How do you correct IOP for CCT?

Subtract the CCT reference of 520 µm from the measured CCT, multiply by 0.07, and subtract that from the measured IOP: corrected IOP = IOP − (CCT − 520) × 0.07. For example, 22 mmHg at 480 µm becomes 22 − (−40 × 0.07) = 24.8 mmHg, and 22 mmHg at 560 µm becomes 22 − 2.8 = 19.2 mmHg.

Is there an IOP correction chart for corneal thickness?

Yes. With the Ehlers formula each 10 µm from 520 µm changes IOP by 0.7 mmHg: 480 µm adds 2.8, 500 µm adds 1.4, 520 µm adds 0, 540 µm subtracts 1.4, 560 µm subtracts 2.8 and 600 µm subtracts 5.6 mmHg. The chart below the calculator lists 440 to 640 µm for the Ehlers, Dresden and Doughty-Zaman formulas.

Which IOP correction formula is best?

None is validated for individual patients. Ehlers (1975) came from manometry in 29 eyes and gives the largest correction, about 0.7 mmHg per 10 µm from 520 µm. The Dresden table (Kohlhaas 2006, manometry in 125 eyes) gives about 0.4 mmHg per 10 µm around 550 µm, and the Doughty-Zaman meta-analysis about 0.5 mmHg per 10 µm from 535 µm. When the formulas disagree by more than 1 to 2 mmHg, treat the corrected value as a range, not a number.

Does correcting IOP for CCT improve glaucoma risk prediction?

No. In the Ocular Hypertension Treatment Study, prediction models using IOP corrected by the Ehlers, Whitacre, Orssengo-Pye, Doughty-Zaman or Kohlhaas formulas were no better than the original model using measured IOP and CCT separately (Brandt et al., Ophthalmology 2012). Thin CCT stays a risk factor in its own right.

How does central corneal thickness (CCT) affect IOP readings?

Goldmann applanation tonometry measures the force needed to flatten a 3.06 mm diameter area of cornea. A thicker, stiffer cornea resists flattening more, so the tonometer records a falsely high pressure. A thinner cornea offers less resistance, leading to a falsely low reading. The reference standard is 520 µm CCT.

What is the Ehlers correction formula?

The Ehlers formula adjusts IOP by approximately 0.7 mmHg for every 10 µm deviation from the 520 µm reference CCT. If CCT is greater than 520 µm, the correction subtracts from measured IOP; if less than 520 µm, it adds to measured IOP. This tool applies that correction automatically.

Is corrected IOP more accurate than measured IOP?

Not necessarily in all cases. The Ehlers formula is a population-level correction that may not perfectly apply to individual patients. Many guidelines recommend using CCT as a risk factor for glaucoma rather than directly adjusting the IOP value. Consult a qualified ophthalmologist for clinical decisions.

What is a normal IOP range?

A commonly cited normal range is 10–21 mmHg. However, the upper limit of normal varies by population and individual risk factors. Ocular hypertension is typically defined as IOP above 21 mmHg without glaucomatous damage. Some patients develop glaucoma at lower pressures (normal-tension glaucoma).

What CCT is considered thin vs. thick?

Average CCT in adults is about 540 to 555 µm by ultrasound pachymetry. In the Ocular Hypertension Treatment Study, eyes with CCT of 555 µm or less had about three times the risk of developing glaucoma of eyes above 588 µm, so 555 µm or less is often called thin and above 588 µm thick. After LASIK or PRK the cornea is thinner because tissue was removed, and Goldmann readings are lower for that reason.

Should I correct IOP after LASIK or PRK?

With caution. Refractive surgery removes stromal tissue and changes corneal stiffness, so Goldmann readings usually fall more than CCT formulas predict, and corrected values can still underestimate true pressure. Compare with pre-operative readings and consider tonometers less affected by the cornea, such as dynamic contour tonometry or the Ocular Response Analyzer.

Should I use this correction for non-contact tonometry?

Non-contact (air-puff) tonometry is also influenced by CCT, but the correction factors differ from Goldmann applanation. This tool applies the Ehlers correction derived from Goldmann tonometry data. For other instruments, consult device-specific literature.

Intraocular Pressure Correction Calculator

Enter a Goldmann IOP and central corneal thickness to compare the Ehlers, Dresden and Doughty-Zaman corrected IOP, the spread between them and the OHTS thickness risk group.

IOP correction for CCT adjusts a Goldmann reading for corneal thickness.

Tonometry Inputs

Measured IOP (mmHg)

Goldmann applanation tonometry reading. Typical range: 8–40 mmHg.

