How to Calculate Class 4 National Insurance When You Have Multiple Self Employed Trades

What Happens to Class 4 NI When You Have Multiple Trades

If you run two or more self employed businesses as a sole trader, HMRC does not calculate your Class 4 National Insurance separately for each trade. Instead, your profits from every trade are added together, and Class 4 NI is charged on the combined total.

This is the aggregation rule, and it matters because it can push you into a higher NI band than you expected. A common mistake is assuming each trade is treated independently, which leads to underpayment and a bill from HMRC later.

For the 2025/26 tax year, Class 4 NI is charged at 6% on profits between £12,570 and £50,270, and 2% on profits above £50,270. If you have two trades each making £30,000, your combined profit is £60,000, not two separate £30,000 calculations. The first £50,270 of the combined profit is charged at 6%, and the remaining £9,730 at 2%.

Who This Applies To

The aggregation rule applies to anyone who is self employed in more than one trade. That includes:

• A freelance graphic designer who also runs a small ecommerce shop on Etsy.
• A builder who takes on domestic work and also does subcontracting through the CIS scheme.
• A personal trainer who takes individual clients and also runs a separate online fitness coaching business.
• A musician who performs gigs and also runs a teaching practice.

If you are a sole trader in any two or more businesses, the rule applies. It does not apply to limited company directors because they pay Class 1 NI through their salary, not Class 4.

How the Aggregation Works in Practice

Let us walk through a real example. A freelance consultant in Manchester has two self employed trades:

• Trade A: a management consultancy turning over £72,000 with expenses of £18,000, giving a profit of £54,000.
• Trade B: a small coaching business turning over £15,000 with expenses of £4,000, giving a profit of £11,000.

For Class 4 NI purposes, HMRC aggregates these profits: £54,000 plus £11,000 equals £65,000 combined profit.

The Class 4 calculation on £65,000 is:

• 6% on the band from £12,570 to £50,270: that is £37,700 multiplied by 6% = £2,262.
• 2% on the band above £50,270: that is £14,730 multiplied by 2% = £294.60.
• Total Class 4 NI due: £2,556.60.

If you had incorrectly calculated each trade separately, you would have applied the lower rate band twice, underpaying by a significant margin. HMRC will correct this when your tax return is processed, but you will then face the bill in one lump sum.

How to Report Multiple Trades on Your Self Assessment

On your self assessment tax return, each trade is reported on a separate SA103 (self employment pages). You complete one SA103 for Trade A and a second SA103 for Trade B. The tax return software then aggregates the profits automatically.

If you file using commercial software like Xero, FreeAgent, or QuickBooks, the aggregation happens behind the scenes. The same applies if you use HMRC's own filing system. You do not need to calculate Class 4 NI manually, but you do need to understand the rule so you can check the calculation is correct.

The risk comes if you file separate returns for each trade or use different software for each. HMRC's systems will still aggregate them at their end, but you might not see the combined figure until the assessment is issued. That is when the surprise bill arrives.

If you use an accountant, they will handle the aggregation. If you file yourself, double check the combined profit figure before submitting.

What About Class 2 NI?

Class 2 National Insurance is different. Class 2 NI was abolished as a compulsory charge from 6 April 2024. You no longer need to pay a flat weekly amount. If your profits are below the small profits threshold (currently £6,845 for 2025/26), you can still protect your state pension record by making voluntary Class 2 contributions.

But the key point is that Class 2 NI is not aggregated in the same way as Class 4. It applies per individual, not per trade. So having multiple trades does not change your Class 2 position.

What Happens If You Also Have Employment Income

If you are employed and also self employed, your Class 4 NI is calculated on your self employed profits only. Your employment income is subject to Class 1 NI through your payroll. The two do not mix.

However, your total income from all sources affects your income tax bands, so the self employed profits can push you into a higher tax bracket. That is a separate calculation from Class 4 NI, but it is worth being aware of.

For example, a teacher in Birmingham earning £35,000 through PAYE also runs a small tutoring business on the side making £15,000 profit. The Class 4 NI on the tutoring profit is 6% on profits above the £12,570 lower profits limit, so on £2,430 in this case (£15,000 minus £12,570). The lower profits limit applies to self-employed profits directly regardless of employment income; the NI calculation only looks at the self employed profits.

How to Plan Around the Aggregation Rule

If you have multiple trades and your combined profit is approaching the upper profits limit of £50,270, you might want to consider whether restructuring your business could save you NI. For instance, if one of your trades is profitable enough to justify incorporating, you could move that trade into a limited company. The company would pay Corporation Tax on its profits, and you would take a salary and dividends, avoiding Class 4 NI on that income entirely.

But incorporation is not always the right answer. It brings additional costs, filing requirements, and complexity. For many people with two modest trades, staying as a sole trader and accepting the aggregated NI charge is the simpler and cheaper option.

Another option is to adjust your profit timing where possible. If you can defer income from one trade into the next tax year, or bring forward expenses, you might keep your combined profit below the upper profits limit for a year. That is a legitimate planning strategy as long as it reflects genuine business decisions, not artificial manipulation.

We cover the pros and cons of different business structures in our incorporation guidance, which walks through the tax implications of moving from sole trader to limited company.

Common Mistakes to Avoid

The most common mistake is assuming each trade gets its own NI threshold. That is wrong. The £12,570 lower profits limit and the £50,270 upper profits limit apply to your total self employed income from all trades combined.

A second mistake is forgetting to include all self employed income. If you have a side hustle that you think of as a hobby, but HMRC considers it a trade, it counts. The test is whether you are carrying on a business with a view to profit. If you sell handmade goods on Etsy, do freelance photography, or rent out a room through Airbnb, those are likely trades and must be declared.

A third mistake is mixing up Class 4 NI with income tax. They are calculated on the same profit figure but at different rates and with different thresholds. Do not assume that because your income tax is handled correctly, your NI is correct too.

What to Do If HMRC Gets It Wrong

HMRC's systems are generally good at aggregating multiple trades, but errors do happen. If you receive a tax calculation that looks wrong, check the combined profit figure on your SA302. If it does not match your total self employed income, you need to correct it.

You can amend your tax return online within 12 months of the filing deadline. After that, you need to write to HMRC. If the error is on HMRC's side, they will usually correct it without penalty. If it is your error, you may face interest and possibly penalties depending on how long it took to spot.

If you are unsure about your calculation, our accounting team can review your position. We regularly help sole traders with multiple trades get their NI and tax calculations right.

Final Check Before You File

Before you submit your self assessment, do this quick check:

• Add up the profit from every self employed trade you ran in the tax year.
• Apply the Class 4 NI rates to that combined figure.
• Compare the result to the NI calculation on your tax return.
• If they do not match, investigate before filing.

That five minute check can save you a lot of hassle later. If your combined profit is close to the £50,270 threshold, it is worth speaking to an accountant to see whether restructuring or timing adjustments could reduce your NI bill legitimately.

For more on how self assessment works for sole traders, see our sole trader and self employment guides.