The average person burns about 100 calories running 1 mile...and about 120-140 calories by walking for an hour. Taking a one hour hot bath burns 140 calories.
The study was conducted by exercise physiologist Steve Faulkner as a way to examine look at the connection between taking a hot bath and blood sugar and calorie burn. His findings showed that a bath burns around 140 calories per hour, and lowers peak blood sugar after eating about 10 percent more than exercising does. [Source]
Now obviously taking hot baths isn't a substitute for exercise, as it won't build muscle or help with endurance...but it does have some pretty awesome benefits and literally all you have to do is sit there and relax!
“Many cultures swear by the benefits of a hot bath,” Steve Faulkner, lead author of the study, told The Conversation. “But only recently has science began to understand how passive heating (as opposed to getting hot and sweaty from exercise) improves health.”