A number of different tests can find out how well your lungs are working. For example, doctors can measure the amount air you normally breathe in or out; the amount you can breathe in or out when you are trying as hard as you can; or the extra you can breathe in when you try after you have breathed in normally. The tests tell doctors about the workings of your lungs and will often be done before surgery to see if you are fit enough to have an operation.