Device comparisons
Forerunner 965 vs Fenix 8: Battery, Water Rating and ECG
Both watches run Garmin's training and recovery features, including HRV Status. Per Garmin's manuals, the Forerunner 965 lasts up to 23 days in smartwatch mode and is rated 5 ATM; the Fenix 8 (47 mm AMOLED) lasts up to 16 days with the gesture display and is rated 10 ATM plus 40 m recreational diving. Garmin names the Fenix 8 among watches with its ECG App. No study has compared their heart-rate accuracy.
Devices
Garmin Forerunner 965, carbon-gray titanium bezel with a black and powder-gray band (part 010-02809-00): product photograph from Garmin. Garmin fēnix 8 47 mm AMOLED, slate gray case with a black silicone band (part 010-02904-00): product photograph from Garmin.
At a glance
This page does not list head-to-head accuracy numbers, so the scoreboard shows hardware differences only.
| Specification | Garmin Forerunner 965 | Garmin Fenix 8 (47 mm AMOLED) |
|---|---|---|
| Battery LifeDiffers | Up to 23 days smartwatch mode | Up to 16 days (gesture); up to 7 days (always-on) |
| Water ResistanceDiffers | Swim, 5 ATM | 10 ATM; dive to 40 m (EN 13319) |
| ECGDiffers | — | ECG App (regional availability) |
Primary rows only. The study tables further down list more results from the accuracy database.
Hardware specs
| Specification | Garmin Forerunner 965 | Garmin Fenix 8 (47 mm AMOLED) |
|---|---|---|
| Battery LifeDiffers | Up to 23 days smartwatch mode | Up to 16 days (gesture); up to 7 days (always-on) |
| Water ResistanceDiffers | Swim, 5 ATM | 10 ATM; dive to 40 m (EN 13319) |
| ECGDiffers | — | ECG App (regional availability) |
Hardware only. Study results are not repeated in this table. Differs marks values that are not the same. An em dash means this page did not publish that figure.
Specs verified on October 17, 2026: Garmin Forerunner 965 owner's manual: battery information, Garmin Forerunner 965 owner's manual: specifications, Garmin Fenix 8 owner's manual: battery information, Garmin Fenix 8 owner's manual: specifications, Garmin: ECG App expands to Australia and the EU (Venu 3, Fenix 8), Garmin: Understanding HRV Status.
Pros and cons
Garmin Forerunner 965
Pros
- Up to 23 days smartwatch battery
- HRV Status and Garmin training features
Cons
- 5 ATM rating with no dive rating
Garmin Fenix 8 (47 mm AMOLED)
Pros
- 10 ATM plus 40 m recreational dive rating (EN 13319)
- ECG App (30-second recording, regional availability)
- Up to 16 days with gesture display
Cons
- Shorter smartwatch battery than Forerunner 965 (16 vs 23 days)
- Up to 7 days with always-on display
What published studies show
One table per metric. These rows come from the accuracy database and are not repeated as a second card list. From the wearable accuracy database (v1.0.0).
Heart rate during exercise2 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Garmin Forerunner 235all exercise conditions | rc = 0.81Lin's concordance correlation | ECG | n = 50, Gillinov: treadmill, bike, elliptical | Gillinov et al. (2017)Funding not reported |
| Garmin Fenix 5 | 2.9%–9.2% (range by intensity)Coefficient of variation | ECG chest strap (Polar H7/ | n = 25, Sitting, walking, running, sprints | Düking et al. (2020)Independent |
Overnight heart rate1 study
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Garmin Forerunner 245 Music | ICC 0.41Intraclass correlation | ECG | n = 53, Overnight, sleep laboratory | Miller et al. (2022)Other maker funded |
| Garmin Forerunner 245 Music | 5.4 bpm (absolute bias)Mean absolute error | ECG | n = 53, Overnight, sleep laboratory | Miller et al. (2022)Other maker funded |
HRV (RMSSD)2 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Garmin Forerunner 245 Music | ICC 0.24Intraclass correlation | ECG | n = 53, Overnight, sleep laboratory | Miller et al. (2022)Other maker funded |
| Garmin Forerunner 245 Music | 33.1 ms (absolute bias)Mean absolute error (absolute bias) | ECG | n = 53, Overnight, sleep laboratory | Miller et al. (2022)Other maker funded |
| Garmin Fenix 6 | 10.52%Mean absolute percentage error | ECG | n = 13, Home, during sleep, 536 nights | Dial et al. (2025)Independent |
