Where they actually differ
The most decisive specs, ranked by how much they matter — before the full spec table.
| Spec | Garmin - Fenix 8 (47mm) | OnePlus Watch 3 |
|---|---|---|
| Battery life | 384 h | 120 h |
| Peak brightness | 1000 nits | 2200 nits |
| Weight | 73 g | 81 g |
| Display size | 1.4 " | 1.5 " |
| Water resistance | 10 ATM | 5 ATM |
Between the Fenix 8 (47mm) and OnePlus Watch 3, the Fenix 8 (47mm) comes out ahead in battery life / weight, while the OnePlus Watch 3 wins on peak brightness / display size. Overall, the Fenix 8 (47mm) scores 71.9 and the OnePlus Watch 3 scores 65.7 on our Mars Score.
Which should you buy?
The Fenix 8 (47mm) edges ahead on the Mars Score (71.9 vs 65.7), but the 6.2-point gap is small enough that the right pick comes down to which spec advantages matter to you.
Pick the Fenix 8 (47mm) if your top frustration with your current device is running out of charge before the day ends — endurance is the lead spec here. The biggest gaps in its favor are battery life, weight, water resistance. The single most decisive one: Battery life — 220.00% more. 384 h vs 120 h.
Pick the OnePlus Watch 3 if the display is what you stare at all day — refresh rate, brightness, or sharpness is where the difference will be most visible. Its strongest claims are peak brightness, display size. The single most decisive one: Peak brightness — 120.00% more. 2200 nits vs 1000 nits.
Both ship with comparable ecg, cellular, spo2, so those specs do not separate the two — focus on the differences below.
Bottom line: the Fenix 8 (47mm) is the safer default. The other model is the right choice only when its specific advantages line up with your priorities.
Which should you buy: Garmin - Fenix 8 (47mm) or OnePlus Watch 3?
Garmin - Fenix 8 (47mm) leads the current Mars Score by 6.2 points.The more useful decision is the trade-off behind that result: compare display size, battery life against the way you will actually use the product.
Choose Garmin - Fenix 8 (47mm) if…
Garmin - Fenix 8 (47mm) has the measurable edge on battery life h, weight g, water resistance atm.
Choose OnePlus Watch 3 if…
OnePlus Watch 3 has the measurable edge on peak brightness nits, display size in.
This is a structured comparison of the published catalog values. It is not a claim of identical real-world performance across every workload; use the linked spec drill-downs and methodology page to inspect the inputs.
Why Garmin - Fenix 8 (47mm) wins
- ▲Battery life — 220.00% more. 384 h vs 120 h.
- ▲Weight — 8.00 g less. 73 vs 81.
- ▲Water resistance — 5.00 ATM more. 10 vs 5.
Why OnePlus Watch 3 wins
- ▲Peak brightness — 120.00% more. 2200 nits vs 1000 nits.
- ▲Display size — 7.14% more. 1.5 " vs 1.4 ".
Spec comparison
Design
| Spec | Garmin - Fenix 8 (47mm) | OnePlus Watch 3 |
|---|---|---|
| Case size | 47 mmlead | 46.5 mm |
| Weight | 73 glead | 81 g |
| Case material | fiber-reinforced polymer | stainless steel |
| Water resistanceⓘ | 10 ATMlead | 5 ATM |
Display
| Spec | Garmin - Fenix 8 (47mm) | OnePlus Watch 3 |
|---|---|---|
| Display size | 1.4 " | 1.5 "lead |
| Always-on display | true | true |
| Peak brightnessⓘ | 1000 nits | 2200 nitslead |
Health & Fitness
| Spec | Garmin - Fenix 8 (47mm) | OnePlus Watch 3 |
|---|---|---|
| ECG | true | true |
| SpO2 | true | true |
| Temperature sensor | true | true |
| Sleep tracking | true | true |
| Blood pressure | false | false |
Battery & Connectivity
| Spec | Garmin - Fenix 8 (47mm) | OnePlus Watch 3 |
|---|---|---|
| Battery lifeⓘ | 384 hlead | 120 h |
| GPS | true | true |
| Cellular | false | false |
| NFC | true | true |
| OS | Garmin OSlead | WearOS |
Frequently asked
Which has longer battery life, Fenix 8 (47mm) or OnePlus Watch 3?+
Fenix 8 (47mm) lasts 384 hours vs 120 hours.
Do Fenix 8 (47mm) or OnePlus Watch 3 support ECG?+
Both support ECG.
Which has cellular, Fenix 8 (47mm) or OnePlus Watch 3?+
Neither offers cellular.