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A close crop of a runner's wrist showing a smartwatch face with distance and pace from a three mile run.

Your Watch's Cardio Fitness Score Is Off by About 13 Percent

Somewhere in your Health app there is a number labeled Cardio Fitness, and if it sits in the Low band, Apple sends you a notification about it every four months until it moves. It looks like a lab result. The validation studies say it’s an estimate with a lot of room in it.

Six points of VO2 max moves you a whole band

Rory Lambe and three co-authors ran the Apple Watch against a metabolic cart in PLOS One last year: 28 adults, a real maximal treadmill test with a mask on, watch estimate on one side and measured VO2 max on the other. The average gap was 6.92 mL/kg/min, or 13.31 percent. Across the whole group the watch read low by about six points.

Six points is most of a fitness category. Apple sorts you into Low, Below Average, Above Average and High using cutoffs that shift with your age and sex, and the bands are only a handful of points wide. An error that size can hand you a warning you don’t need, or hide one you do.

The error leans one way if you’re out of shape

A smaller study is the more useful one. Polona Caserman’s group tested the Series 7 on 19 people for JMIR Biomedical Engineering and split the results by how fit the participants actually were. In the poor-fitness group, the watch read about 3.8 mL/kg/min too high. In the excellent group, it read 12 points too low.

↑ Watch reads high ↓ Watch reads low +3.8 −3.37 −12.0 Poor n=3 Good n=11 Excellent n=5
Mean gap between the Apple Watch estimate and lab-measured VO2 max, in mL/kg/min, by measured fitness level. Data from Caserman et al., JMIR Biomedical Engineering, 2024.

Three people in that first group and five in the second, so hold the exact figures loosely. The direction is the part worth keeping, and it follows from what the algorithm has to do. Your watch never sees oxygen. It watches your heart rate against your pace on outdoor walks, runs and hikes, then fits you to a curve built from thousands of other people. Curves pull toward the middle. So the watch is generous with beginners and stingy with strong runners, which is roughly backwards if you were hoping the number would tell you something you didn’t know.

It also drifts for reasons that have nothing to do with your heart. A hot afternoon, four hours of sleep, three coffees, a loose band, a route with traffic lights on it: all of those change your heart rate at a given pace, and the estimate moves with them.

Your watch may not have looked at you in weeks

There is a second gap that has nothing to do with math. Apple only updates the estimate during outdoor walks, runs and hikes, so if your cardio is a spin class, a pool, a rowing machine or three flights of stairs with a toddler on your hip, the watch never gets a sample. People who train hard indoors end up with a stale, unflattering number sitting in the Health app, then get told every four months that they should probably move more.

That’s worth knowing before you take the notification personally. Check when the reading last changed. If the date is from a walk you took in June, the number is a memory.

Test yourself against yourself instead

The number does one thing honestly, which is show you a line over months on the same wrist, same watch, same kind of route. Give it months and not weeks, because the difference between two nearby readings is usually smaller than the tool’s own error. For everything else, repeat something and time it. That’s what people did before the wrist computers, and it still works.

  • Pick one route you can redo exactly: a hill, four flights of stairs, a lap around the park.
  • Cover it at an effort you could hold for about ten minutes, hard enough that talking gets choppy.
  • Write down the time, and your heart rate at the top if you have it.
  • Do it again in six to eight weeks, same time of day, similar weather.

Faster at the same effort, or the same time at a lower heart rate, means your cardio improved. That comparison is yours, it’s repeatable, and no algorithm is guessing on your behalf.

None of this means throw the watch in a drawer. Something that nags you every four months is doing more for most people than the perfectly calibrated lab test they’ll never book. Just stop reading the number as a verdict on your body. It’s a wide guess wearing the costume of a measurement.

Walk your hill this week and put the time in your notes app. In two months you’ll have something to compare it to.