Eye pressure—also called intraocular pressure or IOP—is a measurement of the fluid pressure inside the eye. Measuring it is like measuring blood pressure.
The eye has a jelly-like substance called vitreous humor filling most of the back part of the eye. A more-watery liquid called aqueous humor also is present. Much of the aqueous humor is in the front part of the eye, behind the cornea and in front of the iris.
In a healthy eye, a small amount of new aqueous humor is always entering the eye while an equal amount drains out. Most of the aqueous humor flows out of the eye through the drainage angle, in front of the iris. This equal flow maintains a stable pressure.
Diagram of aqueous humor and drainage angle system of the eye. The yellow arrow shows the flow of aqueous humor through and out of the front of the eye in a healthy eye.
What Is Normal Eye Pressure?
Eye pressure is measured in millimeters of mercury, like the way a thermometer measures temperature using mercury. Normal eye pressure is usually considered to be between 10 and 20 millimeters of mercury (mmHg). Having eye pressure that’s too low or too high can damage your vision.
Elevated eye pressure with no other symptoms is ocular hypertension. Some people can have higher eye pressure with no damage. Other people may lose vision even if the pressure is in the normal range.
When someone has glaucoma, eye pressure damages the optic nerve. This damage permanently reduces vision. If glaucoma is not treated, it can lead to total blindness.