Medical oxygen cylinder manufacturers use letters to differentiate cylinder sizes. For example, "D" cylinders, the most prevalent size, typically contain between 300 liters and 350 liters of oxygen, although some may have as much as 415 liters, while "E" cylinders carry between 625 liters and 682 liters, explains DeAnza College. A newer system allows manufacturers to label oxygen tanks with the letter "M," which stands for "medical," and an accompanying number indicating the capacity of the cylinder, notes Inogen.

"H" medical oxygen cylinders hold nearly 7,000 liters of gas; "G" cylinders hold slightly over 5,000 liters; and "M" tanks have a capacity of between 3,000 liters and 3,450 liters, according to DeAnza College. Apart from letters indicating capacity, medical oxygen cylinders in the United States also bear the designation U.S.P, standing for United States Pharmacopoeia, which indicates that the relevant regulatory authorities have approved the tank's contents for medical use.

Runtime is not only dependent on the tank's capacity, but also on the flow rate, which is measured in liters per minute, notes DeAnza College. For instance, at a flow rate of 12 liters per minute, a 415-liter tank runs out of gas after 35 minutes. A 300-liter to 350-liter "D" tank supplies oxygen for 30 minutes at a flow rate of 10 liters per minute, but administrators can extend that period by 20 minutes if they reduce the flow rate to 6 liters per minute. A 600-liter "E" tank can supply oxygen for nearly two hours at a flow rate of 6 liters per minute, or for just over an hour at a flow rate of 10 liters a minute.