Back when I was losing weight, I experimented with a variety of swimming calorie calculators and found a 25% margin of error between lowest and highest. I would just use the smallest number, but did not fret about it because ultimately the scale would be more accurate anyway. Some calculators also take into account weight, as well as subjective factors like degree of effort. One guy over at MFP tried to tie various estimates with more objective speeds, but I'm not sure how generalize-able his results are. Anyway, here's links to the calculators/calculations I've seen (from lowest to highest):
caloriesburnedhq.com/.../www.myfitnesspal.com/.../searchwww.everydayhealth.com/Calories-Burned-Swimming.htmwww.active.com/.../how-many-calories-does-swimming-burn
Based on data from the American College of Sports Medicine, a 205-pound person swimming freestyle for one hour will burn 931 calories swimming fast, and 651 calories swimming slower.
www.nutristrategy.com/caloriesburned.htmwww.swimmingcalculator.com/swim_calories_calculator.php
Hope this is helpful, Rev!
