How this service works
This service estimates Self Assessment tax bills. It can also work out an amount to pay through HMRC's Budget Payment Plan. It can keep a customer's forecast up to date during the year.
These pages describe what it does, who it is for and how it has been tested. Every figure in the tests was worked out by hand from GOV.UK rates, then checked against the service.
The service in 4 parts
Self Assessment calculator
- Always on
Works out the tax a customer owes for a tax year, and what to pay by 31 January and 31 July. It covers Income Tax, National Insurance, Capital Gains Tax, student loans and charges.
More tax tools
- Optional
Separate features, each switched on and off by an administrator, and for each organisation and way of use: questions one page at a time, a result, and an API. An organisation's app can also hand any tool to the calculator and get the result back.
Ways to use the calculator
- Always on
Choose which parts of the calculator are used and how: full question pages, question pages inside another app, or the API. The service has its own settings, and so does each organisation.
Making Tax Digital for Income Tax helpers
- Optional
Helpers that work alongside a customer's Making Tax Digital software and HMRC. They never send anything to HMRC: only HMRC-recognised software sends quarterly updates. Each organisation chooses which helpers it offers, and for which way of use.
At a glance
- Features
- 78 features in 8 areas
- Use cases
- 73 use cases for 8 kinds of user
- End-to-end persona tests
- 78 journeys, 263 of 263 checks passed
- Calculator personas
- 137 customers, 381 of 381 checks passed
- Payment plan personas
- 23 customers, 112 of 112 checks passed
- Threshold tests
- 496 of 496 checks passed
- Automated tests
- 1737 passed, 0 failed, 0 skipped
- Last tested
- 16 October 2026
Try it
This test site has dummy versions of GOV.UK One Login, HMRC's APIs, GOV.UK Notify, DVLA and HMRC's Budget Payment Plan service. Try every part of the service with the test customers.