How content gets written, reviewed, and updated

Editorial mission

Most online UK contractor calculators are out of date, quietly biased toward whichever umbrella or accountant runs them, or hidden behind a signup wall that converts the calculator into a lead-generation form. The numbers often reflect last year's rates, silently assume 250 billable working days a year (an unrealistic 14% over- estimate), or skip the personal-allowance taper above £100,000 entirely.

ContractorMaths exists because the maths a UK contractor actually needs, to choose between umbrella and limited company, set the optimal director salary, figure out borrowing capacity, or understand what an IR35 determination would do to take-home, should be free, accurate, and fully shown. Not paywalled. Not approximated. Not biased toward whoever pays the affiliate fee.

This page exists so the editorial process behind the site is auditable. If any of the standards below fail to be observed in any specific piece of content, that's a bug, please email hello@contractormaths.co.uk and we'll investigate.

Three core rules

1. Primary sources only.Every rate, threshold, and band traces back to a primary publisher , gov.uk, HMRC publications, mygov.scot, the Scottish Government, Companies House, the FCA. We don't paraphrase from other contractor calculator sites or accountancy blogs. If a figure can't be cited to a primary source, it doesn't go on the site. The sources page lists every primary URL we cite, organised by topic.
2. Show the working.Every calculator result expands into a band-by-band breakdown. Where assumptions are non-obvious (220 working days, 7.5 hours per day, 20% benefits uplift, £12,570 director salary), they're named explicitly and the user can override them. Worked example pages walk through every line of the maths in plain English with the actual numbers.
3. Disclose the limits.Every calculator has an “Edge cases” section spelling out what it doesn't model. Every guide and blog post names what it's not. None of the calculators or long-form content on this site are regulated financial advice; for specific decisions, see a qualified accountant or financial adviser. The disclaimer covers the site-wide limits.

How a calculator gets built

Every calculator on the site goes through the same five stages before it ships:

1. Sourcing. The applicable rates, thresholds, and bands come from a primary publisher. Income tax rates from gov.uk/income-tax-rates; National Insurance from gov.uk/national-insurance; Scottish income tax from mygov.scot; corporation tax, dividend tax, student loans, VAT FRS, all from gov.uk and HMRC publications. Each source is recorded on the sources page with a direct URL.
2. Implementation. The maths is written as a pure TypeScript function in /lib/tax/ with no UI dependencies, the same function powers the calculator, the unit tests, and any other tool that needs the same computation. Pure functions are easier to test and reason about than calculators with embedded UI logic.
3. Testing. A unit-test suite verifies the output against every HMRC worked example we can find for that calculation. For income tax: HMRC textbook values at £40k / £60k / £105k / £125k / £150k / £200k. For dividend tax: HMRC examples at the band boundaries. For corporation tax: the marginal-relief examples HMRC publishes alongside their online calculator. For Class 1 NI: the rates-and-thresholds-for-employers tables. The test suite is the gate that prevents bad numbers from shipping; the current count is 269 tests across the calculation library, run on every change before deployment.
4. UI. The calculator wraps the underlying function with input fields, sensible defaults, and a fully-itemised result breakdown. Every figure on the result side traces back to one input or one rule, no hidden assumptions. The UI is reviewed for accessibility (label associations, tabular numerics, keyboard navigation) before shipping.
5. Supporting copy.Each calculator page includes a worked example, an “Edge cases” section spelling out what it doesn't model, and 7-8 FAQs covering the questions we've seen contractors actually ask. The point is to give context, not pad the page for SEO.

How long-form content gets written

Guides, comparisons, blog posts, and worked examples follow a similar but less-automated process:

• Topic selection.We write about questions we see contractors actually ask, IR35 status, umbrella mechanics, Ltd setup, Budget changes, the £100k taper trap, mortgage borrowing for the self-employed. We don't write to keyword targets that don't map to a real reader question.
• Sourcing. Same primary-sources rule as calculators. Long-form content links to the authoritative source for each rate, regulation, or precedent it cites. Where we describe HMRC enforcement precedent (the 2017 Loan Charge, the 2023 Pimlico Plumbers ruling, the 2024 off-set provisions for off-payroll), we link to the relevant HMRC publication or court ruling.
• Drafting.First-person voice is avoided in favour of a publisher-as-entity tone — “the calculator”, “our editorial team”, “ContractorMaths”. This isn't about hiding authorship; it's about not making individual-identity claims that aren't relevant to the reader's question.
• Fact-check before publish. Every specific figure (rates, thresholds, dates, monetary amounts) is cross-checked against the primary source cited. Where two primary sources publish differently — gov.uk vs an HMRC manual, or two different HMRC publications, we go to the most recent and most specific.
• Reviewed-date footer.Every published page has a Reviewed date. Updates trigger a date refresh. We don't back-date or pre-date.

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, 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 year 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).
• Reader-reported issues: within 24 hours of confirmation. If you spot a number that looks off, email us; we'll investigate the same day and patch the calculator within 24 hours of confirming the bug.

See the methodology page for the full cadence detail and the cross-site list of things the calculators don't model.

How errors get corrected

We make the channel for error reports deliberately short, there's no separate “corrections form” or ticketing system, just an email address on every page. The process when a report comes in:

1. Acknowledge within 48 hours. If the report is comprehensive (URL, expected number, source) we respond same-day.
2. Investigate. Reproduce the issue, trace it to the underlying calculation, check against the primary source.
3. If it's a real bug: fix the underlying function, add a regression test, deploy. Most fixes ship within 24 hours. Update the page's Reviewed date.
4. If the report turns out to be expected behaviour (e.g. an edge case we don't model, deliberately): reply with the explanation, and consider whether we should add the case to the page's “Edge cases” section to prevent the same question recurring.

Every fix that materially changes a calculator's output is noted in the page's commit history. The site's code is private but every visible numeric change is also visible to anyone running the site against an older version.

Independence from advertisers

ContractorMaths runs on a mix of display ads (kept light, never interstitial) and contextual affiliate partnerships with accountants, umbrella companies, contractor mortgage brokers, and IR35 insurance providers.

Three rules around this:

• Affiliate links are clearly labelled. Every affiliate link carries a “Partner link · paid recommendation” tag. You'll never click an organic-looking link and find an affiliate redirect.
• Affiliates never pay for placement order or favourable wording. If a comparison page calls out an option that has no affiliate relationship, the ordering reflects the maths, not the money. If a calculator surfaces an answer that contradicts what an affiliate would prefer, the calculator wins.
• Editorial decisions are independent of ad network membership.We don't suppress content that might fail an ad-network policy review (e.g. critical pieces about specific tax-avoidance schemes), and we don't add content to please ad networks. The mini-umbrella warning piece was written before any ad-network application; it would still be there if a network rejected it.

See the funding section of /about for the current state of ad-network and affiliate relationships. As of 28 April 2026, no ad networks have been applied to and no affiliate links are live; the site is currently funded by the operator's own time. When that changes, this page and the funding section will be updated to reflect new relationships.

What we're not

ContractorMaths produces estimates from public rates. It is not regulated financial advice. It is not authorised or regulated by the Financial Conduct Authority. It does not provide investment advice, mortgage advice, or tax advice within the meaning of the Financial Services and Markets Act 2000.

For decisions involving your specific circumstances — especially anything mortgage, pension, IR35-determination, or tax-residency related, speak to a qualified accountant or financial adviser. The full disclaimer is at /disclaimer.