Corneal thickness is measured using a quick, painless test called pachymetry, which takes only a few seconds per eye. There are two main approaches: an ultrasound method where a small probe briefly touches your eye, and optical methods that scan your eye without any contact. The average healthy cornea measures about 515 microns (roughly half a millimeter) at its center, and knowing your number matters for glaucoma screening, refractive surgery planning, and detecting conditions like keratoconus.
Ultrasound Pachymetry
Ultrasound pachymetry is the most commonly used method and has long been considered the clinical standard. A small, pen-shaped probe is placed directly on the surface of your cornea. Before it touches your eye, your provider applies numbing drops so you won’t feel discomfort. The probe sends ultrasound waves through the cornea and calculates thickness based on how quickly they bounce back. The device typically takes several readings in rapid succession and averages them to give a final number.
The whole process lasts only a few seconds per eye. Because it involves direct contact, there are a few limitations: the probe needs to be aligned perpendicular to your cornea for an accurate reading, and pressing the probe against the surface can slightly compress the tissue or displace the tear film, introducing small measurement errors. Still, this method remains widely trusted and is the benchmark that newer technologies are compared against.
Optical (Non-Contact) Methods
Several technologies can measure corneal thickness without touching your eye at all. You sit with your chin and forehead resting against a machine while it captures images or scans of your cornea. The main non-contact options include:
- Optical coherence tomography (OCT): Uses light waves to create a cross-sectional image of the cornea. Slit-lamp OCT shows the closest agreement with ultrasound pachymetry, with measurements differing by only about 1.2 microns on average.
- Specular microscopy: Photographs the inner layer of corneal cells and calculates thickness from the image. It correlates well with ultrasound but tends to read slightly higher, around 8 to 9 microns thicker on average.
- Scanning slit topography (Orbscan): Projects a slit of light across the cornea and reconstructs its shape. Readings differ from ultrasound by about 3 microns on average, though individual measurements can vary more widely.
- Scheimpflug imaging (Pentacam, Galilei): Rotates a camera around the eye to photograph the cornea from multiple angles, building a full thickness map from the center to the periphery. This is the go-to method when your provider needs to see thickness across the entire cornea, not just one central point.
All of these non-contact methods correlate strongly with ultrasound (r² of 0.96 to 0.98), meaning they produce very similar results. The choice between them often depends on what equipment a particular office has and what clinical question needs answering.
What to Expect During the Test
No special preparation is needed. If you wear contact lenses, your provider may ask you to remove them before the measurement, since lenses can temporarily alter corneal shape and thickness. For routine pachymetry, removing lenses at least an hour beforehand helps avoid lens-related mechanical changes to the cornea. If you’re being evaluated for refractive surgery, your surgeon will likely ask you to leave contacts out for several days to a few weeks, depending on the lens type.
For ultrasound pachymetry, numbing drops go in first. The probe touches your cornea briefly, and then you’re done. Your vision may be slightly blurry afterward from the drops, so having someone drive you home can be helpful, especially if your pupils were also dilated during the same visit. For optical methods, nothing touches your eye. You simply look into the machine while it scans.
Normal Corneal Thickness
A healthy central cornea typically measures around 515 microns, but normal values range from about 420 to 625 microns. Your cornea is not the same thickness everywhere. It’s thinnest near the center (the thinnest point usually sits about 1.5 millimeters toward the temple side of the center), and it gradually thickens toward the edges. In the peripheral area, thickness can reach 633 to 673 microns.
Because there’s such a wide normal range, a single thickness number doesn’t tell the full story. What matters is how your measurement fits into the context of your other eye health data.
Why It Matters for Glaucoma
Corneal thickness directly affects how accurately your eye pressure can be measured. The standard eye pressure test (Goldmann applanation tonometry) was designed assuming a corneal thickness of 500 microns. If your cornea is thinner than that, the tonometer tends to underestimate your true eye pressure. If your cornea is thicker, it tends to overestimate it.
This is a big deal for glaucoma screening. Someone with naturally thin corneas might have higher true eye pressure than their readings suggest, putting them at risk for undetected glaucoma. Conversely, someone with thick corneas might undergo unnecessary treatment for eye pressure that looks high on the reading but is actually normal. Knowing your corneal thickness lets your eye doctor interpret pressure readings more accurately and make better treatment decisions.
Why It Matters for LASIK and PRK
Laser vision correction works by reshaping the cornea, which means removing tissue. If too much tissue is removed and the remaining cornea is too thin, it can weaken and bulge forward over time, a complication called ectasia. This is why precise thickness measurement before surgery is essential.
The key safety number isn’t just total thickness but how much corneal tissue remains after the laser does its work. Surgeons aim to leave at least 275 to 300 microns of the structural middle layer (the stroma) intact beneath the LASIK flap. A preoperative corneal thickness below 500 microns is considered a risk factor for ectasia, and a final post-surgical thickness below 400 microns has been proposed as a lower safety limit, though the residual stromal bed thickness and a normal preoperative corneal shape are considered even more important than total thickness alone.
For this reason, candidates for refractive surgery get full corneal mapping, not just a single central reading. Scheimpflug imaging or similar tomography devices create a point-by-point thickness map so the surgeon can identify the thinnest spot on the cornea and plan accordingly.
Detecting Keratoconus
Keratoconus is a condition where the cornea progressively thins and bulges into a cone shape, distorting vision. Corneal thickness mapping is one of the most important tools for catching it early, sometimes before symptoms appear.
In manifest keratoconus, the thinnest point on the cornea averages around 438 microns, well below the 515-micron average for healthy eyes. Even in subclinical cases (where standard tests look nearly normal), the thinnest point averages about 458 microns. The difference between the thinnest spot and the thickest spot also widens significantly in keratoconus, because the central or lower cornea thins while the periphery stays relatively thick.
Modern diagnostic approaches look at specific patterns on the thickness map. In keratoconus, the thinnest point on the cornea and the thinnest point on the surface cell layer (the epithelium) tend to cluster in the same area, typically in the lower or lower-temple region. The epithelial layer also shows a distinctive concentric thinning pattern around the thinnest spot. These map patterns, combined with the thickness numbers, let clinicians detect the disease at very early stages, which is critical because early treatment can slow or stop progression.
Contact vs. Non-Contact: Which Is Better?
Neither method is definitively superior. Ultrasound pachymetry is widely available, inexpensive, and well validated. Its main drawbacks are the need for numbing drops and the small variability introduced by probe placement. Non-contact optical methods eliminate the need to touch the eye and are highly reproducible, but the equipment costs more and may not be available in every practice.
For a simple central thickness check during a routine eye exam, ultrasound works perfectly well. For surgical planning, keratoconus screening, or any situation where a full corneal thickness map is needed, Scheimpflug imaging or OCT-based systems are the better choice because they capture data across the entire cornea in a single scan. Many specialty practices use both: a quick ultrasound measurement to confirm a central reading, and a full tomographic scan to see the bigger picture.