The number of calories burned when swimming a mile depends on a variety of factors. The efficiency of your stroke is the largest determinant of calorie consumption, but effort, where you're swimming and even how tight your swimsuit fits all have an effect on the calorie cost of swimming a mile.
A Mile of Freestyle
While freestyle isn't the most efficient competitive stroke (backstroke is) or the biggest calorie burner (that's the butterfly), it is the preferred long-distance stroke because it's the fastest. When swimming 50 yards per minute, an athlete burns about 2.12 calories per pound of body weight per mile. Swimmers who can maintain 75 yards per minute are more efficient and only burn 2 calories per pound per mile. So, a 150-pound swimmer burns 318 calories when swimming a mile in 35 minutes, and 300 calories swimming the same distance in 24 minutes. In the 11 minutes that he saves by swimming faster, he burns the 18-calorie difference showering.
A Mile of Strokes
Swimming a mile of backstroke burns 1.59 calories per pound. Breaststroke burns about 2.27 calories per pound. There is no one-mile breaststroke event because the breaststroke is very fatiguing. If you need more rest, then it decreases the calorie-count per hour. While the energy cost of swimming butterfly for a mile is massive, only elite swimmers could finish a straight mile of butterfly.
Why Be Efficient?
If you can burn more calories per mile swimming freestyle at a slower pace, or swimming a less-efficient stroke like breaststroke, then why would someone swimming to lose weight care about efficiency? Because by swimming efficiently you burn more total calories. If you have 45 minutes to swim, you can cover 1.92 miles (575 calories for a 150-pound athlete) at a 75-yards-per-minute pace compared to 1.28 miles (406 calories) at 50 yards per minute. Even a fast breaststroke is slower than an easy freestyle, so in 45 minutes, breaststroke's calorie expenditure is about equal to that of a slow freestyle.
How Long Is a Mile in a Pool?
If you are in a pool, it is easy to calculate how many lengths you have to swim to cover a mile. Most lap pools in the United States are 25 yards (70 lengths to a mile). See the chart for the number of laps per mile for other pool sizes. The shorter the pool, the more distance you cover gliding off the wall after turns. Swimming a mile in longer pools with fewer turns burns more calories.
How Long is a Mile in Open Water?
Measuring calories per mile in open water can be tricky. There may be currents that force a swimmer to burn more or fewer calories per mile. Marker buoys may drift, and without lane lines it is easy to swim off course, adding distance to your swim. Your body is more buoyant in salt water, so you burn fewer calories swimming a mile in the ocean than in a lake. Finally, in the ocean the tide can vary the distance to shore. When swimming in open water, it is better to estimate calorie expenditure based on swimming time rather than distance.