LASIK can correct up to about -12.0 diopters of nearsightedness, up to +6.0 diopters of farsightedness, and up to 6.0 diopters of astigmatism. Those are the upper boundaries at advanced treatment centers, though. The range your surgeon can safely correct depends on your corneal thickness, pupil size, and the specific laser platform being used.
Correction Limits by Vision Problem
Diopters are the unit eye doctors use to measure how strong your prescription is. The higher your number, the more reshaping the laser needs to do, and the more corneal tissue it removes. Here are the general maximums:
- Nearsightedness (myopia): Up to -12.0 diopters, with or without astigmatism up to 6.0 diopters
- Farsightedness (hyperopia): Up to +6.0 diopters, with or without astigmatism up to 5.0 diopters
- Mixed astigmatism: Up to 6.0 diopters
These figures come from UCLA Health’s refractive surgery program and represent what’s possible at a well-equipped center. Many clinics set their own cutoffs somewhat lower, and FDA-approved indications for specific laser platforms may be more conservative. For example, two widely used guided LASIK systems are approved for nearsightedness up to -9.0 diopters of total correction, with a maximum sphere of -8.0 diopters and cylinder (astigmatism) of -3.0 diopters.
In practical terms, if your glasses prescription is around -8.0 or lower with moderate astigmatism, you fall well within standard treatment ranges. Prescriptions between -8.0 and -12.0 are still treatable but require more tissue removal, which narrows the margin of safety and makes the next factor especially important.
Why Corneal Thickness Matters
LASIK reshapes your vision by removing microscopic layers of corneal tissue with an excimer laser. The stronger your prescription, the more tissue needs to come off. But you can’t remove too much, because the remaining cornea has to stay thick enough to hold its shape permanently. If it doesn’t, the cornea can gradually bulge forward in a condition called ectasia, which causes progressive vision distortion.
The critical measurement is called the residual stromal bed, the thickness of cornea left beneath the LASIK flap after the laser has finished. Most surgeons today aim to leave at least 300 microns (about 0.3 mm) of tissue intact. Some published research supports going as low as 275 microns in carefully selected patients with normal corneal topography and thin flaps, but 300 microns is the more widely accepted safety threshold.
The average human cornea is roughly 540 to 550 microns thick. The LASIK flap itself takes up about 90 to 120 microns depending on the technique. That leaves roughly 420 to 460 microns of tissue to work with, and after subtracting the 300-micron safety floor, you’re left with 120 to 160 microns available for reshaping. Each diopter of nearsightedness requires approximately 12 to 16 microns of tissue removal across the central cornea, so you can see how higher prescriptions start bumping up against the physical limits. Someone with a -10.0 prescription and a thinner-than-average cornea may not have enough tissue for a safe correction, even though -10.0 falls within the theoretical treatment range.
This is why your surgeon measures corneal thickness during your consultation. Two people with identical prescriptions can get different answers about whether LASIK is an option.
How Pupil Size Affects Outcomes
Your prescription might fall within the correctable range and your corneas might be thick enough, but pupil size adds another layer to the equation. When your pupils dilate in dim light, they can expand beyond the zone the laser treated. Light entering through the untreated outer cornea bends differently than light passing through the corrected center, creating halos, starbursts, and glare around lights at night.
Research on patients with moderate to high myopia (averaging about -8.75 diopters) found a significant correlation between large pupil size measured in the dark and worsened night vision after LASIK. Male patients were also more likely to report these disturbances. The visual quality during the day was typically fine; the problems showed up specifically in low-light conditions like nighttime driving.
Modern laser platforms use larger treatment zones than earlier generations, which has reduced this issue. Still, if your pupils dilate to 7 mm or more in the dark, your surgeon will factor that into the treatment plan and discuss the tradeoff between full correction and nighttime visual quality.
When LASIK Can’t Fully Correct Your Vision
For prescriptions near the upper limits, LASIK may intentionally undercorrect rather than push the treatment into an unsafe range. A surgeon might reduce a -11.0 prescription to -1.5 rather than risk removing too much tissue chasing perfect zero. You’d still see a dramatic improvement and could wear much thinner glasses or light-prescription contacts for the remainder.
Prescriptions beyond what LASIK can handle have other options. Implantable lenses sit inside the eye in front of the natural lens and can correct nearsightedness beyond -12.0 diopters. Photorefractive keratectomy (PRK) reshapes the cornea without creating a flap, which preserves more tissue and can sometimes treat people whose corneas are too thin for LASIK. For very high prescriptions, a lens-based procedure combined with laser touch-up can address what neither approach could manage alone.
What Determines Your Personal Range
The published diopter limits are maximums, not guarantees. Your individual correction range is shaped by several factors working together: your prescription strength, corneal thickness, corneal shape and regularity, pupil size in low light, and your age. Corneas that show any early signs of irregular thinning or bulging on topography mapping are typically disqualified regardless of prescription, because LASIK could accelerate that instability.
During a LASIK consultation, all of these measurements are taken and cross-referenced. The surgeon calculates exactly how much tissue removal your prescription requires, subtracts the flap thickness, and confirms the residual bed stays above the safety threshold. If the math doesn’t work, you’ll hear about it before you ever get near a laser. For most people with mild to moderate prescriptions (roughly -1.0 to -6.0 diopters), corneal thickness is rarely the limiting factor. It becomes the critical variable once prescriptions climb above -8.0 or so.