ContractorMaths

Sources & references

Where the numbers come from

Every rate, threshold, band, and rule on this site comes from a primary source. This page lists them all with direct URLs and notes which calculator or guide each one feeds into.

Reviewed 27 April 2026 · See how we calculate for the update process.

How to use this page. If a calculator on the site shows a figure you want to verify against the primary source, find the relevant section below and follow the URL. Every figure on every calculator originates from one of these. Where HMRC publishes a worked example, our test suite checks against it before deployment.

Income tax, England, Wales, Northern Ireland

Income tax, Scotland

Scotland sets its own income tax rates and bands. NIC, dividend tax, and student loan rules remain set by Westminster.

  • Scottish Income Tax

    Scottish Government / mygov.scot

    Used for: Six-band Scottish schedule (Starter / Basic / Intermediate / Higher / Advanced / Top) used in the take-home, umbrella, dividend, and Ltd calculators when residence is set to Scotland.

  • Scottish Budget, Income Tax decisions

    Scottish Government

    Used for: Year-on-year band-cap and rate changes (e.g. Scottish Budget 13 January 2026 changes to the Starter and Basic band caps for 2026/27).

National Insurance

  • National Insurance overview

    GOV.UK / HMRC

    Used for: Class 1 (employee + employer), Class 2, and Class 4 NI rates and thresholds across the take-home, umbrella, and Ltd calculators.

  • National Insurance, HMRC Internal Manual (NIM)

    HMRC

    Used for: Edge-case interpretation: which earnings count, how directors' NI works under the alternative annual basis, and Class 1 above State Pension age.

Dividend tax

  • Tax on dividends

    GOV.UK / HMRC

    Used for: £500 dividend allowance, basic / higher / additional rates, and the rule that Westminster sets the rates regardless of where the recipient lives in the UK.

Corporation tax

  • Corporation Tax rates

    GOV.UK / HMRC

    Used for: Small Profits Rate (19% to £50,000) and Main Rate (25% above £250,000) for the Ltd, salary-dividend-split, and umbrella-vs-limited calculators.

  • Marginal Relief calculator and methodology

    GOV.UK / HMRC

    Used for: The (£250,000 − profit) × 3/200 marginal-relief formula used in our corporation tax computation between £50,000 and £250,000 of profit.

Student loans

  • Repaying your student loan, what you pay

    GOV.UK / Student Loans Company

    Used for: Plan-specific thresholds (Plan 1, 2, 4, 5, Postgrad), 9% / 6% rates, and the multi-plan stacking rule used across every calculator that includes student loan deductions.

VAT

IR35 and off-payroll working

Pensions

  • Pension Tax Manual, Annual Allowance

    HMRC

    Used for: £60,000 annual allowance, the tapered allowance for high earners (£260k+ adjusted income), and the Money Purchase Annual Allowance, referenced in the umbrella SS pension and Ltd-employer-pension copy.

  • Workplace pensions and tax relief

    GOV.UK / The Pensions Regulator

    Used for: Auto-enrolment thresholds and the Net Pay vs Relief at Source distinction explained in the take-home pay calculator's pension copy.

Companies House

  • Set up a private limited company

    Companies House / GOV.UK

    Used for: Incorporation cost (£50), same-day approval, and the Companies House process referenced in /guide/setting-up-limited-company.

  • Annual confirmation statement

    Companies House / GOV.UK

    Used for: The £13 annual filing referenced in the Ltd setup guide and the common-pitfalls section.

  • Companies House size thresholds (small / medium / large)

    Companies House / GOV.UK

    Used for: The small-client thresholds for the off-payroll exemption, both pre-April-2025 (£10.2m / £5.1m / 50) and post-April-2025 (£15m / £7.5m / 50) figures.

Mortgage affordability, industry conventions

The self-employed mortgage calculator uses lender heuristics rather than primary-source publications, these aren't published rates, they're industry conventions assembled from FCA-registered specialist contractor-mortgage brokers. We're honest about this on the calculator page itself.

Insurance

What we don't cite

Other calculator sites, accountancy blogs, and contractor forums. They're useful for sanity-checking but we've found enough disagreement and out-of-date content among secondary sources to make primary-only the stricter standard. If you spot a number on this site that you think traces back to a secondary source, email hello@contractormaths.co.uk and we'll either find the primary source or remove the figure.

Reviewed: 27 April 2026 · See how we calculate · glossary · not financial advice.