If you have it set up already, that is on your weight tracking page (go to My Trackers tab, then select My Weight) and then you will see that option on there to track it. If you don't have it set up yet, you can go to your weight tracking page and then click on the Add/Change What You Measure link near the bottom of the page (under Tools & Settings). You can then select that option and save your changes and it will be added to your weight tracking page so you can track it.
Hope that helps!
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