Methodology

How our tax calculators work.

Where the rates come from, how the figures are tested, who checks them and when they change.

Firm no. 4549994: ACCA
From, per month: £35
Named accountant: 1:1

Four rules every calculator follows.

Step 1
Rates come from the official tables
Every rate, band and threshold lives in one module of the code, copied from HMRC, the Scottish Government, GOV.UK and Revenue Scotland and labelled with its source. Each calculator, every worked example and the published rates file read from that one place, so a figure on a page cannot disagree with the calculator next to it.
Step 2
Worked examples are computed, not typed
Tables such as take-home pay by salary, tax by rent or CGT on a property sale are generated by the calculation engine when the site is built. When a rate changes, every example on every page changes with it.
Step 3
Automated tests guard the arithmetic
Each calculation engine has its own automated tests against figures worked out by hand: tax bands, the personal allowance taper, National Insurance by pay period, student loan plans, Scottish rates, Section 24, BADR, penalty milestones and more. A change that breaks a figure cannot be released.
Step 4
A qualified accountant reviews the method
The firm builds the tools and a named Chartered Certified Accountant reviews the method and the figures. The reviewer is shown on each calculator and linked to their profile.

Calculators are reviewed by Kamran Ishaq FCCA, Founder & CEO.

When the figures change.

Income Tax, National Insurance and student loan thresholds: checked every April for the new tax year (2026/27 is current).
Scottish rates and bands: updated when the Scottish Budget is confirmed, usually in February.
HMRC late payment interest: updated whenever the Bank of England changes Bank Rate (last checked 25 September 2026).
National Minimum Wage: updated each 1 April.
Budget announcements: added once they are legislated, with a note on the page before then.

Every rate the calculators apply is published as machine-readable JSON, generated from the same tables. Each calculator shows the date it was last updated.

What the calculators do not do.

They are estimates for one person, one tax year and the inputs given. They do not see your HMRC record, earlier payslips, other jobs or income you have not entered, and PAYE works payday by payday, so a payslip can differ from an annual estimate. They are not advice. If a figure matters — a job offer, a property sale, an overdue bill — talk to one of our accountants.

Found a figure that looks wrong? Report a calculator issue with the calculator, tax year and inputs used. Please do not send payslips or National Insurance numbers.