| Garmin Fenix 6 | 0.87Lin's concordance correlation | ECG | n = 13, Home, during sleep, 536 nights | Dial et al. (2025)Independent |
Sleep stages (4-stage)1 study
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Garmin Forerunner 245 Music | 50%Epoch-by-epoch agreement | Polysomnography (PSG) | n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together | Miller et al. (2022)Other maker funded |
| Garmin Forerunner 245 Music | κ = 0.25Cohen's kappa | Polysomnography (PSG) | n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together | Miller et al. (2022)Other maker funded |
Sleep vs wake (2-stage)2 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Garmin Forerunner 245 Music | 89%Epoch-by-epoch agreement | Polysomnography (PSG) | n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together | Miller et al. (2022)Other maker funded |
| Garmin Forerunner 245 Music | κ = 0.35Cohen's kappa | Polysomnography (PSG) | n = 53, Sleep laboratory, 1 night, 9 h in bed, all devices worn together | Miller et al. (2022)Other maker funded |
| Garmin Fenix 5S | Worse than actigraphyStudy finding (no single number) | Polysomnography (PSG) | n = 34, Sleep laboratory, 3 nights incl. disrupted night | Chinoy et al. (2021)Funding not reported |
Total sleep time1 study
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Garmin Forerunner 945 | MAE 84.63 min (overestimate)Mean absolute error | Polysomnography (PSG) | n = 49, 1 night PSG + MSLT | Budig et al. (2022)Independent |
Blood oxygen (SpO2)3 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Garmin Fenix 5X Plus | 9.77% (ICC 0.549)Mean absolute percentage error | Arterial blood gas (SaO2) | n = 13, High altitude 4559 m | Schiefer et al. (2021)Independent |
| Garmin Fenix 5X Plus | +3.3% at 12,000 ft (LoA −1.9 to 8.6%)Mean bias (signed) | Medical-grade pulse oximeter | n = 23, Normobaric hypoxia, rest, 900–12,000 ft | Lauterbach et al. (2021)Funding not reported |
| Garmin Fenix 6 | 4.29% (CCC 0.10)Mean absolute percentage error | Medical-grade pulse oximeter | n = 22, Normoxic and hypoxic conditions | Carrier et al. (2025)Independent |
VO2max estimate3 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Garmin Forerunner 245 | 7.9% / 7.2% (two runs)Mean absolute percentage error | Indirect calorimetry | n = 35, Treadmill ramp test | Engel et al. (2026)Funding not reported |
| Garmin Forerunner 920XT | 7.3%Mean absolute percentage error | Indirect calorimetry | n = 24, Lab | Passler et al. (2019)Independent |
| Garmin Fenix 6 | 7.05% (CCC 0.73)Mean absolute percentage error | Indirect calorimetry | n = 19, Graded exercise test and outdoor run | Carrier et al. (2025)Independent |
Energy expenditure (calories)3 studies
| Device tested | Result | Reference | Sample | Study |
|---|---|---|---|---|
| Garmin Forerunner 955physical activity energy expenditure, 10 min | +68.61 kcalMean bias (signed) | Indirect calorimetry | n = 51, 10-min recumbent cycling intervals | Kostrna et al. (2026)Independent |
| Garmin Fenix 5 | 15.9%–34.5% (range by intensity)Coefficient of variation | Indirect calorimetry | n = 25, Sitting, walking, running, sprints | Düking et al. (2020)Independent |
| Garmin Fenix 6walking | 32.0% walk / 21.8% runMean absolute percentage error | Indirect calorimetry | n = 20, Outdoor 2-km walk (6 km/ | Le et al. (2022)Independent |
Verdict
Pick the Forerunner 965 for longer smartwatch battery; pick the Fenix 8 for diving, a 10 ATM rating and the ECG App. Neither Garmin nor an independent study has shown a heart-rate accuracy difference between them.
Frequently asked questions
Is Forerunner or Fenix more accurate for heart rate?
No study has compared the Forerunner 965 and Fenix 8. Garmin's specifications do not establish an accuracy difference.
Does the Fenix 8 have ECG?
Yes. Garmin names the Fenix 8 among compatible watches for its ECG App, which records 30 seconds; availability depends on region.
Which has longer battery life?
The Forerunner 965: up to 23 days in smartwatch mode versus up to 16 days for the 47 mm Fenix 8 AMOLED with the gesture display.