How To Calculate Residual Thickness After Lasik?

What Is Residual Stromal Bed Thickness?

The cornea is made up of five layers. The stroma—the thick, structural middle layer—is where LASIK does its work. During LASIK, a femtosecond laser creates a flap in the upper portion of the stroma, and an excimer laser then removes tissue from the exposed stromal bed to reshape the cornea and correct your prescription.

The residual stromal bed is what remains below the ablation zone once the laser has finished. Think of it as the foundation of your cornea after surgery. If this foundation is thick enough, the cornea maintains its dome-shaped curvature and structural resilience. If it’s too thin, the cornea is at risk of weakening over time.

To understand the structural changes involved, read about how LASIK surgery affects corneal biomechanics.

The Residual Stromal Bed Formula

The calculation is straightforward:

RSB = Central Corneal Thickness – Flap Thickness – Ablation Depth

Central Corneal Thickness (CCT)

This is the total thickness of your cornea at its centre, measured in microns (µm). The average human cornea is roughly 540 µm, but it can range anywhere from 470 µm to over 600 µm. Your surgeon measures this using ultrasound pachymetry or optical devices like the Pentacam.

Flap Thickness

In Femto LASIK, the femtosecond laser creates a corneal flap that is typically between 100 and 120 microns thick. The flap is lifted to expose the stromal bed, and once the correction is complete, it’s repositioned. The flap itself does not contribute to the structural load-bearing capacity of the cornea after surgery—which is exactly why it gets subtracted from the total.

For more on how flap dimensions influence outcomes, see LASIK flap thickness.

Ablation Depth

This is the amount of stromal tissue the excimer laser removes to correct your refractive error. Higher prescriptions require deeper ablations. As a rough guide, every 1.00 dioptre of myopia requires approximately 12–16 microns of tissue removal for a standard 6.5 mm optical zone. So a patient with –5.00 D myopia might need around 65–80 microns of ablation.

A Worked Example

Let’s say a patient has:

• Central corneal thickness: 540 µm
• Planned flap thickness: 110 µm
• Planned ablation depth: 75 µm (for –5.00 D myopia)

The calculation is:

540 – 110 – 75 = 355 µm residual stromal bed

At 355 µm, this patient is well above the accepted safety threshold and is a comfortable LASIK candidate.

Now consider a patient with a thinner cornea (490 µm) and a higher prescription (–8.00 D, requiring approximately 110 µm of ablation):

490 – 110 – 110 = 270 µm

This barely clears the minimum. Most surgeons would either reduce the optical zone, explore a flapless alternative like SMILE Pro, or recommend an implantable lens instead.

Why 250 Microns Is the Accepted Safety Minimum

The widely accepted minimum RSB is 250 microns. Below this threshold, the remaining cornea may lack sufficient tensile strength to resist intraocular pressure, increasing the risk of progressive corneal bulging—post-LASIK ectasia. This complication is rare, but when it does occur, it typically correlates with an RSB that was either miscalculated or pushed too close to the limit.

Many experienced surgeons, including those at Visual Aids Centre, prefer a more conservative minimum of 280–300 microns to provide an additional safety margin, particularly for younger patients whose corneas will need to remain stable for several decades.

Factors That Influence Residual Thickness

Starting Corneal Thickness

Patients with naturally thicker corneas (550 µm and above) have more tissue to work with, allowing higher corrections while maintaining safe RSB values. Thinner corneas (below 500 µm) narrow the margin quickly, especially for moderate to high prescriptions.

Explore your options if your cornea is thin at what if my cornea is too thin for LASIK.

Prescription Strength

Higher myopia, hyperopia, or astigmatism requires deeper ablation. A –2.00 D correction might only remove 30 µm of tissue, while –10.00 D could require 140+ µm. This is why very high prescriptions sometimes fall outside LASIK’s safe range.

See our guide on what is the maximum eye power for LASIK.

Optical Zone Diameter

Larger optical zones require more tissue removal. A 6.5 mm zone consumes more stroma than a 6.0 mm zone for the same prescription. Surgeons sometimes reduce the optical zone slightly to preserve RSB, though very small zones can increase glare and halos at night.

Flap vs. Flapless Procedures

Because a LASIK flap typically uses 100–120 microns, flapless procedures like SMILE Pro—which use no flap at all—preserve significantly more stromal tissue. This is one of the key reasons SMILE Pro is often recommended for patients with borderline corneal thickness.

Compare the structural advantages at SMILE Pro vs Femto LASIK.

What Happens If Your RSB Would Be Too Thin?

If the calculated RSB falls below the safety threshold, your surgeon won’t proceed with standard LASIK. But that doesn’t mean you’re out of options:

1. SMILE Pro – No flap means no 100–120 µm subtracted, leaving a thicker residual bed for the same correction.
2. PRK / TransPRK – Surface ablation removes tissue directly from the corneal surface without a flap, maximising residual thickness.
3. Implantable Collamer Lens (ICL) – A lens is placed inside the eye without altering the cornea at all. Ideal for very high prescriptions or very thin corneas.

How Visual Aids Centre Ensures Safe RSB Calculations

At Visual Aids Centre, every LASIK candidate undergoes corneal pachymetry mapping with the Pentacam—not just single-point thickness readings. This provides a detailed thickness profile across the entire cornea, catching any localised thinning that single-point measurements would miss.

The data feeds directly into the laser’s treatment planning software, and the RSB is verified at multiple checkpoints before the procedure begins.

Want to know your exact corneal thickness and LASIK eligibility? Book your pre-operative assessment today.