Residual volume is the volume of gas left in your lungs after you exhale fully. Even if you exhale as hard as you can, you can never fully empty your lungs of air. Some gets left behind, trapped in the passages and sacs in your lungs. Above-average residual lung volume may mean that you have a pulmonary condition, such as emphysema, that is restricting your exhalation. You can't calculate residual volume at home on your own. A doctor using specialised equipment to measure your breathing needs to calculate it.
Make an appointment with your doctor. Calculating residual volume requires specialised equipment, and the expertise to both use the equipment properly and make sense of the results.
Follow your doctor's instructions to test residual lung volume. Your doctor may use one of two common methods to test residual volume. The first method, called the gas dilution technique, requires that you inhale through a device which provides air containing a known concentration of an inert gas such as helium. When you exhale, the doctor will measure the helium concentration in your breath. The difference between the concentration of helium in the gas you inhale and exhale indicates your residual lung volume.
Alternatively, your doctor may have you breathe in pure oxygen. Filling your lungs with pure oxygen "washes out" any nitrogen left trapped in your lungs. The concentration of nitrogen in your exhalation indicates the residual volume of your lungs.
The final method, called body plethysmography, is more involved but more accurate than gas dilution testing. In a body plethysmography test, you sit in an airtight plastic chamber and breathe through a tube. When you breathe, the air pressure in the chamber changes depending on the volume in your lungs. The air pressure in the chamber when you exhale fully indicates your residual lung volume.