VO2 max
The most oxygen your body can use in hard exercise.
What it measures
Aerobic fitness, in millilitres of oxygen per kilogram of body weight per minute (mL/kg/min).
How trackers estimate it
From heart rate on outdoor walks or runs, GPS pace, and your age, sex and weight.
Apple Watch accuracy
Read 6.07 mL/kg/min low on average against lab testing: a 13.31% mean absolute percentage error (PLOS ONE, 2025).
PLOS ONE, 2025
Trained athletes
Garmin estimates were less valid in highly trained athletes than in moderately trained ones.
Institut für Angewandte Trainingswissenschaft
How it is measured
- The watch measures no oxygen; it calculates an estimate.
- Studies use indirect calorimetry in a lab, analysing breathing gases during a graded treadmill test.
Source: PLOS ONE, 2025
Accuracy
- PLOS ONE 2025 (28 adults analysed): 95% limits of agreement ran from -6.11 to 18.26 mL/kg/min.
- Error varied between people and devices, and small samples are common in this research.
- Individual estimates can be far off, so change over time on one device means more than the number.
Source: PLOS ONE, 2025
Using the trend
- Track the direction of your own estimate over months, alongside how exercise feels.
- New breathlessness or tiredness on activity is a symptom worth mentioning to a GP, whatever the number.
Questions
- Is the VO2 max on my watch measured?
- No. It is calculated from heart rate, pace and personal details, and against lab testing it can be several mL/kg/min out.
- How accurate is it?
- In a 2025 study of the Apple Watch the mean absolute percentage error was 13.31%. Other devices and groups give different results.
- Should I compare my number with other people's?
- Estimates differ between brands, so compare only with your own earlier figures on the same device.
Sources
- Investigating the accuracy of Apple Watch VO2 max measurements - A validation study, PLOS ONE, 2025
- Heart palpitations, NHS
- Validity of VO2max estimates from the Forerunner 245 smartwatch in highly vs. moderately trained endurance athletes, Institut für Angewandte Trainingswissenschaft
When to get advice: The NHS advises 999 or A&E if palpitations do not go away or come with chest pain, breathlessness or feeling faint.
Cheapest trackers that measure it
- 1
- 3 years
- £129.99
- Price
- £129.99
- Subscription
- Optional
- 2
- 3 years
- £139.99
- Price
- £139.99
- Subscription
- Optional
- 3
- 3 years
- £149.50
- Price
- £149.50
- Subscription
- None
- 4
- 3 years
- £159
- Price
- £159
- Subscription
- Optional
- 5
- 3 years
- £178.99
- Price
- £178.99
- Subscription
- None
- Resting heart rate
- Heart rate variability (HRV)
- Sleep stages and deep sleep
- Blood oxygen (SpO2)
- Respiratory rate
- Skin temperature
- Readiness and recovery scores
- Steps
Readings are not a diagnosis. If you are worried about your health, speak to your GP, pharmacist or NHS 111. How we earn.