When running, your heart rate should be between 50% to 85% of your maximum heart rate. To calculate your maximum heart rate, subtract your age from 220. For example, if you are 40 years old, your maximum heart rate would be 180 beats per minute. Most runners between the ages of 20 to 45 will want to stay between 100 and 160 beats per minute on average. However, this average depends on several factors, including your maximum heart rate and current fitness level. You can use the formula and chart provided by the American Heart Association to determine your target heart rate range.

Its important to note that each person is different, and there is no "normal" heart rate when running. Your ideal heart rate while running is influenced by several factors, including age, weight, fitness level, family history, heat and humidity, medications, and stress. Therefore, its essential to listen to your body and adjust your pace accordingly.

To summarize, your heart rate when running should be between 50% to 85% of your maximum heart rate, which can be calculated by subtracting your age from 220. Most runners between the ages of 20 to 45 will want to stay between 100 and 160 beats per minute on average. However, your ideal heart rate while running is different for each person and depends on several factors. Its important to listen to your body and adjust your pace accordingly.