Class 1 employee and employer National Insurance thresholds and rates, Class 2 and Class 4 for the self-employed, and the category letters.
Primary source: National Insurance rates and categories (GOV.UK).
National Insurance is charged on earnings and on the profits of the self-employed, and it funds contributory benefits and the State Pension. It is separate from Income Tax: the thresholds, the rates and the treatment of different kinds of income all differ.
Class 1 contributions are paid by employees and employers on earnings above the primary and secondary thresholds. Class 2 and Class 4 contributions apply to the self-employed. The category letter on a payslip records which rate applies to a particular employment.
The Class 1 employee and employer thresholds and rates, the Class 2 and Class 4 rules for the self-employed, and the category letters are published for each tax year separately. The Tax Rulebook keeps the 2024–25, 2025–26 and 2026–27 editions side by side.