Central Corneal Thickness (µm)

Measured by pachymetry. Adult average is about 540–555 µm.

Headline formula

Previous LASIK, PRK or SMILE in this eye

Ehlers: IOP − (CCT − 520) × 0.07
Dresden: IOP + 23.28 − 0.0423 × CCT
Doughty-Zaman: IOP − (CCT − 535) × 0.05

Result

Enter values and click Calculate.

Corrected IOP (Ehlers)

mmHg

Measured IOP

CCT entered

CCT deviation from 520 µm

Correction applied

Corrected IOP by formula
Formula	Correction	Corrected

Not medical advice. Correction formulas are population-derived, disagree with each other and did not improve glaucoma risk prediction in the OHTS. All IOP interpretation should involve a qualified eye care professional.

IOP Correction Chart by Corneal Thickness

mmHg added (+) to or subtracted (−) from the Goldmann reading. Ehlers and Doughty-Zaman do not depend on the IOP; Dresden is the Kohlhaas 2006 equation.

CCT (µm)	Ehlers	Dresden	Doughty-Zaman	OHTS group
440	+5.6	+4.7	+4.8	Thin (≤555)
460	+4.2	+3.8	+3.8	Thin (≤555)
480	+2.8	+3.0	+2.8	Thin (≤555)
500	+1.4	+2.1	+1.8	Thin (≤555)
520	0.0	+1.3	+0.8	Thin (≤555)
540	−1.4	+0.4	−0.3	Thin (≤555)
560	−2.8	−0.4	−1.3	Middle (556–588)
580	−4.2	−1.3	−2.3	Middle (556–588)
600	−5.6	−2.1	−3.3	Thick (>588)
620	−7.0	−2.9	−4.3	Thick (>588)
640	−8.4	−3.8	−5.3	Thick (>588)

Sources: Ehlers N et al., Acta Ophthalmol 1975; Kohlhaas M et al., Arch Ophthalmol 2006; Doughty MJ, Zaman ML, Surv Ophthalmol 2000; Brandt JD et al. (OHTS), Ophthalmology 2012.

Quick answer

IOP correction for CCT adjusts a Goldmann reading for corneal thickness. Ehlers: corrected IOP = IOP − (CCT − 520) × 0.07, about 0.7 mmHg per 10 µm. The Dresden (Kohlhaas) formula, IOP + 23.28 − 0.0423 × CCT, corrects about 0.4 mmHg per 10 µm around 550 µm, so the formulas can differ by several mmHg for very thin or thick corneas.

Example: Measured IOP 22 mmHg with CCT 480 µm: Ehlers adds (520 − 480) × 0.07 = 2.8 mmHg for 24.8 mmHg; Dresden adds 23.28 − 0.0423 × 480 = 2.98 mmHg for 25.0 mmHg

Enter your IOP and pachymetry reading for the corrected value and CCT category.

Summary

Goldmann applanation tonometry—the clinical standard for measuring intraocular pressure—assumes a central corneal thickness (CCT) of 520 µm. Thicker corneas cause the tonometer to overestimate true IOP; thinner corneas cause underestimation. The Ehlers correction formula adjusts measured IOP by approximately 0.7 mmHg per 10 µm deviation from the 520 µm reference, giving a corrected value that better reflects actual aqueous humor pressure. This correction is relevant when screening for or managing glaucoma, particularly in patients with unusually thin or thick corneas such as those who have had LASIK surgery.

How it works

Enter the IOP measured by Goldmann applanation tonometry (in mmHg).
Enter the central corneal thickness (CCT) measured by pachymetry (in micrometers, µm).
The calculator determines the deviation from the reference CCT of 520 µm.
It applies the Ehlers correction: adjusted IOP = measured IOP − ((CCT − 520) × 0.7 / 10).
It also applies the Dresden (Kohlhaas 2006) formula, IOP + 23.28 − 0.0423 × CCT, and the Doughty-Zaman linear summary of 2.5 mmHg per 50 µm from 535 µm, and shows the spread between the three.
The chosen formula sets the headline value, shown with the OHTS corneal thickness group and a clinical interpretation note.

Use cases

Adjusting IOP readings for patients with thin corneas after LASIK or PRK.
Interpreting borderline IOP in glaucoma suspects with unusual corneal thickness.
Ophthalmology teaching and self-study on tonometry correction methods.
Pre-operative assessment for patients undergoing corneal refractive surgery.
Monitoring glaucoma patients where pachymetry is available.
Comparing corrected vs. uncorrected IOP to assess clinical significance.

Frequently Asked Questions

Last updated: 2026-10-09 · Reviewed by Nham Vu