If your Apple Watch rates your Cardio Fitness as Low, its estimate of your VO2 max falls in the lowest of Apple's four levels for people your age and sex. Below Average is the level just above it. Either one is worth taking seriously, because cardio fitness is one of the strongest predictors of long-term health. Still, it's an estimate, and it can be off by several points, especially if the watch has few qualifying outdoor workouts to work from or you take a medication that slows your heart rate. Most people can raise it with more frequent, harder cardio over a few months.
What Apple Watch Cardio Fitness Measures
Cardio Fitness is Apple's name for an estimate of your VO2 max: the most oxygen your body can use during hard exercise, in milliliters per kilogram of body weight per minute (mL/kg/min). Our VO2 max glossary entry covers the physiology.
A lab measures VO2 max directly, with a mask on while you exercise to exhaustion. Apple Watch can't do that, so it estimates it. Apple Watch Series 3 or later uses its heart and motion sensors during Outdoor Walk, Outdoor Run, and Hiking workouts in the Workout app, and also factors in your age, sex, weight, height, and medications that might affect your heart rate (Apple).
Apple's support page also sets a few rules for when you get a reading:
- Indoor workouts don't count, including treadmills and other gym equipment.
- The first estimate takes time: at least 24 hours of wearing the watch, plus several qualifying workouts and passive measurements. Apple also says Push workouts aren't supported.
- Apple Watch supports a VO2 max range of 14 to 65 mL/kg/min.
The estimate is submaximal: Apple Watch reads how your heart rate responds to a given effort and predicts your maximum from there, so you don't need to go all-out to get one (Apple Developer).
What "Low" and "Below Average" Mean on Apple Watch
Apple sorts Cardio Fitness into four levels: High, Above Average, Below Average, and Low. Your level compares your estimate with people of the same age group and sex, using data from the FRIEND fitness registry (Apple Newsroom, 2020). Levels are available from age 20 (Apple).
So "Low" doesn't mean a fixed number. A VO2 max that reads Low for a 30-year-old may read Below Average or higher for a 60-year-old. Apple doesn't publish the exact cutoffs in its documentation, but the Health app lets you compare your results with each level for your age and sex:
- Open Health and tap Browse, then Heart, then Cardio Fitness.
- Tap Show All Cardio Fitness Levels and tap a level to compare your results with others your age and sex.
- Switch between D, W, M, 6M, and Y to see your averages over time (Apple).
The Low Cardio Fitness Notification
If your level is low for your age and sex, Apple Watch sends a notification, and you'll get more if it stays low (Apple). When Apple launched the feature, it said the notification comes with guidance on improving your fitness over time and talking with your doctor (Apple Newsroom, 2020).
Apple only describes the notification for a Low level, so don't count on one at Below Average. That level still leaves plenty of room to improve.
Is Apple Watch VO2 Max Accurate?
Apple Watch VO2 max is accurate enough to track your trend, but not precise enough to treat any single reading as your true VO2 max. Two small independent studies that compared it with lab tests found it reads lower than the lab on average, with wide differences from person to person.
An earlier study of the Apple Watch Series 7 found a similar average underestimate of about 4.5 mL/kg/min. It also found the error depends on fitness: the watch overestimated VO2 max in people with poor fitness and underestimated it in people with excellent fitness (Caserman et al., 2024). Both studies were small (30 or fewer people each), so the exact numbers may not hold for everyone.
That matters for a Low reading. The "watches read low" finding comes mostly from fit people, so it's no reason to assume your Low level is wrong, and the one study that looked at fitness levels found the watch tended to read high for less fit people. Treat the level as a real signal, then check whether one of the issues below applies to you.
Why Your Cardio Fitness Reading Might Be Off
Apple publishes the conditions its estimate needs. When your workouts don't meet them, you get fewer readings, and the ones you get may rest on limited data.
| Possible cause | What Apple says | What to do |
|---|---|---|
| Few outdoor workouts | It's based on Outdoor Walk, Outdoor Run, and Hiking workouts; indoor workouts don't count | Record a few brisk outdoor walks or runs each week |
| Easy pace or hilly routes | Needs flat ground, sufficient effort, and good GPS and heart rate signal | Use a fairly flat route at a pace that gets you breathing harder |
| Heart rate medication | Medications that limit heart rate can cause an overestimate | Turn on the medication setting in the Health app |
| Outdated health details | Age, sex, weight, and height feed the estimate | Keep your health details current |
Too Few Qualifying Outdoor Workouts
If most of your cardio happens on a treadmill, bike, rower, or in classes, Apple Watch has little to estimate from, since indoor workouts don't count. If you go to spin class three times a week but only record the occasional outdoor dog walk, your level is being estimated from those walks, not your real training, and it may lag behind your fitness. Apple also doesn't produce a reading from your very first qualifying workout (Apple Developer).
