Class 4 National Insurance is profit-based and separate from Income Tax. Use this guide to understand why your estimate changes and what inputs matter most.
Your self-employed estimate is usually made up of multiple parts. Income Tax is based on taxable income. Class 4 National Insurance is a separate calculation tied to profits under the rules for the selected tax year.
If you only look at the total, it is hard to understand what changed. By separating the components, you can see whether a change came from Income Tax bands, Class 4 NIC, or something like a student loan estimate.
Start by entering your annual profit. Then change one input at a time (for example, student loan on/off or pension contribution) to see which components respond. Use the result to plan cash, and verify official guidance for filing.
Not tax advice. This is a simplified estimator designed for planning and clarity, not a substitute for official filing.
Last updated: 2026-04-20
No. In this tool, the relevant input is annual profit after allowable expenses.
Because the tool shows multiple components (Income Tax, NIC, optional student loan), and not every input affects every component equally.
No. It is an estimate based on simplified assumptions. Use it for planning and verify official guidance for your final return.
For official rules and definitions, verify with the references below.