Eye pressure refers to the force of fluid pushing against the inside walls of your eyeball. The clinical term is intraocular pressure, or IOP. Normal eye pressure falls between 10 and 20 millimeters of mercury (mmHg), and keeping it in that range is essential for maintaining healthy vision. When pressure stays too high for too long, it can damage the optic nerve and lead to glaucoma.
How Fluid Creates Pressure Inside the Eye
Your eye constantly produces a clear fluid called aqueous humor. This fluid nourishes the front structures of the eye and helps the eyeball hold its shape. It’s made by a ring of tissue behind the iris called the ciliary body, which filters components from your blood and actively pumps them into the eye’s interior.
Once the fluid has done its job, it drains out through a spongy, mesh-like tissue near the base of the iris. About 75% of the resistance to drainage comes from this mesh structure itself, with the rest coming from the tiny channels beyond it. Your eye pressure at any given moment is the balance between how much fluid is being produced and how quickly it drains away. If drainage slows down or production increases, pressure rises. If too much fluid escapes (sometimes after eye surgery), pressure drops too low.
What Counts as Normal, High, or Low
The American Academy of Ophthalmology considers 10 to 20 mmHg the normal range. A reading above 20 mmHg without any signs of optic nerve damage is called ocular hypertension. It doesn’t automatically mean you have glaucoma, but it does mean your risk is elevated and your eye doctor will want to monitor you more closely.
On the low end, pressure below about 6.5 mmHg is considered statistically abnormal, and the World Glaucoma Association flags pressure at or below 5 mmHg as potentially problematic. Very low pressure can distort the structures inside the eye and threaten vision, though not everyone with low numbers develops complications.
Why High Pressure Damages Vision
The optic nerve exits the back of the eye through a structure called the lamina cribrosa, a thin, sieve-like plate that’s especially vulnerable to pressure. When pressure rises, it compresses the nerve fibers passing through this plate, choking off their supply of oxygen and nutrients. One of the earliest things that happens is a disruption of transport within the nerve fibers themselves. Growth signals that normally travel from the brain back to the nerve cells in the retina get blocked.
Over time, the affected nerve cells lose their branching connections, stop communicating with neighboring cells, and eventually die. This process causes a gradual thinning of the nerve fiber layer in the retina, which translates to blind spots in your peripheral vision. Because the damage builds so slowly, most people don’t notice it until significant vision has already been lost.
High Pressure Usually Has No Symptoms
This is the part that catches people off guard. Chronically elevated eye pressure typically produces zero symptoms. There’s no pain, no redness, no blurred vision in the early stages. The only way to catch it is through a routine eye exam. This is a major reason why eye doctors recommend regular pressure checks, particularly after age 40.
The exception is acute angle-closure glaucoma, a sudden spike in pressure that happens when the drainage pathway gets physically blocked. This is a medical emergency with unmistakable symptoms: severe eye pain, a bad headache, nausea or vomiting, blurred vision, halos or colored rings around lights, and a visibly red eye. If you experience these symptoms together, you need emergency care immediately.
How Eye Pressure Is Measured
The gold standard is Goldmann applanation tonometry. During this test, your eye doctor numbs your eye with drops, then gently touches a small probe to the surface of your cornea. The instrument measures how much force the eye pushes back with, and that reading becomes your pressure number. It’s quick and painless.
The “air puff” test, formally called non-contact tonometry, is what many people encounter at routine screenings. A machine blows a small burst of air at your eye and measures the response. It doesn’t require numbing drops, but it’s less accurate than the Goldmann method, so it works best as a screening tool rather than a definitive measurement. For young children and infants, doctors often use a handheld device called an iCare tonometer, which can take readings without any anesthetic drops at all.
Why Your Reading Might Be Misleading
Corneal thickness is the biggest variable that can throw off a pressure reading. Standard tonometry was calibrated for an average corneal thickness of about 550 micrometers. If your cornea is thinner than that, the instrument will read lower than your actual pressure. If your cornea is thicker, it’ll read higher. The error is roughly 0.4 mmHg for every 10-micrometer deviation from that average. At the extremes, this matters: someone with a very thin cornea (475 micrometers) could have a reading that’s more than 3 mmHg too low, potentially masking dangerously high pressure. Eye doctors can measure your corneal thickness with a separate test and apply a correction factor.
Eye Pressure Changes Throughout the Day
Your eye pressure isn’t a fixed number. It follows a 24-hour cycle, generally peaking during sleeping hours and dropping during the day. This might seem counterintuitive since fluid production actually falls by at least 50% at night. The likely explanation is that lying down changes the pressure in the veins around the eye, making it harder for fluid to drain even though less is being made. Body position plays a large role: much of the nighttime pressure rise disappears if you stay upright.
For people with untreated glaucoma, the pattern can differ. While their nighttime pressures also rise, their peak readings when lying down tend to occur during daytime hours rather than at night. This is one reason why a single office measurement doesn’t always capture the full picture of someone’s pressure profile.
Risk Factors for Elevated Pressure
Several factors increase your likelihood of developing higher-than-normal eye pressure. A large population study found that diabetes roughly doubled the odds of elevated pressure (adjusted odds ratio of 2.44), likely because the disease disrupts the automatic nerve signaling that helps regulate fluid dynamics inside the eye. Obesity carried about twice the risk as well, potentially because excess fat tissue around the eye, thicker blood, and increased oxidative stress on the drainage structures all impair fluid outflow.
High blood pressure is another consistent risk factor, with about a 32% increase in odds in the same study. The connection may involve elevated sympathetic nervous system activity driving both conditions simultaneously. Beyond these, the well-established risk factors include older age, a family history of glaucoma, and African or Hispanic ancestry. Having one or more of these factors doesn’t mean you’ll develop glaucoma, but it does mean regular eye exams with pressure measurements become more important.