Does Cinder work without an Apple Watch?

Partly. Workout logging and GPS run tracking work with just an iPhone. The recovery score needs HRV, resting-heart-rate, and sleep data in Apple Health, which in practice means sleeping with an Apple Watch, or any other device that writes those metrics to HealthKit. With no HRV data for a given day, Cinder shows no score rather than guessing.

How is this different from Whoop?

Whoop pairs its analysis with proprietary hardware worn around the clock and a $199 to $359 yearly subscription, and its scoring model is closed. Cinder computes comparable recovery and strain signals from the Apple Health data your watch already collects, costs $17.99 a year or $30 once, and publishes exactly how its numbers are calculated. What it does not offer is Whoop’s dedicated strap-grade continuous measurement. Cinder’s ceiling is the data your existing devices produce.

What happens before Cinder has enough data about me?

The app shows “Calibrating” until it has at least 4 days of baseline history, and its baselines keep refining up to a trailing 30-day window. Sub-scores with insufficient history, like resting HR in week one, default to neutral rather than skewing the result.