How WHOOP Calculates Your VO2 Max

Most wearables estimate VO2 max from one thing: the relationship between your pace and your heart rate on a run. WHOOP uses that too, but it pulls in a wider set of inputs, including respiration rate, recovery, time spent in each heart rate zone, and GPS running data, and updates the estimate weekly rather than after every activity.

That is a different design with different tradeoffs, and both are worth understanding before you trust the number.

Why a wider net, and why weekly

WHOOP is worn 24 hours a day, which gives it data no watch has: overnight respiration rate, resting physiology, and a continuous record of training load rather than a series of workouts.

Using that breadth is reasonable. Cardiorespiratory fitness genuinely does show up in resting measures, which is the same principle behind Polar's resting OwnIndex test. Combining it with actual running data should beat either input alone.

The weekly cadence follows from the same logic. A model drawing on a week of physiology cannot sensibly update after a single run, and the smoothing means you see fewer of the day-to-day wobbles that make Garmin's number jump after a hilly run.

The cost is transparency. When an estimate comes from one clear relationship, you can reason about why it moved. When it comes from thousands of data points across several domains, you cannot, and a number you cannot interrogate is harder to act on.

Estimate yours independently from a race result or a field test. With any multi-input model, an outside reference point is the only way to tell whether the number is tracking your fitness or tracking your sleep.

The anchor underneath it

Every part of this rests on your maximum heart rate. Time in zone is defined as a percentage of it, and WHOOP seeds that with the Gellish formula and then learns it from your data.

A learned maximum is better than a fixed formula and still fallible, because it moves up on recorded peaks and optical sensors produce artefact spikes at high intensity. If your maximum has been inflated by a bad reading, your zone distribution shifts, and the VO2 max estimate shifts with it.

How to use it

Give it running data. The GPS running input is the part grounded in performance. Train only indoors and the model leans harder on the inferential inputs.

Read the trend, not the value. Weekly updates over a training block tell you something. A single week does not.

Check it against a race. A recent 5k or 10k converted through a VO2 max estimate is a far more direct measurement than anything inferred from resting physiology.

FAQ

How does WHOOP calculate VO2 max? From a combination of GPS running data, respiration rate, recovery and time spent in heart rate zones, updated weekly in the app.

How often does WHOOP update VO2 max? Weekly, rather than after each activity.

Do I need to run outdoors for WHOOP VO2 max? GPS running data is one of the main inputs, so outdoor running gives the model its most direct evidence.

Is WHOOP VO2 max accurate? WHOOP has not published validation data comparable to the independent studies on Apple's Cardio Fitness. Treat it as a trend and check it against a race result.

Why did my WHOOP VO2 max change when my training did not? The model uses recovery and respiration inputs as well as running data, so non-training factors can move it.

Other brands

• Garmin uses Firstbeat and updates after runs.
• Apple Watch is the one with published independent validation.
• COROS uses a rolling six-week window.
• Fitbit shows a range when it has no GPS runs.

Sources

• Notebookcheck on WHOOP's VO2 max feature
• WHOOP on how strain works
• How WHOOP calculates your max heart rate

Cover photo by TheHellRace, CC BY-SA 4.0, via Wikimedia Commons.