Most online calculators bury a 20-line formula under a megabyte of ad scripts and a 2,000-word SEO essay. I wanted the opposite, so I built RaphaAtlas (https://raphaatlas.com) — a hub of health calculators that are self-contained, framework-free, and load instantly.
Why vanilla JS
Every calculator is a single HTML file with no external dependencies. No React, no build step, no CDN calls. A macro calculator doesn't need 300KB of framework to run three arithmetic operations. The payoff is sub-second loads and pages that work even with JavaScript half-broken.
The interesting math
Two examples that were more nuanced than expected:
BMR for the macro calculator. I implemented dual formulas — Mifflin-St Jeor and Katch-McArdle — because they diverge sharply once body-fat percentage is known. Mifflin uses weight, height, age, sex; Katch-McArdle uses lean body mass. For lean, muscular users the difference in daily calories is large enough to matter, so the tool switches based on available inputs.
The Widmark equation for BAC. Blood alcohol estimation looks simple — BAC = (alcohol grams / (body weight × r)) − (β × hours) — but the distribution ratio r and elimination rate β vary by sex and individual, and the standard-drink definition changes by country. I built a jurisdiction selector so the "legal limit" reference and standard-drink grams adjust per region, and I front-loaded the safety framing hard: an estimate is never permission to drive.
What I'd tell anyone building calculator tools
- Show the formula. Users trust a number more when they can see how it's derived, and it's a genuine differentiator against black-box competitors.
- Structured data pays off — JSON-LD on the tool pages makes them eligible for richer search treatment.
- Ship the explanation as progressive disclosure: calculator first, "how this works" collapsible below. Don't make people scroll past an essay to reach the tool.
RaphaAtlas is early and I'm adding calculators regularly. If you've built utility tools, I'm curious how you handle the tension between SEO content depth and keeping the actual tool front-and-center.
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