Apple Watch vs Pixel Watch & Fitbit: VO2max compared
How Apple Watch and Pixel Watch & Fitbit estimate your VO2max, what each needs from you, what else they show and what independent studies say about accuracy.
| Apple Watch | Pixel Watch & Fitbit | |
|---|---|---|
| Called | Cardio Fitness | VO2 max (called Cardio Fitness Score before 2026) |
| Algorithm | Apple in-house | Not disclosed |
| App | Health app (iPhone) | Google Health app (called the Fitbit app until May 19, 2026) |
| From: Outdoor run | Yes | Yes |
| From: Walk | Yes | No |
| From: Treadmill | No | No |
| From: Bike (power meter) | Not confirmed | No |
| From: Resting test | Not confirmed | No |
| From: All-day data | Yes | No |
| Fitness age | Not confirmed | Not confirmed |
| Race predictor | Not confirmed | Not confirmed |
| Published accuracy studies | 3 | 3 |
| What you need to do | Wear the watch for at least 24 hours, then do several Outdoor Walk, Outdoor Run or Hiking workouts in the Workout app. Fairly flat ground, good GPS and heart rate signal help. | An outdoor run of at least 10 minutes with GPS. If no value appears, you may need up to three runs in 30 days. |
Key differences
- Apple Watch can estimate from walk, all-day data; Pixel Watch & Fitbit can't.
- Both have 3 published accuracy studies that we found.
How each one works
Apple Watch: Measures how hard your heart works (heart and motion sensors, GPS) during outdoor walks, runs and hikes, plus passive outdoor walking. It also uses your age, sex, weight, height and heart-rate medications.
Pixel Watch & Fitbit: The app compares your running pace with your heart rate during outdoor GPS runs, then compares that running efficiency with your resting heart rate. Since 2026 it no longer uses height and weight to calculate the score, and it can include runs from connected third-party apps.
Which is better for VO2max?
For VO2max specifically, the best device is the one that sees your hardest efforts: if most of your training is outdoor running, both give an estimate (both use outdoor run). If you cycle with a power meter, walk more than you run, or want a resting test, check those rows above. Whichever you choose, compare the number with itself over time and with the norms for your age.
Accuracy studies
Apple Watch
- Lambe et al., Mayo Clin Proc Digit Health, 2026: Series 10 underestimated VO2max by 6.25 ml/kg/min on average; MAPE 13.2%; poor individual agreement.
- Lambe et al., PLoS One, 2025: Apple Watch underestimated VO2max by 6.07 ml/kg/min on average; MAPE 13.31%.
- Caserman et al., JMIR Biomed Eng, 2024: Series 7 estimate was lower than the lab value (41.37 vs 45.88 ml/kg/min); MAPE 15.79%, ICC 0.47.
Pixel Watch & Fitbit
- Lambe et al., PLoS One, 2026: Pixel Watch 3 vs lab treadmill test (n=34): bias -0.53 mL/kg/min, limits of agreement -12.56 to 11.50, MAPE 8.65%. Funded by Google.
- Klepin et al., Med Sci Sports Exerc, 2019: Fitbit Charge 2 vs lab VO2max (n=60): bias +1.59 (15 s data) and +0.30 mL/kg/min (60 s data), MAPE under 10%.
- Freeberg et al., mHealth, 2019: Fitbit Charge 2 (n=30) overestimated: 52.5 vs 49.9 mL/kg/min measured, ICC 0.87, MAPE 10.2%.
Frequently asked questions
Is Apple Watch or Pixel Watch & Fitbit more accurate for VO2max?
Apple Watch has more published validation (3 vs 3 studies). Watch estimates are typically within several ml/kg/min of lab tests for groups, with larger individual errors, so use either one to track your trend.
Does Apple Watch measure VO2max from outdoor run like Pixel Watch & Fitbit?
Yes: both estimate from outdoor run.
Which is better for VO2max: Apple Watch or Pixel Watch & Fitbit?
Pick the one whose data sources match how you train: Apple Watch uses outdoor run, walk, all-day data; Pixel Watch & Fitbit uses outdoor run.
Built from official documentation (see each brand page for sources). Features vary by model and firmware. Apple Watch and Pixel Watch & Fitbit are trademarks of their owners; VO2max Health is independent and not affiliated with either.