How to calculate Class 4 National Insurance on self-assessment

Calculating Class 4 National Insurance on your self-assessment tax return involves applying the relevant percentage rates to your business profits once they exceed the annual threshold. The exact rates and thresholds change each tax year, so always check the latest figures on GOV.UK before completing your return.

This guide explains the key concepts in practical terms for UK early-stage founders and e-commerce sellers who run online side businesses. You will find clear steps, examples and reasoning to help you understand how these contributions fit into your overall tax position.

What are Class 4 National Insurance contributions?

Class 4 National Insurance is a contribution paid by self-employed individuals on the profits from their business. It is collected through the self-assessment tax system and goes towards your state pension and certain benefits. Unlike Class 2 contributions, which are a flat weekly amount, Class 4 is calculated as a percentage of your taxable profits.

These contributions only apply once your annual profits exceed a lower threshold. Understanding this helps you plan cash flow and avoid surprises when filing your tax return. Rates and thresholds are set by the government and can change, so verify the current rules on GOV.UK each year.

Understanding the percentage rates and thresholds

The main rate of Class 4 National Insurance applies to profits between the lower profit limit and the upper profit limit. A different, lower rate then applies to any profits above the upper limit. Both the rates and the exact profit thresholds change periodically, therefore you must consult the latest GOV.UK guidance before making calculations.

For online side businesses it is common to start with modest profits that may sit below the lower threshold in the first year or two. As turnover grows, a larger portion of profit becomes subject to these contributions. Keeping accurate records of income and allowable expenses is essential because only net profit is used in the calculation.

Step-by-step process to calculate Class 4 NI

Determine your taxable profit for the tax year by subtracting allowable business expenses from your total trading income.
Check the current lower profit threshold on GOV.UK. If your profit is below this figure, no Class 4 National Insurance is due.
If your profit exceeds the lower threshold, apply the main Class 4 rate to the portion of profit that falls between the lower and upper thresholds.
Apply the additional Class 4 rate to any profit above the upper threshold.
Add the two amounts together to find your total Class 4 liability.
Enter this figure in the correct boxes on your self-assessment tax return.

Perform these steps using the exact rates and thresholds published for the relevant tax year. Many accounting software packages will carry out the calculation automatically once you input your profit figure, but it is still important to understand the underlying logic.

Practical examples for online side businesses

Consider a seller running an e-commerce side business who makes £12,000 profit in a tax year. If the lower profit threshold is above £12,000, no Class 4 National Insurance would be payable. This situation is typical for many early-stage online sellers who are still building their customer base.

Now suppose the same business grows and generates £35,000 profit. The first slice of profit up to the lower threshold attracts no Class 4 charge. The amount between the lower and upper thresholds is charged at the main rate. Any profit above the upper threshold is charged at the additional rate. The exact monetary amounts depend on the thresholds and rates in force that year, which must be checked on GOV.UK. This example shows why accurate bookkeeping matters: overstated expenses reduce profit and therefore reduce the Class 4 liability.

Another common scenario is an e-commerce founder who also has employment income. In this case you pay Class 1 National Insurance through PAYE on your salary, and Class 4 only on self-employment profits. The two systems interact through the overall annual maximum contribution rules, but the self-assessment calculation for Class 4 remains based solely on your business profit.

Common mistakes and how to avoid them

Using gross turnover instead of net profit – only profit after allowable expenses is used.
Forgetting to update rates and thresholds each tax year – always download the latest self-assessment notes from HMRC.
Overlooking the interaction with other income – employment and self-employment National Insurance have combined limits.
Delaying record-keeping until January – monthly or quarterly reviews of profit give a clearer picture of likely liability.

Good bookkeeping software or a simple spreadsheet that separates income and different categories of allowable expenses makes the calculation far easier. Retaining evidence of all transactions is important in case HMRC makes enquiries.

How Class 4 NI fits into your wider tax planning

Class 4 National Insurance is only one part of the tax picture for self-employed e-commerce sellers. You must also consider income tax on your profits, VAT if your turnover exceeds the registration threshold, and cash-flow implications of quarterly payments on account once your tax bill grows. Reviewing your profit regularly helps you set aside the right percentage for both income tax and National Insurance.

Many early-stage founders choose to incorporate once profits become more predictable. Moving to a limited company changes the National Insurance rules from Class 4 to Class 1 employer and employee contributions, which have different rates and thresholds. The decision depends on your specific numbers, so compare both structures using current HMRC figures.

Where to get help and final checks

HMRC provides detailed guidance, online tools and a self-assessment helpline to support accurate filing. Free webinars and written examples are updated each tax year. If your affairs become more complex, consider speaking to a qualified accountant who specialises in e-commerce businesses.

Loomworks in West Yorkshire offers selected early-stage founders practical support including mentoring that can help with these financial basics. Ultimately, staying organised, checking the latest GOV.UK rates each year and maintaining clear records remain the most reliable ways to manage Class 4 National Insurance correctly.

Frequently asked questions

Do I pay Class 4 National Insurance if my profits are low?

You only pay Class 4 National Insurance once your annual business profits exceed the lower profit threshold for that tax year. Many side businesses and early-stage e-commerce sellers remain below this level in their first years and therefore pay nothing. Always confirm the current threshold on GOV.UK because it changes.

How do I find the correct Class 4 rates for my tax return?

The rates and thresholds are published each year on GOV.UK in the self-assessment tax return notes and on the National Insurance pages. Download the specific guidance for the tax year you are filing. Accounting software usually imports the latest figures automatically.

Can Class 4 National Insurance be paid in instalments?

Class 4 National Insurance is included in your overall self-assessment tax bill. If that bill is large enough you may need to make payments on account in two instalments. Late payment attracts interest and penalties, so budgeting early based on estimated profit is sensible.

Does Class 4 NI apply to e-commerce income from platforms like Etsy or Shopify?

Yes, profits from any self-employed trading activity, including sales through online marketplaces, are included when calculating Class 4 National Insurance. Deduct allowable expenses such as platform fees, advertising and postage before arriving at the profit figure used in the calculation.

What records do I need to calculate Class 4 National Insurance accurately?

You need a complete record of all business income and a breakdown of allowable expenses for the tax year. Bank statements, sales reports from your e-commerce platform and receipts for purchases should be kept. Accurate records let you calculate true profit and avoid overpaying National Insurance.