Heart rate reserve is the difference between your resting heart rate and your max heart rate. Its one of many heart rate intensity protocols to gauge how hard to work in your training sessions and currently the preferred equation of most personal training organizations. Use the tool below to identify your range.
Step 1- Take your resting heart rate. You can use your activity tracker/Fitbit or you can take it manually. See how here.
Step 2- ID your max heart rate. A simple one is 220 - age.
Step 3- Enter info in the tool below
Step 4- Scroll down to the percentage to determine what heart rate range you'd like to work in.