Contact Lens Power Calculator
Convert your glasses prescription to contact lens power with vertex distance correction. Enter spectacle diopters and vertex distance for the corneal-plane power.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
Estimate for vertex conversion of sphere power only. A contact lens fitting (base curve, diameter, eye health) must be done by an eye-care professional. Not a substitute for a prescription.
How the Contact Lens Power Calculator Works
A glasses prescription is not the same as a contact lens prescription, even for the identical eye. Glasses sit about a centimetre in front of the eye, while a contact lens rests directly on the cornea. That gap, the vertex distance, changes the effective power of the correction. This calculator applies the standard vertex conversion so you can see the corneal-plane power.
Enter the spectacle sphere power in diopters and the vertex distance in millimetres. The tool shows the exact converted power and the nearest available 0.25 D lens step.
The Formula
The effective power at the cornea is:
F_contact = F_spectacle ÷ (1 − d × F_spectacle)
where d is the vertex distance expressed in meters (so 12 mm = 0.012 m) and F is in diopters. Sign matters: minus powers stay minus, plus powers stay plus.
Worked Example
Take a spectacle prescription of -6.00 D at a 12 mm vertex distance:
- d = 12 mm = 0.012 m
- Denominator = 1 − (0.012 × -6.00) = 1 − (-0.072) = 1.072
- F_contact = -6.00 ÷ 1.072 = -5.597 D
- Rounded to the nearest 0.25 D step: -5.50 D
So the -6.00 D glasses lens corresponds to roughly a -5.50 D contact lens, a meaningful change for a strong prescription.
Power and Vertex Effect
| Spectacle power | Approx. contact power (12 mm) |
|---|---|
| -2.00 D | -1.95 D |
| -6.00 D | -5.50 D |
| -10.00 D | -9.00 D |
| +6.00 D | +6.50 D |
| +10.00 D | +11.25 D |
What This Calculator Does Not Cover
This tool converts sphere power only. It does not handle cylinder and axis for astigmatism beyond the spherical component, near-add power for multifocals, base curve, or lens diameter. Contact lens wear also depends on tear film, corneal shape, and overall eye health. A licensed optometrist or ophthalmologist must perform a proper fitting and issue a valid contact lens prescription before you order or wear lenses.
Frequently asked questions
How do I convert my glasses prescription to contact lenses?+
Use the vertex distance formula: contact lens power equals the spectacle power divided by (1 minus vertex distance in meters times spectacle power). For a -6.00 D glasses prescription at a 12 mm vertex distance, the contact lens power is about -5.59 D, which rounds to -5.50 D.
What is vertex distance?+
Vertex distance is the gap between the back surface of your glasses lens and the front of your cornea, typically 10 to 14 mm. Because a contact lens sits directly on the eye, its effective power differs from glasses, and the difference grows with stronger prescriptions.
When does vertex distance correction matter?+
It becomes significant for prescriptions stronger than about ±4.00 D. Below that, the change is usually less than a 0.25 D step and can often be ignored. For strong myopia or hyperopia, skipping the correction can leave the lens noticeably over- or under-powered.
Do minus and plus lenses change in different directions?+
Yes. Minus (myopic) lenses become weaker at the cornea, so a -8.00 D glasses lens needs a slightly weaker contact lens. Plus (hyperopic) lenses become stronger, so a +8.00 D glasses lens needs a slightly stronger contact lens.
Can I order contacts using this result?+
No. This estimates sphere power only. A contact lens prescription also requires base curve, diameter, and an assessment of your eye health and fit by an eye-care professional. Use the result for understanding, not for ordering lenses without a valid contact lens prescription.