A percentage-of-every-payment rule can be a useful habit, but the right percentage changes with profit and personal circumstances. The MoneyCue calculator estimates Income Tax plus Class 4 National Insurance using the 2026/27 bands, with a separate Scotland option.
What the simple estimate does not know
Other taxable income, pension contributions, student loans, benefits, losses, allowances and special reliefs can change the real bill. Use the result as a planning reserve, not a Self Assessment calculation.