Nearly everyone asking this has a working watch. Cardio fitness only updates when you give it a very specific kind of effort, and most people’s training never provides one.
What the watch is actually waiting for
Apple estimates VO₂max from the relationship between your pace and your heart rate during sustained outdoor movement. That requires knowing how fast you are going, which means GPS, which means outdoors.
Qualifying: outdoor walking at a genuine clip, outdoor running, outdoor hiking. Twenty minutes or more, continuous, at a sustained effort.
Not qualifying: treadmills, indoor cycling, outdoor cycling, swimming, rowing, strength training, and any walk gentle enough that your heart rate barely moves. A person who lifts four times a week and cycles to work can go months without a single reading.
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Check your personal details are right
Health app, profile picture, Health Details. Height, weight, age, sex and the wheelchair setting all feed the estimate. A weight several kilos out will bias every reading, and this is the most common cause of a number that looks wrong rather than absent.
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Confirm the feature is enabled
Health, Browse, Heart, Cardio Fitness, and check Cardio Fitness Levels are set up. On the phone, also check Privacy settings allow Motion & Fitness and location for the watch.
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Go and give it a qualifying effort
Start an Outdoor Walk, Run or Hike. Twenty minutes minimum, brisk enough that talking is slightly effortful, on reasonably flat ground without stopping at junctions every two minutes.
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Wear the watch properly for it
Snug, and on the wrist rather than loose over a sleeve. The estimate depends on heart rate accuracy, and a moving watch produces readings the algorithm will discard rather than trust.
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Give it a day
The reading is not always instant. Check the following day before deciding the walk did not count.
Why the number may be lower than you expect
Because a brisk walk is a weak stimulus for this estimate. If all your qualifying efforts are walks, the model has only ever seen you at low intensity and will estimate conservatively.
Runners generally get higher and more stable readings than walkers of equivalent fitness for exactly this reason. If your cardio fitness matters to you, feed it a run rather than a stroll. What the number means and how far to trust it is in Apple Watch cardio fitness, explained.
To move the number rather than just measure it, see how to increase your VO₂max and the VO₂max calculator.