Finance⏱ 6 min read
How National Insurance Works and How to Calculate Your Contributions
National Insurance is often misunderstood as a straightforward percentage of salary — but it has a different threshold structure to income tax, with different rates for employees, employers, and the self-employed.
National Insurance (NI) is separate from income tax and has its own thresholds and rates. The commonly-quoted "12% employee NI" only applies to earnings in a specific band — understanding the full structure reveals why effective NI rates differ significantly from headline figures.
Employee Class 1 NI (2024/25)
On weekly earnings:
Below £242/week (£12,570/year): 0% (Primary Threshold)
£242 - £967/week (£12,570 - £50,270/year): 8%
Above £967/week (above £50,270/year): 2%
Monthly example: £4,000/month salary (£48,000/year)
Below threshold (£1,048/month): £0
£1,048 - £4,000 = £2,952 at 8%: £236.16/month
Total employee NI: £236.16/month = £2,833.92/year
Employer Class 1 NI (2024/25)
Employer NI on same employee:
Below £175/week (£9,100/year): 0% (Secondary Threshold)
Above £9,100/year: 13.8%
Note: Employer threshold is different and lower than employee threshold.
Monthly employer NI on £4,000/month salary:
Below threshold: £758/month (£9,100/12) = £0
£758 - £4,000 = £3,242 at 13.8%: £447.40/month
Annual employer NI: £5,368.80/year
Real cost of employment = salary + employer NI + pension contribution
£48,000 + £5,369 + £1,440 (employer pension 3% min) = £54,809/year total
Self-Employed: Class 2 and Class 4
Class 2 (flat rate, 2024/25): £3.45/week if profits above £12,570
(£179.40/year — counts toward State Pension entitlement)
Class 4 (on profits):
£12,570 - £50,270: 6%
Above £50,270: 2%
Example: Self-employed profit £35,000
Class 2: £179.40
Class 4: (35,000 - 12,570) x 6% = 22,430 x 0.06 = £1,345.80
Total NI: £1,525.20/year
Vs employee earning £35,000:
Employee NI: (35,000 - 12,570) x 8% = £1,794.40
Employer NI also payable — self-employed save on employer's NI
Comparing Tax + NI Effective Rates
SalaryIncome TaxEmployee NICombined
£20,000£1,486£59411.4%
£30,000£3,486£1,39416.3%
£40,000£5,486£2,19419.2%
£50,000£7,486£2,99421.0%
£60,000£11,432£3,01424.1%
£100,000£27,432£3,85431.3%
State Pension: What NI Contributions Buy
New State Pension (2024/25): £221.20/week = £11,502.40/year
Required: 35 qualifying NI years (full State Pension)
Minimum for any State Pension: 10 qualifying years
Each qualifying year adds: £221.20 / 35 = £6.32/week to pension
Annual value of each year's NI contribution: ~£329/year in pension income
If you have gaps in your NI record:
Voluntary Class 3 contributions: £824.20 to buy one missing year
Return on each year purchased: £329/year
Payback period: 2.5 years — one of the best guaranteed returns available