Methodology and Sources

This site publishes tax thresholds, rates and deadlines for the self-employed in the UK and US. Because a wrong number here can cause real financial harm, every figure follows the same verification process before it is used in a calculator.

Primary sources only

Every figure is checked against a primary source, and nothing else counts. For the UK, that means gov.uk and hmrc.gov.uk. For the US, that means irs.gov, specifically Schedule 1-A and its instructions, Form 1040 and Form 1040-ES instructions, and the annual inflation adjustment revenue procedure. Accountancy blogs, software vendor guides and news summaries, however confident they sound, are never used as the source for a figure.

Where a second independent primary source exists, for example a revenue procedure that also confirms a figure carried over from the prior year, it is checked too, and both sources are recorded in the configuration file. A figure with only one available primary source is still published, since the alternative is publishing nothing, but the config comment says so.

Config-first architecture

No rate, threshold, cap, band, percentage or deadline is written directly into a calculator or a page. Every one lives in a dated, sourced configuration file, per tax year and jurisdiction. Calculators are pure functions that take that configuration as an input, which is what lets a single figure change, for example the Making Tax Digital threshold falling to £30,000 in April 2027, without touching any calculator's logic.

Every figure carries a citation

Each configuration object records a source URL, pointing to the specific page rather than a homepage, and a “last verified” date recording when a human checked it. Both render on the tool page that uses that figure. Current examples:

The full set is visible in each tool's “Sources” section, and in the config files themselves, which are open in the site's repository.

What we do when a figure cannot be confirmed

If a primary source cannot be found, or a value cannot be independently confirmed, it is marked internally as unverified and withheld from the live site rather than published as a best guess. A missing feature is preferable to a wrong number.

How correctness is checked

Calculators are unit tested against official worked examples published by HMRC and the IRS, not just against their own formulas, and every test runs before a change ships. This site does not currently have a named, credentialed tax professional reviewing guides and FAQ content before publication. It makes no claim otherwise: see about for what checking accuracy actually means here, and for how to get in touch if you are a qualified reviewer willing to help.

Update policy

Configuration files are revisited whenever HMRC or the IRS announces a change, and at minimum once per UK and US tax year at the point new rates are confirmed. Known upcoming changes, such as the Making Tax Digital threshold drop and the scheduled expiry of the OBBBA deductions after 2028, are tracked against their effective dates in advance.

Correction policy

Anyone can report a suspected error through the contact page. Reported figures are re-checked against the primary source before any change is made. Every correction, along with its date and the figure affected, is recorded on the changelog.