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.
Estimeret VO2max
42.4ml/kg/min
fra 12-minutters Cooper-testen
Cirka hvor det ligger
| Stillesiddende, cirka 30 til 40 | 30 til 40 ml/kg/min |
| Motionsaktiv, cirka 40 til 50Anbefalet | 40 til 50 ml/kg/min |
| Veltrænet, cirka 50 til 60 | 50 til 60 ml/kg/min |
| Udholdenhedsatlet, 60+ | 60+ ml/kg/min |
- Dette er et feltestimat. Et jævnt doseret løb på en flad rute lander inden for et par point af en laboratorietest, men dosering, terræn og vejr flytter alle tallet.
- En sand VO2max kræver en laboratorietest med gasanalyse. Submaksimale muligheder omfatter Uth-Sorensens pulsmetode (VO2max er cirka 15,3 gange HRmax divideret med HRrest) og Rockport-testen med en mils gang.
- Formkategorier varierer med alder og køn. Behandl båndet som en vejledning, ikke en skarp grænse, og sammenlign med dine egne tidligere tests over tid.
En PDF med dine personlige resultater, plus en QR-kode til at åbne dem igen når som helst.
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.
