If you are in business as a sole trader or in a partnership, you are liable for self-employed National Insurance (NI) contributions. That generally means paying flat-rate Class 2 NI and profit-related Class 4 NI
Paying self-employed National Insurance contributions
You register to pay self-employed National Insurance contributions (NICs) as part of registering for tax self-assessment.
Class 4 and Class 2 NI contributions are due at the same time as income tax payments and are both collected through self-assessment - twice a year, by 31 January and 31 July.
You do not have to make Class 4 or Class 2 NICs if you are under 16 or have reached state pension age.
Class 4 NI
Class 4 NI contributions are based on your level of profits. You do not have to pay any Class 4 NI if your profits are below £8,164 (2017/18). You pay 9% Class 4 NI on profits from £8,164 to £45,000, and 2% on profits above that.
If you have more than one business, special rules can apply to the calculation of Class 4 NI contributions in some circumstances - for example, if one of your businesses makes a loss.
Class 2 NI
Class 2 NI contributions are charged at a flat weekly rate (£2.85 a week, 2017/18) and affect your entitlement to some benefits, such as the state pension.
The small profits threshold applies if your earnings are small (below £6,025). Where this is the case, no Class 2 NICs are payable and an exemption is no longer required.