If you're trying to lose weight, it's important to work at a certain level of intensity during cardio exercise. Using the karvonen formula or a target heart rate calculator, you can find the optimum heart rate range to help you reach your goals. The target heart rate zones range from 60-100% of your maximum heart rate. The different ranges include: Low Intensity: 60% to 70%. This zone keeps you at a comfortably low intensity and is a good choice as a warm up or for beginners because it helps you develop aerobic fitness for more intense exercise.