Enter your Age, Resting Heart Rate and Maximum Heart Rate (if known) then click Calculate 

The chart above uses the Karvonen formula.  This formula takes into account your resting heart rate (RHR).  Knowing your resting heart rate helps you to determine your Heart Rate Reserve (HRR) or the difference between the Maximum Heart Rate (MHR) and the Resting Heart Rate (RHR).  This is also known as your Working Heart Rate Reserve (WHR).  The greater the difference of your MHR and your RHR the greater your HHR or WHR.  This is important because this tells you how great your heart rate intensities can be. 

The best way to measure your resting heart rate (RHR) is when you first wake up in the morning and before you get out of bed.  Simply count your pulse for 15 seconds beginning on the zero count and multiply by 4.  You can do this for 5 days and then average the results.  This is your resting heart rate. Another way to guess your resting heart rate is to measure your heart rate while sitting still in a chair quietly during the day and taking a pulse measurement as described above and then subtracting 5 from this number.  

Keep in mind that lack of proper rest, stress and lack of proper nutrition, and medication can influence your RHR.  One sign of over-training is an elevated RHR.  By becoming aware of your RHR you can more closely become in-tuned with with how your body is reacting to different stimulus.