The class you pay depends on your employment status or if you have a gap in your National Insurance record.

National Insurance class Who pays
Class 1 Employees earning more than £153 a week and under state pension age - they’re automatically deducted by your employer
Class 1A or 1B Employers pay these directly themselves on their employee’s expenses or benefits
Class 2 Self-employed people - you don’t have to pay if you earn less than £5,885 and apply for a ‘small earnings exception’
Class 3 Voluntary contributions - you can pay them to fill or avoid gaps in your National Insurance record
Class 4 Self-employed people earning profits over £7,956