The notion that calories in and calories out is all that matters when it comes to being healthy and slim is totally passe. Even so, when it comes to workouts, lots of people from those starting a weight-loss journey to fitness junkies still constantly want to know: How many calories am I burning?? Turns out, the answer’s a little complicated—and super subjective.

To get in the ballpark, more women than ever are sporting discreet trackers like the Up by Jawbone, the Fitbit, and the Nike Fuelband. And some workout hotspots, like Bari Studio and The Studio by Remorca Fitness, have sophisticated tracking systems that allow you to see your heart rate as you sweat and chart your burn progress after.

“Every class is different, but no matter what, it should be an efficient hour of your life,” says Bari head trainer Michelle Pellizzon. “The tracking system’s really kept us honest, and it keeps our clients honest.” That’s important, since lots of workouts will try to tempt you with overarching claims that can’t possibly be accurate, i.e., “You WILL burn 800 calories in 45 minutes without even working hard!”

The truth: You will burn a completely different number of calories than the woman sweating to your right and the dude on your left.

We tracked our own calorie expenditures, chatted with trainers from these metrics-focused studios, and got the scoop from a top exercise physiologist to bring you this simple break down of the many factors influencing your actual—and very personal—calorie burn:

1. Body weight. The list of factors is a long one, but “the biggest thing is your body composition,” says Ciaran Friel, medical director at fitness-meets-medicine center La Palestra. The more you weigh, the more calories you’ll burn as you increase your heart rate, because more mass means more energy needed to get moving. In an example with one person weighing 100 pounds, and another 200, the heavier person “could almost be burning twice as many calories, if they’re working just as hard,” Friel explains.

2. Muscle mass. More muscle mass just equals more calorie burn, which is why lots of trainers advocate heavy weight lifting as an essential part of any weight-loss plan. “When we’re building muscle mass, we’re creating more metabolically active cells,” Friel explains. This is one reason men tend to burn more than women.

3. Fitness level. As you shed pounds and become, say, a better runner, you’re moving through space in a mechanically efficient way that requires less energy, and, therefore, less calorie burn. Remorca co-owner Nedra Lopez says her clients often get frustrated as they get in shape and watch their calorie counts go down. “They complain and say, ‘Now I’m in better shape, but I don’t burn as many calories, and I get drunk faster!'” she laughs. “I tell them, ‘Yes, but you look great and you’re healthy!'”

4. How hard you work. This one’s obvious, but more important than you may think. During class, you’ve got to push it. In my Bari class, for example, a regular client about my size burned 50 more calories than me. But she knew the dance moves and could rock them out, while I kept pausing to try to get the steps right. Working really hard at high levels of intensity has also been shown to increase afterburn, meaning you’ll keep burning more calories once you’re back at your desk (which is one reason HIIT workouts have taken off).

In the end, “calorie burn is pretty individual, and there are lots of variables,” Friel says. “We just have to be realistic about how we arrive at that number of total calories burned.”  —Lisa Elaine Held