How accurate is your watch's VO₂max estimate?

• Apple Watch builds its estimate from your heart rate response during outdoor walks, runs and hikes on fairly flat ground (less than 5% grade). It needs your age, sex and weight, and asks you to record medicines that limit heart rate. Apple's 2021 white paper gives a supported range of 14 to 60 ml/kg/min.³
• Garmin watches use an algorithm from Firstbeat that analyses the relationship between heart rate and speed at many points in a run, keeping only data it judges reliable.^2,4

The model then projects that relationship up to your maximum heart rate. If it uses an age-predicted maximum, any error there flows into the result. A 2025 review noted that, for the Firstbeat method, underestimating maximum heart rate by 15 beats per minute gives an error of about 9%, and overestimating it by 15 beats about 7%. With a known maximum, the error dropped to about 5%.²

Some devices also offer a resting test, based on resting heart rate data and your profile rather than exercise. These are a different, weaker kind of estimate (see below).¹

The big picture

The 2022 INTERLIVE meta-analysis pooled 14 validation studies.¹

• Watches using exercise data: bias −0.09 ml/kg/min, limits of agreement −9.92 to +9.74.
• Watches using resting data: bias +2.17 ml/kg/min (overestimating), limits −13.07 to +17.41.

In plain terms: across a group, exercise-based estimates average out close to the truth. For one person, the watch could be around 10 ml/kg/min off in either direction. The authors concluded the estimates suit population-level use, but "the estimation error at the individual level is large".¹

Individual device studies

MAPE is the mean absolute percentage error: the average size of the miss, ignoring direction. Samples are small, 19 to 35 people in the studies above, so treat any single figure as a guide. Most Garmin studies in the 2025 review also paired the watch with a chest-strap heart-rate sensor, so results with wrist heart rate alone may differ.²

The fitness pattern

The clearest pattern is a pull towards the middle. Watches tend to read high for less-fit people and low for fit ones. Düking's runners showed it directly.⁷ Caserman's authors described the same pattern for the Apple Watch.⁵ In Engel's study, the underestimate grew in athletes with VO₂max around 60 ml/kg/min and above.¹⁰ If you are a trained endurance athlete, expect your watch to undersell you.

Apple's own numbers

Apple's white paper reports a mean error of 1.4 ± 4.7 ml/kg/min in 221 validation participants. Note the reference: VO₂max was projected from submaximal heart rate and VO₂ data to age-predicted maximum heart rate, not taken only from maximal tests.³ The independent studies above compared against maximal lab tests.

An estimate is usually fine for

• General awareness of your fitness and whether it is broadly rising or falling.
• Motivation and long-term habits.
• Research on large groups, where individual errors average out.¹

Measure with gas exchange when

• You need training zones. A watch VO₂max doesn't give your ventilatory thresholds, and age-based heart-rate zones carry their own error. See Ventilatory thresholds VT1 and VT2, explained.
• You are highly trained, where watch error is largest.¹⁰
• You take medicines that change heart rate, or have a condition that does.³
• You mainly cycle, row or swim, and your watch estimates only from walking or running.³
• The number feeds a clinical, research or selection decision. As Engel and colleagues put it, "Laboratory-based gas analysis remains the preferred method when precision is required."¹⁰

For how a lab test works, see VO₂max testing: how it's measured, and what the number means.

QuestionHow accurate is Apple Watch VO₂max?

Two independent studies found Apple Watch underestimated lab-measured VO₂max on average: by 4.5 ml/kg/min in 19 adults (Series 7) and by 6.1 ml/kg/min in 30 adults. Average absolute errors were about 13% to 16%.^5,6 Apple's own white paper reports a smaller error, but against a reference projected from submaximal tests.³

QuestionHow accurate is Garmin VO₂max?

Studies of Garmin watches found average absolute errors of roughly 5% to 8% in recreational runners and general adults.^7,9 Accuracy was worse in very fit athletes: in highly trained endurance athletes, one study found an average underestimate of 6.9 ml/kg/min.¹⁰

QuestionWhy is my watch VO₂max lower than my lab test?

If you are fit, that is a common pattern. Several studies found watches overestimate VO₂max in less fit people and underestimate it in fitter people.^2,7 Your lab result, measured from your breath, is the better number.

QuestionHow can I make my watch estimate more accurate?

Keep your age, sex and weight up to date, and enter a measured maximum heart rate if your watch allows it. A review noted that a 15 beat per minute error in maximum heart rate can shift the estimate by 7% to 9%.² Record medicines that limit heart rate, run outdoors on flat ground, and give the watch several runs.^3,8

QuestionCan I trust the trend even if the number is off?

Partly. Apple reports good run-to-run consistency in its own data, so large, lasting changes probably mean something.³ But independent long-term studies of whether watch trends match lab-measured changes are still lacking, so treat small shifts with caution.²

1. Molina-Garcia P, Notbohm HL, Schumann M, et al. Validity of estimating the maximal oxygen consumption by consumer wearables: a systematic review with meta-analysis and expert statement of the INTERLIVE network. Sports Med. 2022;52(7):1577–1597. PMC9213394
2. Železnik Mežan L. Accuracy of wearables for determining the maximal oxygen uptake and lactate threshold: a qualitative systematic review. Front Sports Act Living. 2025;7:1707991. PMC12748164
3. Apple. Using Apple Watch to estimate cardio fitness with VO₂ max. White paper, May 2021. apple.com (PDF)
4. Firstbeat. Fitness level (VO₂max). firstbeat.com
5. Caserman P, Yum S, Göbel S, Reif A, Matura S. Assessing the accuracy of smartwatch-based estimation of maximum oxygen uptake using the Apple Watch Series 7: validation study. JMIR Biomed Eng. 2024;9:e59459. PMC11325102
6. Lambe R, O'Grady B, Baldwin M, Doherty C. Investigating the accuracy of Apple Watch VO₂ max measurements: a validation study. PLoS One. 2025;20(5):e0323741. PMC12080799
7. Düking P, Van Hooren B, Sperlich B. Assessment of peak oxygen uptake with a smartwatch and its usefulness for training of runners. Int J Sports Med. 2022;43(7):642–647. PMC9286863
8. Düking P, Ruf L, Altmann S, et al. Assessment of maximum oxygen uptake in elite youth soccer players: a comparative analysis of smartwatch technology, Yo-Yo Intermittent Recovery Test 2, and respiratory gas analysis. J Sports Sci Med. 2024;23(2):351–357. PMC11149075
9. Carrier B, Marten Chaves S, Navalta JW. Validation of aerobic capacity (VO₂max) and pulse oximetry in wearable technology. Sensors. 2025;25(1):275. PMC11723475
10. Engel FA, Masur L, Sperlich B, et al. Validity of V̇O₂max estimates from the Forerunner 245 smartwatch in highly vs. moderately trained endurance athletes. Eur J Appl Physiol. 2026;126:591–603. doi:10.1007/s00421-025-05923-x