Eye Pressure (IOP) Calculator: Corneal-Thickness Adjustment and Glaucoma Risk
Interpret an eye pressure (IOP) reading against the normal 10–21 mmHg range, adjust it for central corneal thickness (pachymetry), and optionally estimate the 5-year risk of glaucoma in ocular hypertension with the OHTS-EGPS points system. A risk note, not a diagnosis.
How it works
Adjusted IOP = measured IOP + (545 − CCT) × 0.02 mmHg per µm (0.5 mmHg per 25 µm) · normal 10–21 mmHg, over 21 ocular hypertension, 30 or more markedly elevated · OHTS-EGPS points: 0–4 each for age, IOP, CCT, vertical cup/disc and PSD; total 0–6 ≤4%, 7–8 10%, 9–10 15%, 11–12 20%, 13+ ≥33% five-year risk
Goldmann applanation tonometry, the standard eye-pressure test, assumes a cornea of average thickness. A thinner cornea is easier to flatten, so the pressure reads lower than it really is; a thicker one reads higher. A meta-analysis by Doughty and Zaman found that in normal eyes a 10% difference in corneal thickness shifts the reading by about 1.1 mmHg, roughly 0.2 mmHg for every 10 µm. This calculator applies that rule around a 545 µm reference (you can change it under More options) and places the result in the usual bands: 10–21 mmHg normal, above 21 ocular hypertension. Pressure alone does not diagnose glaucoma. Many people with ocular hypertension never develop it, and some get glaucoma at normal pressure. For adults with untreated ocular hypertension, the Ocular Hypertension Treatment Study and European Glaucoma Prevention Study (OHTS-EGPS) built a validated risk model. Its points version gives 0–4 points each for age, pressure, corneal thickness, vertical cup-to-disc ratio and visual-field pattern standard deviation (PSD), using the average of both eyes. Turn on the risk option to add it. The model uses the measured pressure and thickness, because adjusting the pressure did not improve its predictions. A thin cornea is a risk factor in its own right. All of this is a guide for a conversation with your eye doctor, who looks at the optic nerve, visual field and history before deciding on monitoring or treatment.
Worked example
A measured pressure of 16 mmHg with 545 µm corneas needs no adjustment: (545 − 545) × 0.02 = 0, so the adjusted IOP is 16.0 mmHg, in the normal 10–21 mmHg range. With thin 495 µm corneas the same reading would gain (545 − 495) × 0.02 = +1.0 mmHg, giving 17.0 mmHg. For the risk score, the study’s own example is a 55-year-old with a pressure of 24 mmHg, 540 µm corneas, cup-to-disc 0.4 and PSD 2.2 dB. That scores 2 + 2 + 3 + 2 + 2 = 11 points, an estimated 5-year glaucoma risk of about 20%.
Sources: Gordon MO et al. Validated prediction model for the development of POAG in ocular hypertension (OHTS-EGPS). Ophthalmology 2007 · OHTS: points system for estimating 5-year risk of POAG · Brandt JD et al. Adjusting intraocular pressure for central corneal thickness does not improve prediction models for POAG. Ophthalmology 2012 · Eye (Nature): CCT and IOP, citing the Doughty & Zaman meta-analysis
Frequently asked questions
What is a normal eye pressure?
Usually 10 to 21 mmHg. Values above 21 are called ocular hypertension and raise glaucoma risk, though the pressure at which nerve damage starts varies between people.
Why adjust for corneal thickness?
The standard tonometer is calibrated for an average cornea. Thin corneas make the reading underestimate the true pressure and thick corneas overestimate it, so thickness (pachymetry) helps interpret borderline readings.
What is a normal corneal thickness?
Most adult corneas are about 520–580 µm, averaging roughly 540–555 µm. In the OHTS, people with corneas of 555 µm or less had a clearly higher risk of developing glaucoma than those above 588 µm.
How is the 5-year glaucoma risk worked out?
The OHTS-EGPS points system gives 0–4 points for each of age, eye pressure, corneal thickness, vertical cup-to-disc ratio and visual-field PSD. The total maps to a 5-year risk, from 4% or less (0–6 points) to 33% or more (13 or more points).
Does normal pressure mean I do not have glaucoma?
No. Normal-tension glaucoma occurs at normal pressures, and high pressure does not always cause glaucoma. Diagnosis needs an eye specialist to assess the optic nerve and visual field.
When is high eye pressure an emergency?
A sudden painful red eye with blurred vision, halos, headache or nausea can be acute angle-closure glaucoma. It needs same-day emergency care.
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Educational clinical estimate only, not a diagnosis, medical advice or treatment. Eye pressure and risk scores must be interpreted by an eye-care professional alongside an optic-nerve and visual-field examination. Never start, stop or change glaucoma treatment based on this tool.