I open-sourced a comprehensive fitness calculator library for TypeScript

Vedad Burgic — Sun, 01 Feb 2026 15:42:28 +0000

Every fitness app needs the same calculations — BMI, TDEE, one-rep max, macros, heart rate zones. But the npm ecosystem for fitness math is surprisingly bad. Most packages are abandoned, incomplete, or lack TypeScript support.

So we built @finegym/fitness-calc and open-sourced it.

What's included:

Module	Details
BMI	WHO classification, healthy weight ranges, metric + imperial
BMR	3 formulas (Mifflin-St Jeor, Harris-Benedict, Katch-McArdle)
TDEE	6 activity levels from sedentary to extra active
1RM	7 formulas (Epley, Brzycki, Lombardi, Mayhew, O'Conner, Wathan, Lander)
Macros	6 diet presets + custom ratios, maintenance/cutting/bulking
Body Fat	US Navy method + BMI-derived estimation
Heart Rate	Standard + Karvonen models, 5 training zones
Calories	30+ activities with MET values
Pace	Running pace, speed, race time prediction (Riegel formula)
Ideal Weight	4 formulas (Robinson, Miller, Devine, Hamwi)
Water Intake	Activity-adjusted daily recommendations
Conversions	Weight, height, distance, temperature

Quick example:

import { calculateOneRepMax, calculateAllFormulas } from '@finegym/fitness-calc';

// Bench press: 100kg for 5 reps
const result = calculateOneRepMax(100, 5);
// { oneRepMax: 116.7, formula: 'epley', percentages: { 100: 116.7, 95: 110.9, ... } }

// Compare all 7 formulas
const all = calculateAllFormulas(100, 5);
// { epley: 116.7, brzycki: 112.5, lombardi: 117.5, ... }

Technical specs:

Zero dependencies
~20KB bundle
Dual CJS/ESM
Full TypeScript declarations
85 tests
CI on Node 18/20/22

Install:

npm install @finegym/fitness-calc

All formulas are backed by peer-reviewed research (Mifflin 1990, Tanaka 2001, Epley, Brzycki, etc.). Every input is validated with descriptive error messages.

We built this at FineGym (https://www.finegym.io) while building our gym management platform. Rather than keeping it internal, we decided the fitness developer community would benefit from having a single, well-maintained library.

Links:

MIT licensed. PRs welcome — especially for new formulas, additional activities, and sport-specific calculations.

DEV Community: Vedad Burgic

I open-sourced a comprehensive fitness calculator library for TypeScript