Honestly, it depends on the donut. Taking glazes, frostings, and toppings into account,a typical Dunkin' Donuts donut can be anywhere from 250-400 calories. A good rule of thumb to consider when ordering is: 5 munchkins = 1 donut. So if you're popping munchkins thinking it's less unhealthy than eating a donut, think again.
It depends a lot on many factors, such as the type of donut (cake, dough, flaky, crispy, ect.), whether is it baked or fried, the amount of creme in it, and the flavor of both the donut and the creme. Generally (assuming it is a mornal size and all), about 250 to 350 calories. (Fried, more creme, chocolate, or sugar coated will make the caloire total higher, to name a few.)