We built a calculator suite, then tested it the way we'd test yours.
FinToolbox is a trading name of Axiom Testing Ltd — 57 free UK financial calculators, built and maintained in-house. It's the clearest example we have of how we work, because we own every decision in it.
npm run build runs the unit suite before it compiles anything. A failing test doesn't produce a build, so it can't produce a deploy.
All four coverage figures, rather than only the flattering one: 92.81% statements, 98.56% lines, 96.9% functions, 85.99% branches. Measured on 14 August 2026 and taken from FinToolbox's published build report, which states its own scope: Coverage of the calculation engine and data registry only. It does not cover the Astro page templates or their browser scripts — those are exercised by the Playwright suite instead, reported separately under tests.e2e.
What it is
FinToolbox is a set of 57 free calculators covering UK tax, VAT, property, savings, business finance and marketing metrics. No sign-up, no paywall, and every calculation runs in the browser — nothing a visitor types is sent anywhere.
It's a real product with real users, which is the point. Testing something we ship and maintain ourselves keeps the standards we sell honest.
Why we're showing it
Financial calculators are unusually unforgiving. A rounding error or a stale tax threshold doesn't crash anything — it quietly returns a wrong number that looks entirely plausible. There's no stack trace to catch it.
That's exactly the class of defect risk-based testing exists to find, so the approach below is the same one we bring to client work.
How it's tested
Tests block the build
The build command runs the full suite first and stops on the first failure. A broken calculation can't reach production because the deploy simply never happens — the gate isn't a policy someone has to remember.
Every calculator is covered by its own tests
522 unit tests across 64 files, covering the 57 calculators and the shared rate tables they all read from. Each covers the ordinary path plus the boundaries that actually bite: band thresholds, zero and negative inputs, and the rounding rules HMRC specifies.
Rates pinned against reversion
A dedicated suite pins each statutory rate and threshold for the tax year, with the GOV.UK page it came from cited alongside it on the calculator itself. Rates last verified 6 April 2026.
A few of the calculators
Representative of the range — each one is backed by its own test suite.
The VAT calculator
Adds or removes VAT at the standard and reduced rates.
Funding comparison
Weighs loan, overdraft and equity options side by side.
Emergency fund planner
Sizes a buffer against real monthly outgoings.
Risk and return
Compares expected return against volatility.
Return on ad spend
Revenue against spend, per campaign or in total.
Social media ROI
Nets campaign cost off the return it generated.
FinToolbox is operated by Axiom Testing Ltd (company no. 17267026) — we built it, we own it, and we maintain it. Our work on it is software testing, not financial advice: the calculators are general information tools and are not reviewed by a qualified financial or tax adviser.
Want this standard on your release?
Tell us what you're shipping and we'll scope the coverage that fits it.