Eye pressure, also called intraocular pressure (IOP), is a measure of how much fluid force exists inside your eye. Normal eye pressure falls between 10 and 20 mmHg. Readings outside that range can signal problems with fluid drainage in the eye, increased risk for glaucoma, or other underlying health conditions, though a single reading doesn’t tell the full story on its own.
How Eye Pressure Works
Your eye constantly produces a clear fluid called aqueous humor that nourishes the front of the eye and maintains its shape. An equal amount of this fluid drains out through a tiny mesh of tissue near the base of the iris. When production and drainage stay in balance, pressure holds steady. When drainage slows down or fluid production increases, pressure rises. When too much fluid escapes, often after surgery or injury, pressure drops.
This balance is surprisingly dynamic. Eye pressure is highest during sleeping hours and lower while you’re awake. In people without eye disease, the peak typically occurs at night while lying down. These natural fluctuations mean a single pressure reading is just a snapshot, which is why eye doctors sometimes track your pressure at different times of day if they’re concerned.
What High Eye Pressure Indicates
A reading above 20 mmHg with no other signs of eye damage is classified as ocular hypertension. This is not the same as glaucoma. It means the pressure inside your eye is elevated, but your optic nerve (the cable that carries visual information to your brain) still looks healthy and your visual field is intact. Roughly 3 to 6 million people in the United States have ocular hypertension, and most of them never develop glaucoma.
That said, elevated pressure is the single biggest risk factor for glaucoma. Glaucoma permanently damages the optic nerve, and it typically causes no symptoms until vision loss has already occurred. Doctors distinguish between the two using a five-factor model developed in a landmark clinical trial. The model considers your age, the pressure reading itself, corneal thickness, the structure of your optic nerve, and any irregularities in your peripheral vision. Only when there’s measurable nerve damage or visual field loss does the diagnosis shift from ocular hypertension to glaucoma.
Several health conditions raise your odds of developing elevated eye pressure. These include high blood pressure, diabetes, and extreme nearsightedness. Long-term use of steroid medications, whether eye drops, inhalers, or oral steroids, can also push pressure upward. Two less common conditions, pigment dispersion syndrome and pseudoexfoliation syndrome, where tiny particles clog the eye’s drainage system, increase risk as well.
What Low Eye Pressure Indicates
Low eye pressure gets less attention than high pressure, but it carries its own concerns. The statistical threshold for abnormally low pressure is below 6.5 mmHg, though international glaucoma guidelines use a cutoff of 5 mmHg or lower to define non-physiological pressure. This condition, called ocular hypotony, can cause blurry or distorted vision because the eye loses the internal support it needs to maintain its shape.
Low pressure most commonly occurs after eye surgery, particularly glaucoma procedures designed to improve fluid drainage. It can also result from trauma, inflammation inside the eye, or a retinal detachment. In rare cases, it signals a leak in the wall of the eye itself. If your doctor finds unusually low pressure, they’ll typically investigate whether fluid is escaping somewhere it shouldn’t be.
Why Your Reading Might Be Misleading
The most common tool for measuring eye pressure, called a tonometer, works by pressing against the surface of your cornea. The problem is that corneas vary in thickness from person to person, and this directly affects the reading. A thicker cornea resists the instrument more, producing a number that’s artificially high. A thinner cornea offers less resistance, making the reading appear lower than the true pressure inside.
The corrections are significant. Someone with a very thin cornea (around 445 microns) may need up to 7 mmHg added to their measured reading to get the real number. Someone with a very thick cornea (around 645 microns) may need 7 mmHg subtracted. Average corneal thickness sits around 535 to 545 microns, where little to no correction is needed. This is one reason many eye doctors measure your corneal thickness at least once, especially if your pressure reading is borderline.
Thin corneas also appear independently in the five-factor risk model for glaucoma. So a thin cornea doesn’t just mask higher true pressure; it’s a risk factor in its own right.
High Pressure Without Glaucoma
If your eye doctor tells you your pressure is elevated but you don’t have glaucoma, you’ll likely enter a monitoring schedule rather than start treatment immediately. Not everyone with high pressure needs medication. The decision depends on how elevated the pressure is, what your other risk factors look like, and whether there’s any hint of optic nerve changes over time.
For people who are diagnosed with open-angle glaucoma, follow-up visits become more frequent. When a target pressure is reached and the disease isn’t progressing, visits might happen every 6 to 12 months. If the nerve damage is worsening despite treatment, visits may be as often as every 1 to 2 months until the situation stabilizes. The goal is always to lower pressure enough to stop further nerve damage, since any vision already lost to glaucoma cannot be recovered.
What Your Eye Pressure Doesn’t Tell You
It’s worth understanding what eye pressure alone cannot reveal. Some people develop glaucoma at pressures that would be considered normal, a condition called normal-tension glaucoma. Others carry pressures above 20 mmHg for decades without any nerve damage. Pressure is one piece of the puzzle, not the whole picture.
This is why a comprehensive eye exam includes more than just the pressure check. Your doctor will also examine the optic nerve directly, often with imaging that maps its structure over time, and test your peripheral vision. Together, these measurements paint a much fuller picture of your eye health than any single number can.