We’re more than happy to shell out for one of the city’s niche fitness classes when we know we’ll be walking out with some good soreness and a sense of achievement.

Too bad the whole experience is often more of a gamble than it’s worth.

Or so says former Wall Street analyst Gillian Casten, who, after years of trying out less-than-stellar instructors, set out to eliminate the gym-class guesswork.

Think of her new, free site, RateYourBurn, as a Yelp for the fitness community. Searchable by class, instructor or gym, the service offers specific reviews from its network of in-the-know bloggers (who must be in tip-top shape) and regular exercisers like you.

For instance, before signing up for barre in Chelsea, spin on the Upper East or yoga in Park Slope, you can easily browse the class’s ratings, gleaning useful info about a studio’s certain high-energy instructor, killer playlist or not-so-updated equipment.

Meanwhile, if you decide to post your own anonymous or named review, expect replies from instructors happy to receive feedback, plus comments from other RateYourBurn users including Casten herself.

Now if she could only figure out a way to make those last reps a little less difficult.

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