Gas cylinders are identified using the letters A to M; cylinder volume increases as the letters of the alphabet proceeds.
For example, an H size cylinder holds about 6,900 liters.

Larger cylinders are most often used in hospitals and long-term care facilities as back up to the central gas systems. Smaller, more portable cylinders are commonly used to transport patients between rooms, for emergency medical services and homecare.

An E sized cylinder holds 660 - 690 liters (size varies slightly by provider) and is the most commonly used size in medical settings.