Apple’s specification sheet for the Apple Watch Ultra 4 lists 11 sensors, and four of them exist for the outdoors: a depth gauge rated for recreational scuba to 40 metres with accuracy of plus or minus 1 metre, a water temperature sensor, an always-on altimeter and a compass. The watch starts at $799 and reaches stores on Friday, September 18.
The heart sensors have taken most of the attention this week, because the new Health Sensing System is the headline change. The environmental sensors are the quieter half of the list, and they are the reason a long day out produces data the phone in your pocket cannot collect on its own.
Key facts: every sensor Apple lists for Ultra 4
| Sensor | What Apple says about it |
|---|---|
| Depth gauge | Recreational scuba to 40 m under EN13319, accurate to plus or minus 1 m |
| Water temperature sensor | Listed, with no range or accuracy figure |
| Always-on altimeter | Listed, with no altitude range or accuracy figure |
| Compass | Listed, with no accuracy figure |
| High-g accelerometer | Listed, with no g range published |
| High dynamic range gyroscope | Listed, with no range published |
| Ambient light sensor | Listed |
| Third-generation electrical heart sensor | Supports the ECG app |
| Optical heart sensor | Heart rate every five seconds, HRV as often as every five minutes |
| Blood Oxygen sensor | Listed, with a Blood Oxygen app |
| Temperature sensor | Wrist temperature sensing |
Set against Apple’s Ultra 3 pages, the outdoor half of that list is unchanged. Apple lists the same 40 metre depth gauge, the same water temperature sensor, the same 100 metre case rating and the same ECG, Blood Oxygen and wrist temperature sensing on last year’s model. What is new for 2026 is the heart sensing hardware and the rate at which it reads, which is why Apple’s own comparisons this week are about frequency rather than a longer list of parts.
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The sensors built for water
The case is rated to 100 metres under ISO 22810:2010, which Apple describes as covering swimming, snorkelling, scuba diving and high-speed water sports. That case rating and the depth gauge are separate claims: the gauge is certified to EN13319 for recreational scuba to 40 metres.
Plus or minus 1 metre is the only accuracy figure Apple attaches to any sensor on the watch. For contrast, the depth gauge on Apple Watch Series 12 is listed to 6 metres, in a case rated to 50 metres.
The water temperature sensor appears on both watches and carries no numbers at all on either. Apple publishes no measurement range, no accuracy and no sampling interval for it.
The altimeter, the compass and the motion sensors
Apple calls the altimeter always-on, which describes when it runs rather than how high it reads. There is no altitude ceiling in the specification sheet, and no accuracy figure, so elevation gain over a long climb is a number the watch reports without a published tolerance.
The high-g accelerometer and the high dynamic range gyroscope are named the same way, without ranges. Positioning is handled separately from the sensor list, by precision dual-frequency GPS working with GPS, Galileo, QZSS and BeiDou, which Apple calls the most accurate on-device GPS in a sports watch without publishing the test behind the claim.
How often the heart sensors read
The Ultra 4 gets the same Health Sensing System as the Series 12. Apple measures heart rate every five seconds and heart rate variability as often as every five minutes, and describes the result as 60 times more frequent background heart rate readings than Ultra 3 and 24 times more frequent HRV readings.
Those rates feed the Readiness app, sleep score, hypertension notifications, sleep apnea notifications and the ECG app, as Apple lists them. Apple labels the Vitals app as a wellness feature rather than a medical one, and how any of this applies to you is a question for a clinician, not a spec sheet.
Accuracy figures Apple publishes, and the ones it does not
| Measurement | Published figure |
|---|---|
| Depth | Plus or minus 1 m, to 40 m |
| Heart rate interval | Every five seconds |
| HRV interval | As often as every five minutes |
| Altitude | Not published |
| Water temperature | Not published |
| Compass heading | Not published |
| GPS position | Not published, only a comparative claim |
| Step count | Not published, only a most accurate claim |
What Apple has not said
- No sampling rate for the altimeter, compass or water temperature sensor.
- No test conditions or comparison set for the GPS accuracy claim.
- No statement on which sensors keep working in Low Power Mode or the workout modes.
- No barometric pressure reading listed separately from the altimeter.
- No sensor is described as a diagnostic instrument, and we do not treat any of them as one.
Orders for the Apple Watch Ultra 4 opened on September 9, and the watch is in stores on Friday, September 18. watchOS 27 arrives on Monday, September 14 as a free update alongside iOS 27, and our event recap covers the rest of the September 9 announcements.
