How to Calculate UK Income Tax for 2025/26 (With Free Tools for Developers)

If you're a developer living or working in the UK — or building a fintech/payroll app — understanding how HMRC calculates income tax is essential. In this post, I'll break down the 2025/26 UK tax system and show you how to implement the logic in JavaScript. UK Income Tax Bands for 2025/26 The UK uses a progressive tax system . For the 2025/26 tax year, the rates are: Band Taxable Income Rate Personal Allowance Up to £12,570 0% Basic Rate £12,571 – £50,270 20% Higher Rate £50,271 – £125,140 40% Additional Rate Over £125,140 45% Note: The Personal Allowance is reduced by £1 for every £2 earned over £100,000. National Insurance (NI) — Don't Forget This Income tax is only part of the picture. Employees also pay National Insurance: 12% on earnings between £12,570 and £50,270 2% on earnings above £50,270 This significantly reduces your take-home pay, so any salary calculator must include NI. Implementing a Basic UK Tax Calculator in JavaScript Here's a simple function to calculate income tax