Slow Walks and Hilly Routes
Apple's developer documentation spells out what a qualifying workout needs: relatively flat ground (less than a 5% incline or decline), adequate GPS and heart rate signal, and enough effort that your heart rate stays at roughly 130% of your resting heart rate or more (Apple Developer). For someone with a resting heart rate of 60, that's a heart rate of about 78 or higher.
A slow stroll may not reach that, and a hilly hike may not count because of the grade. If your walks are short, gentle, or on steep trails, record some on flatter routes at a brisk pace. Our guide to turning walks into real cardio explains how to pick that pace by heart rate.
Medications That Lower Your Heart Rate
Because the estimate depends on how your heart rate responds to effort, anything that blunts that response affects it. Apple says conditions or medications that limit your heart rate might cause an overestimate of your VO2 max (Apple). Beta blockers are the best-known example. Apple's documentation says people taking a medication that may lower their peak heart rate can turn on a medication setting in the Health app for a more accurate estimate (Apple Developer).
If you take a heart rate medication, treat your Cardio Fitness level as a rough guide, and ask your doctor how to read it.
Outdated Health Details
Your age, sex, weight, and height are inputs to the estimate, so keep them current in the Health app, especially after a weight change. If the Health app finds your earlier estimates could be more accurate, an Update Your Data option appears on the Cardio Fitness screen; tap Update to recalculate them (Apple).
Why Low Cardio Fitness Matters for Your Health
Even with its error margin, a Low level deserves attention, because cardio fitness is closely tied to how long and how well people live.
- In a meta-analysis of 33 studies with about 103,000 people, each 1-MET increase in fitness (about 3.5 mL/kg/min of VO2 max) was linked to 13% lower all-cause mortality and 15% fewer heart disease and cardiovascular events. People in the low-fitness group had 70% higher all-cause mortality than those in the high-fitness group (Kodama et al., 2009).
- In a study of more than 120,000 patients who had treadmill tests, the mortality risk tied to low fitness was as large as or larger than the risk tied to smoking, diabetes, or coronary artery disease (Mandsager et al., 2018).
- The American Heart Association recommends that doctors assess cardio fitness as a clinical vital sign, noting that it may predict mortality better than smoking, high blood pressure, high cholesterol, or type 2 diabetes (Ross et al., 2016).
These studies show associations, and they measured fitness with exercise tests rather than a watch. They don't mean a Low level predicts anything specific for you. They do mean fitness is one of the health numbers you can change most directly, and starting low means you have the most room to gain.
When to Talk to a Doctor
Apple Watch Cardio Fitness is a fitness estimate, not a medical test, and FITIV doesn't diagnose anything either. Stop exercising and get medical help right away if you get chest pain or pressure, unusual shortness of breath, dizziness, or fainting during exercise; call emergency services if it's severe. Otherwise, talk to your doctor:
- before you start exercising harder if you have a heart condition, high blood pressure, diabetes, or another chronic condition, or you take medication that affects your heart rate
- if your level drops and stays down without a clear reason, such as time off training, or you feel more tired or breathless than usual
The Physical Activity Guidelines also advise adults with chronic conditions or symptoms to be under the care of a health care provider when planning their activity (Physical Activity Guidelines for Americans, 2018). If you want a direct measurement, a doctor can refer you for a cardiopulmonary exercise test, the lab test that measures VO2 max directly.
How to Raise a Low Cardio Fitness Level
Apple's own advice is short: most people can raise their cardio fitness by doing cardio more often and at a higher intensity (Apple). Our complete VO2 max training guide has the full protocols and a 12-week plan.
Build Up to the Weekly Guideline
If you're starting from Low, the first goal is simply more regular cardio. Work up to at least 150 minutes a week of moderate activity, like brisk walking, or 75 minutes of vigorous activity, like running, spread across the week (Physical Activity Guidelines for Americans, 2018). Most of it should be easy enough to hold a conversation: that's Zone 2 training, and it builds the aerobic base everything else rests on.
