what metric do they measure with watch

A watch can measure different metrics depending on whether you mean a smartwatch or a regular wristwatch. For a smartwatch, the most common metrics are steps, heart rate, calories burned, distance, sleep, blood oxygen (SpO2), and sometimes ECG or stress ; for a regular watch, the main “metric” is usually the case size in millimeters.

Smartwatch metrics

• Steps and activity level.

• Heart rate and heart-rate variability.

• Calories burned and distance.

• Sleep tracking and recovery-related scores.

• SpO2, ECG, and skin temperature on some models.

Regular watch measurement

• Watch size is usually given in mm , which refers to the case diameter.

• Lug width is another common metric, and it tells you the strap size where it attaches to the watch.

If you mean a specific watch brand or a fitness watch, I can narrow it down to the exact metrics it tracks.