What VO2 max actually measures
VO2 max is the most oxygen your body can pull in, deliver, and use during hard effort, expressed in millilitres of oxygen per kilogram of body weight per minute. In a lab it's measured with a mask on a treadmill, ramping the intensity until your oxygen uptake plateaus. On your wrist, it's an estimate — but the underlying thing it estimates is the same: how efficient your heart, lungs, and muscles are at the gas-exchange job that keeps you moving.
The Apple Watch reports values between 14 and 65 mL/kg/min. To anchor those numbers: the average reading across Apple Watch users sits around 29; the top 1% reach about 51. World-class endurance athletes can clear 70 in a lab. A sedentary 60-year-old might sit in the high teens. That range — from "struggles to climb stairs" to "runs ultras" — is what this single number is summarising, which is why it's worth caring about even if you're not chasing a race.
How Apple Watch estimates it (and why it's an estimate)
Apple's algorithm doesn't measure your oxygen uptake — it infers it. The Watch looks at your heart-rate response during outdoor walks, hikes, and runs, combines that with your demographics (age, sex, height, weight) and GPS-derived pace and grade, and fits a model that maps that response to a VO2 max number. The model was trained against treadmill VO2 max tests, but it's still a model, not a measurement.
A peer-reviewed 2025 accuracy study compared Apple Watch readings against gold-standard lab gas-exchange and found Apple under-estimates by about 6 mL/kg/min on average, with the gap growing for fitter users. So if your Watch shows 42, your true VO2 max is probably closer to 48. The practical takeaway: the absolute number is conservative, but the direction is reliable. Trust the trend, not the digit.
Reading your number — what counts as "good" at your age
Apple uses age and sex bands to classify readings as Low, Below Average, Above Average, or High. Roughly speaking, for someone in their late 30s to mid-40s: high-30s and up is genuinely fit; mid-40s+ is athletic; above 50 is rare on the Watch and likely closer to 56+ in a lab. Subtract roughly five years for each decade older — a 65-year-old above the high-20s is doing well; above the mid-30s is exceptional.
Peter Attia's framing of the "Centenarian Decathlon" is useful here: ask what physical tasks you want to do at 90, then back-calculate the VO2 max you need at 50 to still have headroom. Most everyday tasks — climbing two flights of stairs, carrying a suitcase up a curb, getting up off the floor without using your hands — require a VO2 max somewhere in the high-teens to mid-20s. If you're sitting at 38 today, you have a generous buffer. If you're at 22 at 45, the buffer is thin and worth attending to.
Moving the number — what the data says actually works
Two interventions have the strongest evidence for raising VO2 max in non-elite adults.
The first is Zone 2 cardio: 45–60 minutes at a conversational pace, three to four times a week, at RPE 5–6 (you can talk, but a song is hard to sing). This builds the aerobic base — the mitochondrial density and capillary network that lets your muscles use oxygen efficiently. It's unglamorous and feels too easy, which is why most people skip it.
The second is brief high-intensity intervals, most famously the Norwegian 4×4: warm up, then four sets of four minutes near-max effort with three minutes easy in between. Once a week is enough. This pulls the ceiling up while Zone 2 widens the base.
Doing both beats doing either alone. A weekly mix of three Zone 2 sessions plus one interval session, sustained over 8–12 weeks, is what the literature consistently shows moves the number for non-athletes.
Strength training doesn't raise VO2 max directly — but it protects the muscle you'll lose otherwise, which in turn protects your metabolic floor and your ability to actually do the cardio. Two strength sessions a week is the floor I'd suggest, anchored around compound lifts. I look at your overnight HRV trend before suggesting a Zone 2 day versus a strength day versus a rest day — recovery decides which your body's ready for, and the answer changes week to week.
What the number is NOT telling you
A single low reading isn't a verdict. Day-to-day fluctuations on Apple Watch are mostly measurement artifacts: how snug the Watch sat on your wrist, whether GPS was good, how warmed up you were before the walk that triggered the estimate. The Watch also tends to under-report relative to a chest strap, and skin tone, tattoos, and cold weather all affect the optical heart-rate read.
Treat anything inside ±2 mL/kg/min as noise. Look at the six-month trend in the Health app — that's where the signal lives.
The healthspan frame: why this number predicts how well you'll age
A widely cited 2018 study (Mandsager et al.) followed more than 120,000 adults and found that higher cardiorespiratory fitness was associated with substantially lower all-cause mortality — the effect size rivalled or exceeded that of not smoking and not having diabetes. The gap between "low" and "elite" fitness was associated with a 5× difference in mortality risk over the follow-up.
That doesn't mean your Apple Watch reading is destiny, and it doesn't mean a number below the mean dooms anyone. What it means is that VO2 max is one of the very short list of things you can move — through training that's free and available — that meaningfully changes how long you stay functional. Frame it as raising your floor, not chasing a ceiling. The number you hold steady at 60 matters far more than the peak you hit at 35.
A simple weekly rhythm to actually move it
If your recovery markers look healthy, a workable week looks like this:
- Mon — Strength (compound lifts, 45 min)
- Tue — Zone 2 cardio, 50 min
- Wed — Rest or easy walk
- Thu — Intervals (Norwegian 4×4 or 10-min high-intensity block)
- Fri — Strength (45 min)
- Sat — Longer Zone 2, 60–75 min outdoors
- Sun — Mobility / easy walk
This isn't sacred. On a low-HRV morning I'd shift Thursday's intervals to a Zone 2 walk and push the hard session to Saturday; on a stressed week I'd cut volume by 30% and let the trend hold. The plan exists to be adjusted — that's the whole point of looking at the Watch in the first place.
The number on your wrist is not a verdict. It's a question: given what your body is showing this week, what's the smartest next session? That's the question I'm built to answer.