National Insurance Calculator

UK National Insurance (NI) is a tax on earnings and self-employed profits, which builds your entitlement to certain state benefits, including the State Pension. Your total income is calculated as the sum of your employment income and net self-employment income: total_income = employment_income + self_employment_net_income. For employees, Class 1 contributions (ni_c1) are deducted automatically through PAYE based on earnings above the Primary Threshold. For self-employed individuals, contributions are split into Class 2 (ni_c2), a flat rate for profits above a small profits threshold, and Class 4 (ni_c4), which is charged as a percentage of profits falling within specific lower and upper profit limits.

Consider a taxpayer in the 2023/24 tax year who earns an employment income of £30,000 and has a net self-employment income of £18,000, bringing their total annual income to £48,000. First, the calculator evaluates the employment earnings against Class 1 thresholds, applying the standard main rate percentage to earnings between the primary threshold and upper earnings limit. Next, it assesses the £18,000 self-employment profit against Class 2 and Class 4 parameters. Because £18,000 exceeds the lower profits limit, Class 4 contributions are calculated on the profit amount sitting between the lower profits limit and upper profits limit at the designated legislative rate. The final national insurance figure aggregates these distinct Class 1, Class 2, and Class 4 liabilities into your total annual contribution.

Keep track of your exact profit thresholds annually, as HM Revenue and Customs frequently adjusts these figures alongside tax brackets. If you have both a regular job and a side business, remember that both income streams contribute toward your overall social security record, though each class of NI operates independently. Always verify your National Insurance record online via your personal tax account to check for any gaps that might affect your future State Pension entitlement.