Remember absorbent surfaces such as bare plaster take more paint, than surfaces which are merely to be repainted. Textured and relief materials also take considerably more than a smooth surface. Changing from one colour to another where there is a considerable difference may call for two coats of undercoat. Top coats have very little, if any, covering power. When measuring up, divide surfaces into Measurable rectangles then add them all together. Window frames are hard.
Count a window as a solid area by multiplying width by height. Shop around for paint, as quality materials can be bought at very considerable discounts. And buying one large can is usually less expensive than a lot of smaller ones. It also (insures that the colour is consistent, as batches can and do vary slightly. You will find the batch number on the can. Remember also that paint coverage can vary from type to type, so don't generalise. Choose your paint, then check on the can to find the recommended coverage.