What's the target heart rate during aerobic exercise?

Usual formula. Assuming you're in generally good health, the usual formula used is 80% of your age-determined maximum predicted heart rate. A good rule of thumb for determining your mphr is 220 minus your age. For example, if you're 40, your target heart rate would be 144 (220-40=180), then, (180 x 0.8=144). Go for it!