Heart rate has been used for many years as a way to determine exercise intensity. While it does have its limitations it can be useful for anyone completing steady-state exercise (exercise at the same pace).
Exercise intensity is usually prescribed based on a percentage of maximum heart rate. Traditionally, maximum heart rate was determined with the simple formula 220 - age. Many Exercise Professionals believe that this underestimates maximum heart rate in certain individuals and prefer to use the Karvonen formula (also known as the "heart rate reserve formula").
The calculator below will determine various heart rate zones, as well as a predicted maximum heart rate, using the Karvonen formula.