I built a free, privacy-first Heart Rate Zone Calculator with Vanilla JS

Building a Free, Privacy-First Heart Rate Zone Calculator with Vanilla JS

Most fitness applications require user registrations, log health data to cloud databases, or charge monthly subscriptions just to display basic heart rate training zones.

I wanted a simple, zero-friction alternative—so I built Heart Rate Zone Calculator Pro.

Why I built this:

1. Privacy First: The tool runs 100% on the client side using pure JavaScript. No health data is stored or sent to any server.

2. Accurate Math: It utilizes the Karvonen Formula (Heart Rate Reserve) rather than basic estimations, factoring in Resting Heart Rate (RHR) and Age.

3. Lightweight UI: Optimized for instant mobile and desktop performance with zero bloat.

Check out the live tool here:

https://www.globaltoolsbox.online/2026/09/heart-rate-zone-calculator.html

I'd love to get feedback from the Indie Hackers community! What utility features or health algorithms should I integrate next?