Variables: Intensity, Weight and Time. The amount of calories that you burn when swimming this stroke will depend a lot on the intensity that you put forth, how much you weigh, and the amount of time that you swim. The more you weigh and the harder that you swim, the higher the amount of calories you will burn doing the elementary backstroke. A 120-pound person swimming an easy pace will burn 120 to 220 calories in 30 minutes. A 200-pound person swimming at a vigorous pace will burn 200 to 400 calories in 30 minutes.