Can your smartwatch tell how much you sweat?

A 2025 study tested watch sweat-loss estimates against a scale. They followed the trend well and missed the amount by a lot.

Several running watches now show an estimated "sweat loss" after a workout, sometimes with a suggestion for how much to drink. It's a useful-looking number. A 2025 study in Sports Health checked how good it is.¹

What the study did

Researchers had 111 participants complete up to four running sessions each, indoors and outdoors, from short runs to longer efforts and intervals. They compared the sweat-loss estimates from two commercial smartwatches against the standard reference: body-mass change before and after exercise, corrected for fluid intake.

What they found

• The trend was decent. The watches' estimates moved in the right direction: longer, hotter, harder sessions produced higher estimates, with agreement statistics in the moderate-to-strong range.
• The amount was not. Average absolute error was roughly 25 to 33%. If you actually lost a litre, your watch could plausibly say anything from about 670 ml to 1,330 ml.

That gap is the difference between "you're fine" and "you're at the edge of the 2% loss level where performance usually drops", for a lot of people in a lot of sessions.

Why watches miss

A watch on your wrist doesn't measure sweat. It estimates it from things it can sense, such as heart rate, pace, duration and sometimes weather data, and runs those through a model. Models are built on averages, and sweat rate is one of the most individual numbers in exercise physiology. Two people with identical heart rate and pace can sweat very different amounts.

Measuring instead of estimating

The broader pattern in hydration tech is a move from estimating to measuring. On the loss side, that means sweat patches that collect and analyse sweat directly, which have been tested with industrial workers.² On the intake side, it means devices that weigh a bottle to record what was actually drunk.

Disclosure: The Hydration Ledger is published by the team behind syp, which makes an intake-tracking bottle sleeve of that second kind. We mention the category here because it's relevant, not as a recommendation. See About.