Assuming you are doing a moderate stoke (rather than say the butterfly or you are swimming vigorously), you weigh 190 pounds (the average weight of an American male), you are swimming in a normal pool filled with water without a wave machine and you are wearing normal swim wear, then you’d burn approximately eight calories a minute. This would increase to 14 calories is you doing a more vigorous stroke.
Decrease this figure if you weigh less than 190 pounds or you prefer the breaststroke to say the front crawl. Increase this figure if you swimming in the sea, are heavier or are wearing normal clothes.
Swimming is a very good way to burn calories as moving through water offers much more resistance than air so you have to work harder and expend more energy. Also it is non-impact, unlike running or doing weights, so you’ll be putting less stress on your joints and are less likely to injure yourself.