As a developer who started lifting seriously two years ago, I kept running into the same problem: to understand my body composition, I needed to visit 5 different sites.
One for BMI. Another for BMR. A third for TDEE. A fourth for ideal weight. A Navy body fat calculator on some .edu domain from 2007.
Each one asked for the same inputs — height, weight, age, measurements — and each gave me back a single number with zero context. None of them explained what the number meant or how it related to the others.
So over a weekend, I built bodycalctool.com — a single page that takes your stats once and outputs everything: BMI, BMR (3 formulas), TDEE, ideal weight (5 formulas), and Navy body fat percentage. No signup, no paywall, no tracking beyond basic analytics.
The Stack
Pure HTML/CSS/JS with no framework. All calculations run client-side — your measurements never leave your browser. The PDF report generation uses jsPDF, and the optional email delivery goes through Resend's free tier via a Cloudflare Worker.
The whole thing is hosted on Cloudflare Pages (free tier) and costs about $0/month to run.
Why I Built It
Most health calculators are wrapped in ad-heavy, SEO-optimized pages that make you scroll through 800 words of filler before showing your result. Some lock the "detailed report" behind an email wall. Others show your number but don't explain what to do with it.
I wanted something that gives you the numbers instantly and then gets out of your way.
The tool has grown to include a plateau diagnostic (if the scale is stuck, it checks 7 possible causes), body fat visualizations, and comparison pages that show how it stacks up against MyFitnessPal and Calculator.net.
What I Learned
- BMR formulas disagree by up to 218 calories — Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle give different results from the same inputs. Showing all three side by side solves the "which one do I trust" problem.
- People want context, not just numbers — A BMI of 27.8 means nothing by itself. A BMI of 27.8 with 16% body fat and a BMR of 1,950 tells a completely different story.
- No-signup tools convert better — The free report tool sees high conversion from calculator use to email opt-in because users already see their results before being asked for anything.
Open to feedback. If you've built a health/fitness tool, I'd love to hear what you learned.
