Jillian Michaels OnlineTo do cardio right, you must first figure out your maximum heart rate (MHR). This is done by subtracting your age from 220. For example, If you're 32, check it out: 220 - 32 = 188. (Now you know your MHR is 188.) Then take 85% of 188, which gives you 160—that's your target heart rate, the ideal number of times your heart should beat in one minute while training.