We get this question a lot. And we don't list sex in our cardio tracker for several reasons.
For reference, a cardio workout is anything that raises your heart rate for an extended period of time, usually 20-30 minutes. It works all of the major muscle groups, and you should find yourself sweating and breathing hard during the most intense part of the workout.
Cardio works large muscle groups repetitively (such as how running uses your large leg & arm muscles and swimming uses your whole body). Sex does not typically use major muscle groups (or at least not in a full range of motion the way that these examples do).
Cardio must reach a specific level of intensity, meaning that you must elevate your heart rate to an aerobic level and sustain it there—about 60-85% of your max heart rate, or on a Rate of Perceived Exertion Scale (where you estimate how hard you are working on a scale of 1-10, about a 6-8.5). Although you may be breathing heavily during sex, this isn’t necessarily because your heart rate is up. It has more to do with hormones and what’s going on with your nervous system. (Think of how you may breathe harder or feel your heart rate increase if you’re scared—that doesn’t mean you’re getting a cardio workout.)
How many calories you're burning during sex really depends on too many factors. It's like asking "does walking burn a lot of calories?" It depends on your speed, distance, duration, intensity, etc.
Now, if you really feel that your sexual activity meets all of these requirements—although I'd recommend taking your heart rate several times during it to make sure—then you could probably count it as cardio. But according to most calorie counters, the calorie burn is pretty minimal—less than walking and not much more than just sitting. Plus, how accurate is that?
It’s important to distinguish that not all activities are a workout. Sure, being "active" burns more calories than not being active. But just because you’re burning a few additional calories does not mean you are exercising or that you’re receiving the same benefits of a true workout.
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