I got tired of searching "gas mark 4 to celsius" and landing on ad-heavy US sites that had never heard of a British oven. So I built the site I wanted to use myself.

Here's how it went, what I learned, and why the stack matters.

The problem
Most conversion sites are US-centric and ad-riddled. They don't cover:

Gas Mark — the temperature scale on every British cooker
UK shoe sizes — totally different from US
Multi-band VAT — standard, reduced, zero rates per country
UK gallons — not the same as US gallons
And the ones that do cover some of these are buried in SEO spam with 5 ad placements per page.

The stack
Layer Choice Why
Framework Astro 5 Ships zero JS by default. Every tool page is static HTML — instant load, perfect Lighthouse scores
Components React 19 Only where interactivity matters (calculators, inputs). Astro islands keep the rest static
Styling Tailwind 4 Utility-first, no context-switching to CSS files, tiny bundle
Hosting Cloudflare Pages Free tier, global CDN, git-push deploy
Currency API frankfurter.dev Free, open ECB data. No API key needed
Feedback Formspree Form backend without writing a single server route
Analytics GA4 + Consent Mode v2 Default denied, opt-in only. No cookies without consent
Architecture: data-driven static pages
Every tool follows the same pattern:

src/data/tools.ts → tool definitions (name, category, route, icon)
src/pages/tools/[id].astro → one template, renders all tools
src/components/*.tsx → React islands for interactive bits
Adding a new tool is literally adding one entry to an array. No new routes, no new templates. The [id].astro page reads the slug and renders the right component.

Privacy as a feature, not an afterthought
This was non-negotiable for me:

All calculations run in the browser. No input ever touches a server.
Consent Mode v2 — analytics default to denied. Users opt in, not out.
No sign-ups, no accounts, no tracking pixels.
GDPR-friendly by design, not by cookie banner hack.
The footer says it plainly: "All tools run in your browser — no data uploaded."

What I'd do differently
Start with GSC data before picking tools. I built 9 tools based on my own needs. Two months of Search Console data would have told me exactly which UK conversion keywords have demand vs. which ones are dead.
Ship with 3 tools first, then expand. 9 tools upfront meant more surface area for bugs. Launching leaner would have been faster.
Pinterest earlier. The Gas Mark conversion chart has real "save and reference later" potential. I should have made those pins day one.
Numbers (after week 1)
~20-50 daily visitors (organic, zero paid)
~12 GSC impressions/day and climbing
Product Hunt launch pending
$0 revenue — it's a free tool. Maybe AdSense later, maybe not.
The site
👉 ukconvert.com

All static, all free, all private. Source on GitHub.

If you're building a similar tool site, happy to answer questions in the comments.