How we calculate

Sources

Every rate, threshold, and band on the site comes from a primary source. We don't paraphrase from secondary calculator sites, we go direct.

• Income tax bands and personal allowance— gov.uk/income-tax-rates and HMRC's Income Tax manual
• National Insurance (Class 1, 2, 4)— gov.uk/national-insurance and HMRC's NI manual
• Scottish income tax bands: mygov.scot and gov.scot's published rates
• Student loan thresholds and rates — gov.uk/repaying-your-student-loan/what-you-pay (Plans 1, 2, 4, 5, Postgrad)
• Dividend tax rates and allowance — gov.uk/tax-on-dividends
• Corporation tax rates and marginal relief , gov.uk/corporation-tax-rates
• VAT Flat Rate Scheme percentages: VAT Notice 733 (HMRC)
• Pension annual allowance and tapering— HMRC's Pension Tax Manual
• Mortgage affordability heuristics — published lender criteria from FCA-registered contractor-mortgage specialists (CMME, Halifax contractor product, Saffron)

Where HMRC publishes a worked example, we test our calculators against it as part of an automated test suite. 269 tests run on every change before deployment; a wrong number gets caught in CI before it ships.

Tax years covered

Each calculator defaults to the current tax year (running 6 April → 5 April in the UK). Every calculator has a tax-year toggle so you can switch between the available years for comparison.

• 2025–26
• 2026–27 (current)

When the next tax year arrives, it gets added as a new option on the toggle and the previous years remain available. Older years aren't hidden behind separate URLs, every calculation history is one click away on every calculator.

Update cadence

• Spring Budget / Autumn Budget: when a Budget changes rates, we commit to updating affected calculators within 24 hours of the Chancellor's statement and republishing them with a fresh Reviewed date. Tax-year-aware code makes a Budget update a single configuration change in one file, so the bottleneck is verification against the published rates rather than the engineering work.
• 6 April (start of new tax year): the new tax year is added to every calculator's toggle and becomes the default; previous years stay accessible.
• Continuously: we monitor HMRC publications, the Scottish Budget, and other rate-setting announcements for mid-year changes (rare but they happen). Each calculator's footer carries a Reviewed date so you can see exactly when the figures were last checked.

What the calculators don't cover

Honest disclosure of gaps. Each calculator's “Edge cases” section spells out its specific limits; this is the cross-site summary.

• Benefits in kind: company car, private medical, etc. are not modelled. If you have a P11D, your real figure will differ.
• Marriage allowance transfer: referenced in the take-home calculator but not auto-applied; calculate separately if you or your partner are transferring £1,260 of unused allowance.
• Blind person's allowance: not currently supported.
• Pension annual allowance taper: flagged but not auto-calculated for very high earners (£260k+ adjusted income); a dedicated pension calculator is on the roadmap.
• High-income child benefit charge: claws back child benefit between £60k and £80k of adjusted net income; not included in the take-home figure.
• Mid-year residence changes (e.g. moving between Scotland and the rest of the UK), calculations assume a full year in one residence.
• Multiple concurrent contracts: most calculators assume a single income source; combine manually for multi-contract years.
• Day-rate market benchmarks: the site currently doesn't publish role-and-location benchmark figures. A separate aggregation tool is on the roadmap; in the meantime use CWJobs, IT Job Board, and Reed directly for live market ranges.

Mortgage affordability heuristics

The self-employed mortgage calculator uses heuristics, not the precise underwriting any specific lender will apply. Three modes, day-rate contractor, sole trader, Ltd director, match the three most common lender approaches in the UK market. We model the income basis (e.g. day rate × 5 × 46 for the contractor mode) and apply industry-standard income multiples (4× / 4.5× / 5×) to give a borrowing range, not a specific offer. Real lender decisions also factor credit score, deposit, debt-to-income ratio, stress tests, and underwriter judgement, none of which the calculator can know about. Use the output as orientation before talking to a broker, not as a guaranteed offer.

Errors and corrections

If you spot a number that looks off, email hello@contractormaths.co.uk with the URL and what you expected. We respond within 48 hours. Real bugs get fixed within 24 hours of confirmation and the page is republished with an updated Reviewed date.