Resting Heart Rate (RHR) is defined by the Mayo Clinic as the heart rate "when the heart is pumping the lowest amount of blood you need because you're not exercising".
This is typically between 60 to 100 beats per minute, with a lower amount usually indicative of increased efficiency in pumping blood as a result of exercise and healthy lifestyle habits.
There is no industry standard algorithm for calculating this, however default values can be estimated using age, gender, and athletic ability.
To find your resting heart rate manually (ideally as soon as you wake up):
Place your index and third fingers on your neck to the side of your windpipe. To check your pulse at your wrist, place two fingers between the bone and the tendon over your radial artery — which is located on the thumb side of your wrist.
When you feel your pulse, count the number of beats in 15 seconds. Multiply this number by four to calculate your beats per minute.