Honest contractor maths.

Why this site exists

Most online UK contractor calculators are either out of date, quietly biased toward whichever umbrella or accountancy runs them, or hidden behind a signup wall that converts the calculator into a lead-generation form. The numbers themselves often reflect last year's rates, quietly assume you bill 250 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, to set the optimal director salary, to figure out how much they could borrow, to understand what an IR35 determination would do to their take-home, should be free, accurate, and fully shown. Not paywalled. Not approximated. Not biased toward whoever pays the affiliate fee.

What this site is

• Free, accurate UK contractor calculators. Take-home pay, IR35 status, umbrella vs limited, dividend tax, salary-vs-dividend split optimiser, day-rate ↔ salary, hourly ↔ day rate, self-employed mortgage. No signup wall.
• Long-form guides on the messy stuff. Plain-English explainers on how umbrella companies actually work, what IR35 really determines, how to set up a Ltd company end-to-end, and a first-time contractor “start here” hub.
• Tested against HMRC published examples. Every calculator has a unit test suite that verifies outputs against HMRC's own worked examples; 269 tests run on every change before deployment. A wrong number gets caught in CI before it ships.
• Honest about limits.Every calculator has an “Edge cases” section spelling out what it doesn't model. The methodology page lists the cross-site gaps.
• Transparent about sourcing. Every rate, threshold, and band traces back to a primary publisher (gov.uk, HMRC publications, mygov.scot, Companies House, FCA). The sources page lists each source with a direct URL.

How a calculator gets built

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

1. Sourcing. The applicable rates, thresholds, and bands come from a primary publisher — gov.uk for the UK-wide rules, mygov.scot for Scottish income tax, HMRC manuals for edge cases. Each source is recorded on the sources page.
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 tests, and any other tool that needs the same computation.
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: HMRC's rates-and-thresholds-for-employers tables. The test suite is the gate that prevents bad numbers from shipping.
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.
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 to pad the page for SEO.

Editorial standards

Three rules the site holds itself to:

• Primary sources only.We don't paraphrase from other contractor calculator sites or accountancy blogs. If a figure can't be traced to gov.uk, HMRC, mygov.scot, Companies House, or another primary publisher, it doesn't go on the site. The sources page lists every URL.
• Show the working.Every result expands into a band-by-band breakdown. Where assumptions are non-obvious (220 working days, 7.5 hours per day, 20% benefits uplift), they're named explicitly and the user can override them.
• Disclose the limits.Every calculator names what it doesn't model. Every long-form guide names what it's not, none of the calculators or guides on this site are regulated financial advice; for specific decisions, see a qualified accountant.

How the site is funded

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. Every affiliate link is clearly labelled with a “Partner link · paid recommendation” tag, you'll never click an organic-looking link and find an affiliate redirect.

Affiliates pay only if you sign up to a partner. They never pay for placement order or favourable wording. If a comparison page calls out an option that has no affiliate relationship, that ordering reflects the maths, not the money.

The site has not yet applied for ad-network membership; display ads will appear after AdSense or Mediavine approval. Until then, the only revenue is affiliate commissions on the partner links you choose to click.

What we don't do

• Regulated financial advice.Calculators produce estimates from public rates. They aren't a substitute for a qualified accountant or financial adviser when your situation is non-obvious. See the disclaimer.
• Newsletter or email capture.No signup wall, no email capture, no “join 10,000 contractors” popups. If you want to email us about something specific you found, the address is hello@contractormaths.co.uk.
• Tracking individuals. Analytics is Cloudflare Web Analytics, no cookies, no fingerprinting, no individual visitor tracking. See the privacy policy.

Get in touch

Spotted a wrong number? A missing edge case? A calculator you wish existed? A glossary entry that should be there? Email hello@contractormaths.co.uk , replies within 48 hours.