UPDATED · News · 25 May 2026 · MTW Editorial Team
The Apple watchOS 27 heart rate update is the headline change in Apple’s reportedly polish-heavy 2026 watch software release, with Bloomberg’s Mark Gurman confirming the upgrade on 24 May 2026 while flagging that Apple’s planned AI health coach, internally codenamed Mulberry, will not ship at watchOS 27 launch. Apple will unveil watchOS 27 alongside iOS 27 at the WWDC 2026 keynote on 8 June 2026, with general availability scheduled for September 2026.
- Bloomberg Power On newsletter (Mark Gurman) reports a heart-rate accuracy upgrade in watchOS 27.
- AI health coach Mulberry delayed; not at launch, partial rollout possible later in cycle.
- WWDC 2026 keynote: 8 June 2026, 10 AM PT.
- watchOS 27 general availability: mid-September 2026, alongside iOS 27.
- Modular Ultra watch face arrives on non-Ultra Apple Watch models for the first time.
- Apple Watch Series 6 and later supported; Series 5 dropped.
What the Apple watchOS 27 heart rate update changes
The Apple watchOS 27 heart rate update changes how the Apple Watch interprets the photoplethysmography (PPG) signal that the optical heart-rate sensor produces. Apple has not detailed the algorithm changes, but Gurman’s reporting indicates the upgrade focuses on accuracy during high-intensity workouts and during environments with vibration or motion artefacts (cycling, weightlifting, running on uneven terrain). These are the cases where every Apple Watch since the Series 4 has been off by 5 to 12 beats per minute compared with a chest-strap reference, and they are the most-reported accuracy complaints in the Apple Watch user base.
If the watchOS 27 update closes the high-intensity gap, it changes the Apple Watch’s positioning against the Garmin Fenix 8 and the Polar Vantage V3 series for serious athletes. Apple Watch already wins on smart-feature integration, app ecosystem and overall convenience; the heart-rate accuracy gap has been the persistent technical concern that pushed runners and cyclists toward dedicated sport watches. The watchOS 27 upgrade is the right priority. Compare it against the analysis in our Apple Watch Ultra 3 vs Garmin Fenix 8 Pro piece if you are choosing between the two.
Mulberry AI health coach delayed
The Mulberry codename refers to Apple’s planned AI health coach, an Apple Intelligence-powered assistant designed to surface personalized health and fitness guidance based on Apple Watch sensor data. Gurman reports that Mulberry has slipped past the watchOS 27 launch window, although a partial rollout is possible later in the iOS 27 cycle (likely a watchOS 27.2 or 27.3 update in early 2027). Apple’s reasoning is sound: an AI health coach has a higher safety bar than most consumer Apple Intelligence features, because the failure mode is medical. Apple needs Mulberry to clear regulatory review and internal medical-board sign-off before it ships, and that work is not complete.
The Mulberry delay is the right call. AI health coaches that ship hot will produce wrong guidance that real users will follow, and the resulting health consequences are the worst-case outcome for any consumer AI product. Apple’s brand position depends on getting this right the first time. Compare the deliberate pace Apple is taking on Mulberry with the faster but lower-stakes Gemini Spark rollout we walked through in our Gemini Spark setup guide. Both are agentic AI; only one has medical-grade consequences if wrong.
watchOS 27 other rumoured features
| Feature | Status | MTW read |
|---|---|---|
| Heart-rate accuracy upgrade | Confirmed for watchOS 27 | Best practical change; closes the long-standing gap. |
| Modular Ultra watch face | Coming to non-Ultra Watch | Removes Ultra exclusivity; reasonable democratization. |
| Mulberry AI health coach | Delayed past launch | Correct call; medical-grade AI needs to be perfect. |
| Stability / performance polish | The dominant theme | Polish year; do not expect new headline features. |
| watchOS 26 design carry-forward | Yes | The Liquid Glass design language continues. |
The bigger-picture takeaway from the watchOS 27 leak set is that 2026 is a refinement year for the Apple Watch, not a leap year. The Apple Watch Series 11 hardware launched in September 2025 with the most ambitious health hardware Apple has ever shipped (the new blood pressure trend feature, the redesigned Wrist Body Temperature sensor, the upgraded haptics motor), and the 2026 software release is about extracting more value from that hardware. Hardware-led refinements show up in 2027.
Apple Watch supported devices for watchOS 27
The watchOS 27 supported device list has not been formally confirmed, but Gurman reports Apple Watch Series 5 will be dropped while Series 6 and later remain supported. That includes Apple Watch SE 2nd gen, Apple Watch Ultra 1, Apple Watch Ultra 2, Apple Watch Ultra 3 and the Apple Watch Series 11. If you are still on a Series 5 the upgrade window is now closing, and the Series 5 lifecycle ends with watchOS 26. Anyone running Series 4 or older is already past end-of-support.
The five-year window of software support is the Apple Watch’s structural advantage over Wear OS. Google’s Wear OS 7 update we covered last week supports Pixel Watch 1 onwards, which is roughly a three-year window. Samsung’s Galaxy Watch 7 and Watch 8 commit to four years of software support. Apple’s five-year window is the longest in the smartwatch market and the single best argument for buying Apple Watch hardware over any Android alternative. Read our comparison in the Apple Watch Series 11 vs Galaxy Watch 8 head-to-head for the brand-by-brand picture.
What current Apple Watch owners should do
If you own an Apple Watch Series 6 or later, install watchOS 27 on day one for the heart-rate accuracy improvements alone. If you own an Apple Watch Ultra 1, Ultra 2 or Ultra 3, the upgrade is meaningful because the watchOS 27 changes also tweak the Always-On display and the Workout Buddy assistant introduced in watchOS 26. If you own an Apple Watch Series 5, this is the last cycle you will see new features; the upgrade window closes with watchOS 26 and you should budget for a Series 11 or Ultra 3 in the next six months.
If you are considering buying a new Apple Watch right now, wait for the September 2026 release of the Apple Watch Series 12 (rumoured) rather than buying the Series 11 today. The Series 12 launch will coincide with watchOS 27 GA and will include any hardware refinement Apple has been working on alongside Mulberry. The exception is if you want the Apple Watch Ultra 3 specifically, in which case buying now is fine because the Ultra refresh is on a less predictable cycle. Compare current models against our best iPhone UK 2026 picks for the complete Apple ecosystem buying decision.
For UK readers
UK readers: the watchOS 27 heart-rate update will roll out to UK Apple Watch users on the same day as the global release expected at WWDC 2026. The Apple Heart Study UK arm runs through the Imperial College London partnership Apple announced in 2025, so UK AFib detection alerts will continue under MHRA medical-device sign-off. UK Apple Watch users on EE, VodafoneThree and Virgin Media O2 Watch plans will see the update push automatically.
MTW verdict
The Apple watchOS 27 heart rate update is the single most useful change Apple could make to the Apple Watch software in 2026, and the Mulberry delay is the right call. Install watchOS 27 on day one if you own a Series 6 or later; budget for a Series 12 in September 2026 if you are still on a Series 5. WWDC 2026 on 8 June will confirm the full feature list.
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