How Fitbit Calculates Your VO2 Max (Cardio Fitness Score)
If Fitbit shows you a range like 42-46 instead of a number, it has estimated your VO2 max from resting heart rate alone. Here is how to get a real one.
Fitbit reports VO2 max as your Cardio Fitness Score, in ml/kg/min like a lab test, alongside a Cardio Fitness Level that ranks you from Poor to Excellent against people of your age and sex.
The thing to check first is whether you are looking at a number or a range. If Fitbit shows you 42 to 46 rather than 44, it is telling you it has not seen you run.
The range means it guessed from your resting heart rate
Fitbit produces the score two ways.
With GPS runs, it compares your running efficiency, the ratio of your pace to your heart rate, against your resting heart rate. That is a performance-based estimate in the same family as Garmin's, and it produces a single number.
Without GPS runs, it falls back to a non-exercise model built from your resting heart rate, age, sex, weight and other profile data, and reports a range rather than a value. That is Fitbit being honest about its uncertainty, and most people never notice the distinction.
The gap between the two is large. A performance estimate is grounded in something you actually did. A resting-heart-rate model is a population regression applied to your profile, in the same category as Polar's OwnIndex, and it carries the same tendency to be confidently wrong about individuals.
Geschatte VO2max
42.4ml/kg/min
uit de 12-minuten-Cooper-test
Ongeveer waar dat ligt
| Inactief, ongeveer 30 tot 40 | 30 tot 40 ml/kg/min |
| Recreatief actief, ongeveer 40 tot 50Aanbevolen | 40 tot 50 ml/kg/min |
| Goed getraind, ongeveer 50 tot 60 | 50 tot 60 ml/kg/min |
| Duuratleet, 60+ | 60+ ml/kg/min |
- Dit is een veldschatting. Een goed verdeelde loop op een vlak parcours komt binnen een paar punten van een labtest, maar tempo, terrein en weer verschuiven het getal allemaal.
- Een echte VO2max vereist een laboratoriumtest met gasanalyse. Submaximale opties zijn de Uth-Sorensen-hartslagmethode (VO2max is ongeveer 15,3 keer HRmax gedeeld door HRrest) en de Rockport-wandeltest over één mijl.
- Conditiecategorieën variëren met leeftijd en geslacht. Behandel de band als richtlijn, niet als harde grens, en vergelijk met je eigen eerdere tests in de tijd.
Een PDF met je resultaten, plus een QR-code om ze altijd opnieuw te openen.
Estimate yours from a race result or a field test. If Fitbit is showing a range and your field test lands outside it, trust the field test.
How to turn the range into a number
Run outdoors with GPS. Fitbit only measures VO2 max during outdoor runs with GPS enabled. Walking, cycling and treadmill running do not produce an estimate, so a treadmill-only winter leaves you on the resting-heart-rate model indefinitely.
Make sure your resting heart rate is real. It is an input to both versions of the score. A device worn loosely, or not worn overnight, gives Fitbit a poor resting figure and shifts everything built on it.
Fix your max heart rate. Fitbit defaults to 220 − age and places its zones inside your heart rate reserve. Every heart-rate-based fitness estimate inherits whatever you leave in that field.
What the score is good for
Not for comparing against a friend's Garmin, and not as a clinical measurement. As a trend measured the same way each time, it works: same kind of run, same conditions, same wear habits, and the direction it moves is meaningful even when the absolute value is not.
The Cardio Fitness Level ranking is the more robust half of the feature. Moving from Fair to Good is a bigger, slower change than a two-point wobble in the raw number, and it is harder to produce by accident.
FAQ
How does Fitbit calculate VO2 max? From your running efficiency, the ratio of pace to heart rate on outdoor GPS runs, compared against your resting heart rate. Without GPS runs it estimates from resting heart rate and profile data instead.
Why does Fitbit show a range instead of a VO2 max number? Because it has no GPS run data. A range means the estimate came from resting heart rate alone.
Why is my Fitbit VO2 max not tracked? Fitbit only measures it during outdoor runs with GPS. Treadmill, walking and cycling sessions do not count.
What is a good Fitbit Cardio Fitness Score? The Cardio Fitness Level shown alongside it already answers that, ranking you from Poor to Excellent against your age and sex.
Is Fitbit VO2 max accurate? The GPS-run version is a reasonable trend indicator. The range version is a population estimate and should not be treated as a measurement.
Other brands
- Garmin uses Firstbeat and short credible segments.
- Apple Watch has published independent validation.
- COROS uses a rolling six-week window.
- Polar offers both a resting test and a per-run estimate.
Sources
- Google Health Help: learn about your VO2 max
- Fitbit Web API: Cardio Fitness Score
- How Fitbit calculates your max heart rate
Cover photo by Raysonho, CC0, via Wikimedia Commons.
