Why this exists
If you've ever Googled "TDEE calculator" or "one rep max formula," you've seen the pattern: a generic tool, no sources, wrapped in ads. Maybe it uses Mifflin-St Jeor. Maybe it uses Harris-Benedict from 1919. You can't tell because nobody bothers to say.
FitnessCalcs started because that's not good enough. Every formula on this site comes from published research. Every calculator shows you which equation it's using and why. If a method has known limitations - like the Navy body fat formula underestimating in lean individuals - we tell you.
What makes this different
Transparent math. We don't hide behind "proprietary algorithms." The Epley formula is the Epley formula. The Katch-McArdle equation needs lean body mass. We show the work so you can judge the output.
Built for people who train. This isn't a wellness app trying to sell you a subscription. It's a reference tool for people who already know what RPE means and want accurate numbers to program with.
No data collection. Every calculation runs in your browser. We don't store your weight, your lifts, or anything else. We use Plausible for privacy-focused analytics - no cookies, no personal identifiers.
The origin story
This site was built during the 2025 NBA Playoffs - specifically during commercial breaks of a Pacers-Knicks Game 6. It's a collaboration between @juddlyon (a developer in Atlanta who was annoyed by bad fitness tools) and Claude Code (Anthropic's AI coding assistant that made it possible to ship 30 calculators before the final buzzer).
The playoff energy wore off. The site kept growing. Now it's 30 calculators and 14 articles, all free, all open about their sources.
Our standards
- Formulas sourced from peer-reviewed research or established sports science
- Limitations and edge cases documented, not hidden
- Context provided for every result - numbers without context are noise
- Updated when better methods emerge
- No paywalls, no sign-ups, no dark patterns
Medical disclaimer
These calculators are educational tools, not medical advice. They provide estimates based on population-level research - your individual results will vary. Consult qualified healthcare professionals before starting any exercise program or making significant dietary changes.