exercise ring app on Apple Watch

You need to get your heart rate up for it to count as exercise

Make sure that your information is up to date and correct in the Health app.

Age/sex/height/weight

The calculations are based on that.

For exercise you are supposed to get your heart into the 70-90% off your max heart rate.

Max heart rate is calculated as 220 -your age

For example say you are 50 years old, 220 -50 = 170

170 is your maximum heart rate

Your exercise range is 70 - 90% of that

So it is 119 to 153 bpm

This is the basic rule of exercise calculations.

It goes without saying that everyone is different so it is always advised to consult your medical professional for what they say is best for you instead of taking medical knowledge or advice off an internet forum

To earn exercise or workout credit you must move at the speed of a brisk walk.

To earn 1 minute of exercise credit(green ring);

Travel a full minute at a brisk walking pace 3.3 mph or 18 minutes per mile approx.or 5.3kph or 11.3 minutes per kilometer.

Raise your heart rate to approx 75% of your maximum.

When you're at around or above this heart rate and speed for the entire minute you'll receive 1 exercise minute credit.

If you pause or slow down during this minute then you won't get any exercise credit for that minute,it will still measure your steps and calories burnt but will not give you any credit towards the exercise ring.

Source;Successful phone call I had with apple support.