Compare Apple Watch or Garmin Fenix

Apple Watch + Stamio, or Garmin Fenix 8?

Runners ask it all the time: keep the Apple Watch Ultra for everyday life, or switch to a Fenix to train seriously? I asked myself for five months, with both watches on my wrist. Here is what I found, with the numbers.

Facts and prices checked on October 29, 2026

Apple Watch + StamioYour everyday watch, with the math of a running watch.Ultra 3 from $799 · Stamio free, Pro $49.99 a year

Garmin Fenix 8The benchmark sports watch, weeks of battery.From $999.99 · Fenix 8 Pro from $1,199.99

The deciding question

Do you want one watch for the whole day, reachable and open to apps, or the longest possible battery life, even if it means wearing a separate sports watch?

Criterion	Apple Watch Ultra 3 + Stamio	Garmin Fenix 8
Watch price	$799 (Ultra 3). Already have an Apple Watch Series 6 or newer? Stamio runs on it: $0 of watch.	$999.99 (Fenix 8), $1,199.99 (Fenix 8 Pro).
Battery	Partly 42 h claimed, 72 h in Low Power Mode. Running, on my Ultra 3: 6 to 9% an hour with map and voice.	Yes Up to 27 to 29 days as a smartwatch, depending on the model.
Morning readiness, training status, load	Yes Calculated by Stamio from the watch's measurements.	Yes Training Readiness, Training Status, acute load.
Overnight HRV, energy, sleep score	Yes HRV status, Energy, sleep score. Stages: the Apple Watch's, the closest to the sleep lab among six watches in a 2025 study.	Yes HRV status, Body Battery, sleep score. In the same study, the Garmin tested (Vivosmart 4, older than the Fenix 8) counted 44 minutes too much deep sleep.
VO2 max and race predictions	Yes Calculated by Stamio: 0.28 average gap with my Fenix 8 over 109 runs.	Yes VO2 max and race predictor.
A plan that follows your fitness	Yes From your first 5K to the marathon, adjusted every Monday from your readiness and your actual runs.	Yes Garmin Coach and daily suggested workouts, free.
Offline maps and navigation	Yes OpenStreetMap areas downloaded to the watch, turns announced by voice, even during intervals.	Yes Preloaded topo maps, turn-by-turn navigation.
Leave the phone, stay reachable	Yes Cellular on every Ultra (carrier plan): calls, messages, music.	Partly Fenix 8 Pro: LTE and satellite with a Garmin subscription. Fenix 8: no.
Safety	Yes Emergency SOS, fall detection. Ultra 3: Emergency SOS via satellite where available.	Partly Incident detection with the phone. Fenix 8 Pro: satellite SOS with a subscription.
In the rain	Partly Stamio locks the screen; the Action button and double tap keep working.	Yes Physical buttons, touchscreen can be turned off.
Everyday watch	Yes App Store, Apple Pay, Siri, message replies.	Partly Garmin Pay, Connect IQ apps, notifications.
Subscription	Daily metrics free. Pro: $49.99 a year (goal plans, maps, full analysis).	Metrics free. Optional Garmin Connect+: $6.99 a month or $69.99 a year.
Phone	iPhone only.	Yes iPhone and Android.

Facts and prices checked on October 29, 2026. Sources ↓

What it changes day to day

01
One watch, from wake-up to the finish line
Before Stamio, I ran with both: the Fenix for the numbers, the Apple Watch for everything else. Now the same watch wakes me up, tells me if I can push, guides my intervals and takes my calls.
02
Your landmarks, on the Apple Watch
Readiness, training status, load, HRV status, energy, VO2 max, threshold, race predictions: the numbers that keep people on a Fenix. Stamio calculates them its own way, from the Apple Watch's sensors, and I compare them to my Fenix 8 every day.
03
Checked with the same strap
Since October 22, an HRM 600 chest strap sends the same heartbeats to both watches. Same average heart rate to the beat, VO2 max within 0.1 to 0.3: what's left is a calculation gap, published as is.
04
The hardware was already there
Dual-frequency GPS, cellular, satellite SOS on the Ultra 3: the watch that can call for help didn't have the math of a running watch. Only the software was missing.
05
The same HRV, give or take the night
Stamio calculates HRV from heartbeats, like a sports watch: 2.9 ms median gap with my Fenix on well-measured nights, less than it changes from one night to the next. The rest comes partly from what each watch calls “the night”.
06
Sleep stages closer to the sleep lab
In a 2025 study of six watches, the Apple Watch agreed best with a sleep lab recording. Stamio keeps its stages for your sleep score and readiness, instead of imitating the Fenix, which on my nights counts almost an hour more deep sleep.

Where the Fenix is better, honestly

Battery: weeks versus two days. For an ultra over 15 hours or several days without a charger, get the Fenix.
Buttons: the Fenix works entirely without touching the screen. Stamio narrows the gap in the rain, it doesn't close it.
Preloaded topo maps with relief. Stamio's maps download by area, without relief for now.
Multisport: swimming, triathlon, skiing. Stamio covers running, trail, walking, hiking, cycling and strength.
Android, and years of math refined on millions of athletes. Stamio is checked on one runner, me, until the beta.

Choose the Fenix 8 if

you run ultras or outings longer than a day;
you want to charge your watch every two or three weeks;
you swim or do triathlon;
you want buttons for everything, or you're on Android.

Choose Apple Watch + Stamio if

you want one watch, every day and for running;
you want to leave your phone and stay reachable;
you already have an Apple Watch: nothing to buy;
you run from 5K to the marathon and want a plan that follows your fitness, with the map in your workout.

Questions

Does Stamio use Garmin's algorithms?

No. Stamio has its own calculations, based on published research and tuned on Apple Watch measurements. The Fenix is only a point of comparison. Garmin is not affiliated with Stamio.

Do I need an Apple Watch Ultra?

No. Stamio works from Apple Watch Series 6, SE (2nd generation) and every Ultra. The Ultra has the biggest battery and the Action button, both handy for running.

Will the battery last a marathon?

Yes. On my Ultra 3, 4 hours with map and voice take a quarter to a third of the battery. A test published by Tom's Guide (October 2025) measured 24 points for the Ultra 3 and 22% for a Fenix 8 Pro over a marathon.

Can I keep my Fenix and add Stamio?

Yes. By default, Stamio only counts Apple Watch workouts; a setting also counts those from other apps that reach Health.

Why isn't my overnight HRV exactly my Garmin's?

Because each watch decides on its own when you fell asleep and woke up, and they don't measure at the same pace: the Fenix continuously, the Apple Watch in one-minute series. On my nights, when both agree within 15 minutes, the average gap is 3.2 ms; otherwise, 4.2 ms. Details in “How I checked”.

Are the Apple Watch's sleep stages reliable?

No watch replaces a sleep lab. But in a 2025 study comparing six watches with polysomnography in 62 adults, the Apple Watch Series 8 agreed best (kappa 0.53, vs 0.21 for the Garmin tested, a Vivosmart 4). It underestimated deep sleep by 25 minutes; the Garmin overestimated it by 44.

Isn't the Apple Watch's VO2 max enough?

The one in the Health app is often underestimated for runners: by 6 ml/kg/min on average in a study published in 2025. Stamio calculates its own. Details in “How I checked”.

Sources

Prices and features checked on October 29, 2026 on the pages cited; they may change. Spotted a mistake? Email contact@stamio.app and I'll fix it.

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Other comparisons

Estimates for wellness and training, not a medical device.

What it changes day to day

One watch, from wake-up to the finish line

Your landmarks, on the Apple Watch

Checked with the same strap

The hardware was already there