Strapped to the same runner with a chest monitor as referee, both watches landed within 2 bpm. So the choice comes down to battery life, apps and outdoor tools.
A side-by-side run test found the Apple Watch Ultra 4 and Garmin Fenix 9 Pro equally accurate for heart rate. On a hilly 30-minute run, the Garmin averaged 159 bpm and the Apple Watch 160 bpm, against 158 bpm from an ECG chest strap.
With accuracy a tie, the decision comes down to daily use. The Ultra 4 has better software, more apps and tighter iPhone integration, but needs charging every two to three days. The Fenix 9 Pro lasts for weeks and adds a flashlight, offline maps and optional satellite messaging.
Either one is accurate enough for training by heart rate zones.
Put the Apple Watch Ultra 4 on one wrist, the Garmin Fenix 9 Pro on the other and an ECG chest strap around your ribs, then run up a steep hill for half an hour. The result: the two watches finished within 2 beats per minute of the strap, and within a single beat of each other.
If you’re about to spend serious money on a premium sports watch, that’s good news, because it means heart rate accuracy no longer separates these two. It also means your choice should rest on other things: how often you’re willing to charge, whether you live in Apple’s world, and how far from a power outlet you tend to wander.
The test was a 30-minute run on a hilly route with a long climb in the middle, designed to push heart rate up and down quickly, which is where wrist sensors usually struggle. Both watches use optical sensors, which shine LED light into the skin to estimate pulse. As a benchmark, the runner also wore a Frontier X2, an ECG-capable chest strap that reads the heart’s electrical signal directly and is generally more reliable.
The average heart rates were:
A gap under 5 bpm is usually treated as an insignificant margin of error, good enough even for people who train by heart rate zones. Under 10 bpm is fine for casual exercisers watching broad trends. Both watches cleared the stricter bar easily.
Exact second-by-second matches are impossible, because every company processes sensor data with its own algorithms. What matters is that a watch tells the same story as the chest strap over a whole workout, and here all three devices did.
Calorie estimates, which often vary wildly between brands, were also close: 388 on one watch and 375 on the other. The chest strap’s calorie figure was very different, but it has far less data to work with than a watch, so only its heart rate number counted.
Apple redesigned the heart rate hardware for the Ultra 4 and the Series 12. Its new Health Sensing System pairs a new optical sensor with a second-generation electrical heart sensor and takes readings far more often through the day. Apple says it tested the system against an electrical chest monitor, and it points to a study ranking it the best among wearables.
The extra data feeds measures such as heart rate variability, a common stress indicator, and a new Readiness feature. Some of the Ultra 4’s new health features were not switched on at launch and were due later in the year.
This is where the two watches split. Apple’s official figures for the Ultra 4 are 50 hours in everyday use, stretching to 84 hours with Low Power Mode switched on, and 18 hours when tracking an outdoor workout. That’s a big step up for an Apple Watch, and one reviewer found they needed to charge it roughly every third day.
Garmin plays a different game. It rates the Fenix 9 Pro with an AMOLED screen at up to 31 days in smartwatch mode on the 51mm model, and solar versions at up to 57 days. In the head-to-head test, the practical gap was roughly 10 days for the Garmin against about two days for the Apple Watch.
Both are built for serious sport. Each has a dive mode, satellite communication, a titanium case and a screen that reaches 3,000 nits so you can read it in bright sun.
The Apple Watch Ultra 4 is the better smartwatch. Its software, third-party apps and everyday smartwatch features are far ahead, and it pairs tightly with an iPhone.
The Fenix 9 Pro leans into the outdoors. It has a built-in LED flashlight with white, red and green modes, offline maps with faster panning, 64GB of storage, and optional inReach satellite and LTE connectivity for calls, texts and SOS messages without your phone. That service starts at $7.99 a month. It comes in 43mm, 47mm and 51mm sizes, while the Ultra 4 comes in one 49mm size. Garmin lists the Fenix 9 range from $999.99, with the Pro at up to $1,099.99.
For most people who wear a watch every day, run a few times a week and take the odd weekend hike, the Apple Watch Ultra 4 is nicer to live with, especially with an iPhone. You’ll simply need to charge it every couple of days.
If you go on multi-day trips, race long distances or spend time far from a charger, the Fenix 9 Pro is the safer pick. Its battery life alone can decide a week in the backcountry. Since heart rate accuracy is effectively a tie, pick the watch that matches how you live.
Neither has a meaningful edge. On a hilly run, both were within 2 bpm of an ECG chest strap, and within 1 bpm of each other.
The Fenix 9 Pro, by a wide margin. Garmin rates it in weeks, while Apple rates the Ultra 4 at up to 50 hours of normal use.
Yes, Garmin watches pair with iPhones and Android phones, but the Apple Watch offers deeper iPhone integration and far more apps.
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