinitely, which makes the higher-income calculation less punishing than many assume.

Who Does Not Pay Class 4 NI?

Not every self-employed person is liable. You are exempt if:

• Your net profits are at or below the Lower Profits Limit of £12,570.
• You have reached State Pension age before the start of the tax year (currently age 66).
• Your trade is classified by HMRC as a business not subject to Class 4, such as certain diving professionals covered by diver's regulations.

If you are employed and self-employed simultaneously, your Class 4 NI is calculated only on your self-employment profits. Your PAYE National Insurance contributions from your employment are entirely separate.

Class 4 NI and Making Tax Digital

From April 2026, Making Tax Digital for Income Tax will require sole traders with income above £50,000 to submit quarterly digital updates to HMRC instead of a single annual return, with those earning above £30,000 following in April 2027. Class 4 NI will still be calculated on an annual basis within that system, but the shift means you will have a much clearer, more frequent view of your accruing liability throughout the year rather than one large shock in January. We cover the practical implications in detail over on the TapTax blog.