How many calories you burn while swimming depends on your stroke, effort, speed, distance, duration, gender, weight and skill. Because most people swim a mix of strokes at various intensities, computing your exact calorie count is difficult.

For example, a 150 pound athlete burns about 272 calories by swimming 1500 yards in 30 minutes; swimming butterfly for 30 minutes burns 38 percent more calories, breaststroke burns 25 percent more and backstroke 12 percent less. Compare that with a 120 pound athlete who burns only 218 calories swimming the same 1500 yard workout (60 lengths in a 25 yard pool).

Generally, the faster an athlete swims, the more calories he burns in an hour. For example, if the 150 pound swimmer above swims one more length per minute, he burns 102 more calories in the same 30 minute period, but it works out to nearly half a calorie LESS per length. This anomaly occurs because he swims more efficiently.

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