Buried in your Health app is one of the most meaningful numbers your Apple Watch tracks: Cardio Fitness. It sounds vague, but it's actually Apple's label for your VO2 max — the best single measure of how fit your heart and lungs are, and a metric large studies link to longevity. Most people never look at it, or see it and don't know what "42 ml/kg/min" is supposed to mean. This guide fixes that: what it is, what's good for your age, how accurate it is, and how to move it in the right direction.
The one-line version: Cardio Fitness = VO2 max = how much oxygen your body can use at full effort. Higher is better, it declines with age, and unlike most metrics you can meaningfully improve it with training. Your Apple Watch estimates it automatically from outdoor walks, runs, and hikes.
What Cardio Fitness actually measures
VO2 max is the maximum volume of oxygen (VO2) your body can take in and use per minute during all-out exercise, expressed relative to your body weight in millilitres per kilogram per minute (ml/kg/min). The more oxygen your heart, lungs, and muscles can move and burn, the longer you can sustain hard effort — and the more resilient your cardiovascular system tends to be day to day. It's why VO2 max is considered the gold-standard snapshot of aerobic fitness, and why Apple surfaces it front-and-centre as "Cardio Fitness."
How Apple Watch measures it
You don't take a test. Apple Watch estimates your Cardio Fitness in the background from your heart rate during outdoor walks, runs, and hikes, where it can pair effort against GPS-tracked speed and incline. That's the key detail most people miss: it needs outdoor cardio to calibrate. Log a few brisk outdoor sessions and the estimate sharpens; rely only on indoor workouts and it may stay stale or low. You'll find the number in the Health app under Browse → Cardio Fitness.
What's a good Cardio Fitness level for your age?
There's no universal "good" number, because VO2 max naturally falls with age and differs between men and women. Apple classifies yours as high, above average, average, below average, or low relative to people of your age and sex. As a rough orientation:
| Age | Men — solid level | Women — solid level |
|---|---|---|
| 20–29 | ~45+ ml/kg/min | ~37+ ml/kg/min |
| 30–39 | ~42+ | ~34+ |
| 40–49 | ~39+ | ~32+ |
| 50–59 | ~35+ | ~29+ |
| 60+ | ~31+ | ~26+ |
These are rough guides, not strict cut-offs. To see exactly where your number lands and what it implies, our free VO2 max calculator rates it by age and sex, and the Fitness Age calculator turns it into an age you can feel — a more intuitive way to read the same value.
How accurate is it?
It's an estimate, not a lab test, so a single reading can differ from a clinical VO2 max measurement by several points. But Apple's estimate has been validated against clinical testing and is reliable for what matters most: the trend. Watching your Cardio Fitness drift up over a training block, hold steady, or slip is far more useful than fixating on one morning's figure. Give it good data — a few outdoor cardio sessions — and correct health details like your age and weight, and it becomes a trustworthy trend line.
How to improve your Cardio Fitness
This is the best part: unlike your age, VO2 max responds to training, and most people can raise it with a consistent routine that mixes two intensities:
- Zone 2 base work. Easy, conversational cardio — you can still talk in full sentences — for 30–60 minutes builds the aerobic foundation that lets your ceiling rise. Most of your weekly cardio should sit here.
- Intervals. One or two weekly sessions of harder efforts near your maximum, alternated with recovery (for example, repeated 3–4 minute pushes), are the most direct stimulus for lifting VO2 max.
- Log it outdoors. Recording outdoor walks, runs, and hikes both trains you and gives the watch the data it needs to update the estimate.
Progress comes from weeks and months of consistency, not any single workout — and simply walking more each day nudges your baseline up. If tracking one clear number keeps you consistent, that's exactly what a daily score is for.
Turn your fitness into one daily number
Health Genie reads your Apple Watch steps, resting heart rate, and sleep and builds one Vitality Score each morning — the simplest way to stay consistent enough to move numbers like Cardio Fitness. Free and private.
Download Health Genie FreeFrequently Asked Questions
What is Cardio Fitness on Apple Watch?
It's Apple's name for your VO2 max — the maximum oxygen your body can use during hard exercise, and the best single measure of aerobic fitness. Apple Watch estimates it automatically from outdoor walks, runs, and hikes and stores it in the Health app in ml/kg/min. Higher is better.
What is a good Cardio Fitness level?
It depends on age and sex, since VO2 max declines with age. Apple rates yours as high through low relative to your peers. Roughly, a middle-aged man above ~42 ml/kg/min or a woman above ~34 is doing well, with thresholds falling a few points each decade.
How accurate is Apple Watch Cardio Fitness?
It's an estimate, so a single reading can differ from a lab test by several points, but it's validated and reliable for tracking your trend over weeks and months — which matters more than any one figure.
How do I improve my Cardio Fitness?
Regular aerobic training: a base of easy Zone 2 cardio plus one or two weekly interval sessions near your maximum. Consistency over months matters most, and logging outdoor cardio gives the watch data to refine the estimate.
Why is my Apple Watch Cardio Fitness low?
It can reflect genuinely low fitness, or too little outdoor cardio data to estimate well. Record a few brisk outdoor walks or runs to calibrate and check your age is correct. If it stays low despite activity, a gradual aerobic program raises it over time.