The Garmin Fenix 8 and Apple Watch Ultra 2 are two of the best sports watches available, and two of my favorite devices, having worn them both for long stretches to track my marathon training in particular.
I ran the London Marathon 2025 wearing both watches, clocking 2:26:48. Both impressed me for GPS accuracy in particular, helping me to pace the race in the first half — the fact I went out too quickly and had to slow down was certainly my fault, not the watches!
Both watches are at the more expensive end of the sports watch spectrum, but the Apple Watch Ultra 2 is cheaper at $799 compared with $999 for the cheapest Fenix 8 model — the 47mm sapphire model I wore retails at $1,099.
Given the price and impressive specs on both watches, you expect a high level of performance. How did they perform at the London Marathon?
GPS Accuracy
The official marathon distance is 42.195km, but you usually have to run further than this distance in the race itself as you dodge around people and take a wider route around corners than the measured route.
That said, I was (mostly) able to stick to the blue lines on the road marking the fastest route around London on race day, and the Apple Watch’s overall distance of 42.26km is impressively close to the official distance.
The Garmin Fenix 8 measured 42.47km, which is still very good, and the GPS tracks of both watches are nearly identical.
That is aside from around Canary Wharf, which is where watches tend to go wrong in the London Marathon. Both watches were still pretty good here, but the Garmin had a slightly more accurate GPS track.
Overall I could pace the race using either watch and the lap alerts were closely aligned with the markers on the course, such was the high level of accuracy they offered thanks to the dual-band GPS available on both.
Winner: Apple Watch Ultra 2
HR accuracy
I didn’t wear a chest strap heart rate monitor in the race, which I normally do during training for the most accurate heart rate possible. But I have worn chest straps in past marathons and know what my heart graph should roughly look like during a long race.
I’d expect to see a steady rise throughout the race, as the effort gets harder and harder, and that’s exactly what the Fenix 8 produced on the day, rising sharply to near my max heart rate as I tried to sprint down the home straight.
In contrast, the Apple Watch Ultra 2 struggled to lock onto my heart rate in the first half of the race, mostly showing no data and reading far too high when it did.
After around an hour, however, it did start to read the heart rate correctly and mostly matched up closely with the Garmin for the remainder of the race.
Winner: Garmin Fenix 8
Battery Life
I charged both watches overnight, so they were at 100% at the start of the day. I had the always-on screen enabled and put both in the most accurate and power-intensive GPS mode to track the run.
At 6 pm on Sunday, the Garmin Fenix 8 was at 84% and the Apple Watch Ultra 2 was at 67%, so the Garmin certainly wins the day on battery life.
I’d expect to be able to use the Fenix 8 for another week without charging, partly because I’ll be a lot less active than usual as I recover after the marathon.
The Apple Watch Ultra 2 would still easily last me two days while including a marathon, which is impressive for a smartwatch with such a large and bright screen. However, battery life is still an area where sports watches like the Fenix 8 have the edge.
Winner: Garmin Fenix 8
Verdict
Aside from the Apple Watch Ultra 2’s dodgy first hour of heart rate tracking, I’d say that both watches did an excellent job at the London Marathon, and their GPS accuracy particularly impressed me on a tricky course.
The Garmin Fenix 8 is my main sports watch and will remain so, partly because of its battery life. But I also enjoy the wealth of analysis it offers when training for a marathon.
Come race day itself, however, I’d be delighted to rely on either of these watches to help me get round the course.
More from Tom's Guide
- I ran a marathon with the Garmin Fenix 8 vs. Garmin Forerunner 965 — here’s the winner
- I'm running the London Marathon — here are the shoes, watches and fueling products I’m using
- 8 hidden Garmin watch features that you need to know about
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