Add Some Harder Efforts
Once a few weeks of regular cardio feel comfortable, add one session a week with harder efforts, such as brisk uphill walking or short running intervals. Intervals are the most direct way to raise VO2 max: in one well-known trial, 4-minute intervals at 90–95% of maximum heart rate raised VO2 max significantly more than long slow running or threshold running did over 8 weeks (Helgerud et al., 2007). Our interval training guide shows how to build them. If you're older or new to exercise, check with your doctor before adding hard efforts.
Give Apple Watch Data to Work With
You can get fitter on a bike or treadmill, but Apple Watch won't see it in your Cardio Fitness level. Include at least a few Outdoor Walk or Outdoor Run workouts each week on fairly flat routes, at a pace that gets your heart rate well above resting. That way your level keeps up with your real fitness.
Expect Weeks, Not Days
Your level won't change after one good week. Fitness gains build over several weeks to a few months, and they come at any age: in a meta-analysis of 41 trials, endurance training raised VO2 max by about 16% in sedentary adults aged 60 and over (Huang et al., 2005). Check your trend in the Health app monthly with the M or 6M view rather than reacting to single readings.
How FITIV Shows Your Cardio Fitness
FITIV Pulse shows your VO2 max on the Cardio Fitness card in the post-workout report for workouts with heart rate data, from one of two sources:
- From Apple Health: if your Apple Watch recorded a Cardio Fitness reading at the end of the workout, as it can after qualifying Outdoor Walk, Outdoor Run, and Hiking workouts, FITIV shows Apple's value.
- FITIV estimate: otherwise, including indoor workouts and workouts with a Bluetooth heart rate monitor, FITIV estimates it from your peak heart rate during the workout and your resting heart rate setting. It only does this for workouts long and hard enough to give a reliable estimate; if a workout falls short, the card says why, for example "Workout too short for a reliable estimate."
That second source helps if most of your cardio happens indoors, where Apple Watch doesn't record Cardio Fitness. Because FITIV's estimate rises with how hard you worked, compare similar workouts over time rather than an easy walk with a hard run.
FITIV rates the value as Poor, Active, Fit, or Elite for your age and sex. These are FITIV's own levels with their own cutoffs, not Apple's, so the labels won't always line up: a reading Apple calls Below Average isn't automatically "Poor" in FITIV. The full table is in our VO2 max guide, and the VO2 max tracking page explains the feature.
FITIV also tracks your time in each heart rate zone, so you can check that your week includes both easy Zone 2 work and some harder efforts.
Apple Watch and Apple Health are trademarks of Apple Inc. Other product names are trademarks of their respective owners. FITIV is an independent app and has not been authorized, sponsored, or otherwise approved by Apple Inc.
Frequently Asked Questions
Why doesn't my Apple Watch show a Cardio Fitness level?
Apple Watch needs at least 24 hours of wear plus several Outdoor Walk, Outdoor Run, or Hiking workouts before the first estimate, and indoor workouts don't count. It also needs Apple Watch Series 3 or later, and levels are only shown for people 20 and older. Apple says Push workouts aren't supported.
How long does it take to raise a low cardio fitness level?
Expect several weeks to a few months of regular cardio before your level moves. In studies, structured training raises VO2 max measurably within about 8 weeks, and people starting from a low level usually have the most room to improve. Check your monthly trend rather than individual readings.
Does losing weight raise Apple Watch Cardio Fitness?
It can. VO2 max is measured per kilogram of body weight, and Apple uses your weight in the estimate, so losing weight while keeping your fitness raises the number. Keep your weight current in the Health app so the estimate reflects it.
Should I get a lab VO2 max test?
Most people don't need one to get fitter, since the watch trend is enough to track progress. A lab test, called a cardiopulmonary exercise test, gives a direct measurement and is worth asking your doctor about if you have a heart or lung condition, take medication that affects your heart rate, or want a precise baseline.
Does FITIV use the same levels as Apple Watch?
No. Apple uses High, Above Average, Below Average, and Low, while FITIV uses Poor, Active, Fit, and Elite, each with its own cutoffs by age and sex. FITIV shows Apple's VO2 max value when your watch recorded one, but rates it on FITIV's own scale.
