To calculate the low end of your THR zone, 50% of your heart rate reserve, you'll use the following formula:
206.9 - (0.67 x age) = Max Heart Rate (MaxHR)
MaxHR - (resting heart rate) = Heart Rate Reserve (HRR)
HRR x 50% = training range %
training range % + resting heart rate = low end of THR zone
The following example shows the low end of a THR for a 35-year-old person with a resting heart rate of 60 bpm:
206.9 - (0.67 x 35) = 183.45
183.45 - 60 = 123.45
123.45 x 50%= 62
62 + 60 = 122 beats per minute
For this person, the low end THR starts at 122 bpm, which would be a light, warm up pace. Keep in mind that these numbers are often underestimated, meaning a heart rate of 122 bpm may feel too easy to our sample exerciser. He may need to adjust that number a bit higher, something I'll discuss in later steps.
Make a note of your own heart rate and go on to the next step: calculating the high end (85%) of your THR zone.