An eye pressure range chart is read by understand that "mmHg" is millimeters of mercury, which is the scale for recording eye pressure, according to Glaucoma Research Foundation. Normal eye pressure varies throughout the day, but average pressure is somewhere between 10 and 21 millimeters of mercury, states WebMD.
When eye pressure is greater than 21 millimeters of mercury, this is referred to as ocular hypertension, according to WebMD. Not every person who has increased eye pressure develops glaucoma, states the National Institutes of Eye Health. Eye pressure lower than 10 millimeters of mercury is referred to as ocular hypotony, but it normally isn't a problem unless the eye pressure drops below 5 millimeters of mercury, according to Glaucoma Research Foundation. Low eye pressure can be caused by many things such as surgery, inflammation or retinal detachment.
Pressure inside the eye is measured during two or more visits to the eye doctor with a tonometer, states WebMD. The cause of eye pressure is important, and an ophthalmologist uses varying techniques to why the eye has a buildup or decrease in pressure. People with thin corneas are thought to be at a greater risk for an increase in eye pressure. Most often, high pressure in the eye does not cause any symptoms, which is why it is important to have checkups.