Why your iPhone heart rate lags your Apple Watch
Your watch face updates your heart rate constantly. Open almost any iPhone app, even Apple's Health app, and the number is often a few minutes old. I measured why while building a heart rate app on an Apple Watch Ultra 4.
What I found
- At rest, the Watch shows heart rate constantly but saves it only every few minutes. In my logs, saved readings were about 5 minutes apart (for example 19:56, then 20:01). Apps can only read saved readings.
- The iPhone only gets those readings when the Watch syncs. Opening or switching back to an app on the iPhone often brings in a new batch, which is why closing and reopening an app sometimes "catches up". Pull-to-refresh inside an app usually doesn't trigger a sync.
- Opening the Heart Rate app on the Watch takes a fresh reading and syncs it right away.
- During a workout, everything changes: readings come every few seconds.
How to see live heart rate on your iPhone
- Start a workout. Workout apps that mirror to the iPhone show your heart rate every few seconds.
- Keep an app open on the Watch. A Watch app that's on screen can send readings straight to its iPhone app.
- Keep that app on screen longer: Watch app → General → Return to Clock → choose the app → Custom → After 1 hour. Otherwise the Watch goes back to the watch face after 2 minutes.
These observations come from my own testing on one Apple Watch Ultra 4 with watchOS 27. Apple controls how often the Watch saves and syncs, and it can differ between models and software versions.
Zone 2 Training Coach
Open it on your Watch and your iPhone shows live heart rate and your current zone, or start a live Zone 2 workout that taps your wrist when you drift out of zone.
