An eye pressure test, formally called tonometry, measures the fluid pressure inside your eyes. The result helps your eye doctor assess your risk for glaucoma, a group of conditions that damage the optic nerve and can lead to permanent vision loss. Normal eye pressure falls between 10 and 20 millimeters of mercury (mmHg), and the test takes only a few seconds per eye.
Why Eye Pressure Matters
Your eyes constantly produce and drain a clear fluid that maintains their shape. When drainage slows or fluid production increases, pressure builds inside the eye. That pressure can compress and damage the optic nerve over time, which is the core mechanism behind glaucoma. Eye pressure is the single most important modifiable risk factor for the disease, making it the number your doctor watches most closely.
Pressure inside the eye also fluctuates throughout the day and can peak outside of office hours, which means a single reading doesn’t always capture the full picture. If your doctor suspects glaucoma or sees borderline numbers, you may be asked to come back for repeat measurements at different times of day.
What Happens During the Test
There are several ways to measure eye pressure, but two methods account for most routine exams: the air puff test and the contact method done at the slit lamp.
Air Puff Test (Non-Contact Tonometry)
This is the version most people recognize. You rest your chin on a support, stare at a target light, and a small burst of air hits the surface of your eye. The device measures how long it takes the air to flatten a tiny area of your cornea, then calculates the pressure from that timing. No drops, no instruments touching your eye. The puff is startling but not painful. Because each reading covers a very short instant, the technician usually takes several puffs and averages them.
The air puff test is convenient and can be performed by non-medical staff, which is why it’s common in optometry offices as a screening tool. Its readings can differ from the gold-standard method by several mmHg, especially when pressures are in the low 20s or above.
Contact Method (Goldmann Applanation Tonometry)
This is considered the gold standard. Your doctor places numbing drops and a yellow-orange dye called fluorescein in your eyes, then positions you at the slit lamp (the chin-rest microscope used during most eye exams). A small, flat-tipped probe gently touches the surface of your cornea and measures exactly how much force is needed to flatten a tiny circle about 3 millimeters across. You’ll see a blue light during the measurement but won’t feel the probe because of the anesthetic drops.
The whole process is quick. After the test, avoid rubbing or touching your eyes until the numbness wears off, typically within 15 to 30 minutes. While your eyes are still numb, you won’t feel pain normally, so it’s easier to accidentally scratch the cornea without realizing it.
Handheld Devices
Portable tonometers come into play when a patient can’t sit upright at the slit lamp. People with mobility issues, hospitalized patients, children, and anyone being examined under anesthesia may have their pressure checked with a pen-sized device instead. Some newer handheld models work so quickly and gently that they’re especially useful for young children or uncooperative patients, since the measurement can be taken through a barely open eyelid in any position, including lying down.
What Your Numbers Mean
A reading between 10 and 20 mmHg is generally considered normal. A reading above 20 mmHg without any signs of optic nerve damage is classified as ocular hypertension. That doesn’t mean you have glaucoma, but it does put you in a higher-risk category that warrants closer monitoring.
It’s worth knowing that some people develop glaucoma even with pressures in the normal range (called normal-tension glaucoma), while others maintain healthy optic nerves despite elevated readings for years. Pressure is one important data point, not the whole diagnosis. Your doctor will also examine the optic nerve directly, test your peripheral vision, and sometimes measure the thickness of your cornea to interpret your pressure reading more accurately.
Why Corneal Thickness Changes the Reading
The standard tonometer was designed around an average corneal thickness of about 520 micrometers. If your cornea is thicker than average, the test overestimates your true pressure because the instrument needs more force to flatten stiffer tissue. If your cornea is thinner, the opposite happens: your reading comes back artificially low, potentially masking a problem.
The error can be meaningful. Research on cannulated eyes (where true internal pressure was directly measured) found that deviations from the assumed corneal thickness could skew readings by as much as 7 mmHg per 100 micrometers of difference. In a large clinical trial studying ocular hypertension, corneal thickness turned out to be the single strongest baseline predictor of who would go on to develop glaucoma. Many patients with thick corneas and elevated readings actually had normal true pressures and didn’t need treatment. Conversely, patients with thin corneas were sometimes undertreated because their readings looked reassuringly low.
This is why your doctor may perform a separate, painless measurement of corneal thickness called pachymetry, particularly if your pressure is borderline or you have risk factors for glaucoma.
How Often You Need the Test
Eye pressure is checked as part of a comprehensive eye exam, so the recommended testing schedule follows the broader exam guidelines. The American Academy of Ophthalmology recommends that adults with no risk factors get a baseline comprehensive eye exam at age 40. After that:
- Ages 40 to 54: every 2 to 4 years
- Ages 55 to 64: every 1 to 3 years
- Age 65 and older: every 1 to 2 years
If you’re at higher risk for glaucoma, the timeline accelerates. African Americans, who have a significantly elevated risk for glaucoma, are advised to start screening earlier and return more frequently: every 2 to 4 years before age 40, every 1 to 3 years from 40 to 54, and every 1 to 2 years from 55 to 64. Other risk factors that may warrant earlier or more frequent screening include a family history of glaucoma, very high nearsightedness, previous eye injuries, and long-term corticosteroid use.