Contact Lens Vertex Calculator
Convert a glasses prescription to the effective power at the cornea for contact lenses. Enter the spectacle sphere power in diopters (minus for nearsighted, plus for farsighted) and the vertex distance in mm (12 mm is typical).
Example: with Spectacle power (diopters) -8 · Vertex distance (mm) 12 → Effective power at cornea: -7.30 D.
- Nearest 0.25 D contact power-7.25 D
- Does it matter here?At this power the vertex shift is clinically meaningful — confirm the final power with your fitter.
Computed by the calculator below using its default values. Change any input to see your own numbers.
Vertex formula: Fc = F ÷ (1 − d·F), with d the vertex distance in meters. Minus (myopic) lenses need less power as contacts; plus (hyperopic) lenses need more.
Why vertex distance changes the power
A lens sitting 12 mm in front of your eye bends light differently than one resting on the cornea. That small gap, the vertex distance, matters because a lens's effect depends on how far it is from the eye. Move a strong lens closer, and its effective power changes: minus lenses that correct nearsightedness act stronger up against the eye, so a contact needs less minus power. Plus lenses for farsightedness act weaker at the eye, so a contact needs more plus power.
The size of the shift grows with the prescription. Below about 4 diopters it is under a quarter diopter and gets rounded away. Beyond that — a −8.00 glasses lens becomes roughly −7.25 as a contact — ignoring it would leave the wearer meaningfully under- or over-corrected.
How it’s calculated
Effective (contact) power Fc = F ÷ (1 − d × F), where F is the spectacle power in diopters and d is the vertex distance in meters (12 mm = 0.012 m). The result is rounded to the nearest 0.25 D, the step contact lenses come in.
Applies to the spherical component; strong cylinder (astigmatism) and toric fitting need separate handling. Actual vertex distance and fit vary, so the final contact power is set by an eye-care professional, not this estimate.
Vertex shift at 12 mm (spectacle to contact)
| Spectacle power | Effective at cornea | Nearest contact power |
|---|---|---|
| -4.00 D | -3.82 D | -3.75 D |
| -8.00 D | -7.30 D | -7.25 D |
| -12.00 D | -10.49 D | -10.50 D |
| +4.00 D | +4.20 D | +4.25 D |
| +8.00 D | +8.85 D | +8.75 D |
Computed with Fc = F/(1 − 0.012·F); rounded to the nearest 0.25 D.
Common mistakes
- Skipping vertex compensation on strong prescriptions above about 4 diopters.
- Getting the sign wrong: minus lenses lose power as contacts, plus lenses gain power.
- Entering vertex distance in cm instead of mm, inflating the shift tenfold.
- Applying this to the cylinder axis without proper toric calculation.
Frequently asked questions
What is the vertex distance formula?
Effective power equals the spectacle power divided by (1 minus the vertex distance in meters times the spectacle power): Fc = F/(1 − d·F). At 12 mm, a −8.00 glasses lens becomes about −7.30 D at the cornea.
When does vertex conversion matter?
Roughly at 4 diopters and above. Below that the change is less than a quarter diopter and is usually ignored. Strong prescriptions, near ±8 D and beyond, shift enough to affect vision noticeably.
Why do minus and plus lenses move opposite ways?
Bringing a lens closer to the eye increases the effective power of a minus lens and decreases that of a plus lens. So myopic contacts need less minus, and hyperopic contacts need more plus, than the glasses.
Can I order contacts from this number?
No. Use it to understand the math. A proper contact lens fitting also accounts for astigmatism, tear film, curvature, and comfort, and must be done by an optometrist or ophthalmologist.