Many people believe that the more you sweat, the more weight you lose, and that if you sweat a lot during a workout, it means you’re working out harder than someone who may not be sweating as much. Both of these beliefs are incorrect.
The purpose of sweating is to cool your body down, therefore helping to regulate your core body temperature. Sweat is excreted from sweat glands in the layer beneath your skin. We are each born with between 2 million and 4 million sweat glands, and how much you sweat depends on the number of sweat glands you have (more glands equals more sweat). Men's glands are more active than women's, so they tend to sweat more.
On a superficial level, when you sweat, you do lose some water weight in the form of sweat. But all you're losing is water and electrolytes, and that will be gained right back as soon as you eat again or drink water. It's true that some athletes who are trying to make a certain weight class work out in sweatsuits or garbage bags right before they weigh in — and it can have an effect — but this is just to achieve a temporary number on the scale and does not reflect true weight loss.
This also applies to beliefs about sweating unaccompanied by the physical exertion of a workout. For example, sitting in a sauna or steam room, or participating in a workout class where the temperature of the room is raised before you even start to move (e.g., hot yoga or certain spin classes where they pack the room with people).
In these instances, you aren't necessarily burning extra calories by generating extra sweat.