Depends on the slice. This is an excellent question as you need to have an idea of how many calories you're eating. In this case, it's going to depend on many things including the toppings on the pizza and the size of the slice. Many (if not most) restaurants now include nutritional information about their foods. I would go to the website for the pizza in question and see what you can find out.
One slice pizza. An average slice of cheese pizza has about 250 calories. Add more for the toppings, extra cheese and thick crust.