What is back vertex distance (BVD)?

The distance from the back surface of a spectacle lens to the front of the cornea — commonly assumed to be about 12 mm. It is part of how a glasses prescription is defined, which is why the same eye needs a different number at the cornea.

Why are my contact lens and glasses prescriptions different?

A lens moved closer to the eye changes its effective power. Converting between the spectacle plane and the cornea uses vertex compensation, the calculation this tool performs.

When does vertex distance actually matter?

As a rule of thumb, above about ±4.00 D. Below that, the compensation is smaller than a 0.25 D trial-lens step and is usually ignored clinically.

What formula does the calculator use?

Fc = F ÷ (1 − d·F) going from glasses to the cornea, and F ÷ (1 + d·F) going the other way, with d as the vertex distance in meters. Cylinder is compensated by converting each principal meridian separately.

What BVD does the tool assume?

It starts at the common 12 mm assumption and lets you change it — preset chips for 10, 12, 13.75 and 14 mm, or any value from 5 to 20 mm.

Can I order contact lenses from this result?

No. Vertex math is one ingredient; a contact lens prescription also depends on corneal measurements, lens design, material and an on-eye evaluation. We fit all of that in office, including scleral and specialty designs for high prescriptions.

Medically reviewed by Dr. Brandon Cohenmehr, OD Last reviewed October 24, 2026

Back Vertex Distance Calculator.

Convert a prescription between the spectacle plane and the cornea — the math behind why glasses and contacts never share the same numbers.

Starts at the standard 12 mm vertex distance — change it to whatever was measured. Sphere and cylinder are compensated by meridian, shown exact and rounded to the nearest 0.25 D.

Set it to the prescription

Same eye, two planes, one conversion.

Book a contact lens fitting Or call 310-828-2010

01The idea in one sketch

Move a lens, change its power.

A spectacle lens does its work about 12 millimeters in front of your eye. Slide that same power to the cornea, where a contact lens sits, and its effect changes: minus lenses act weaker as they approach the eye, plus lenses act stronger.

01
Below ±4.00 D: The difference is smaller than a trial-lens step; glasses and contact numbers usually match.
02
Beyond ±4.00 D: Compensation becomes mandatory: a −8.00 spectacle wearer needs roughly −7.25 at the cornea.
03
High prescriptions: The gap grows fast: the strongest prescriptions can shift a full diopter or more, which is why vertex distance is printed on some prescriptions.
04
Scleral & specialty lenses: Every custom lens we design is vertex-compensated from your measured refraction, which is one reason fittings beat conversions.

Specialty contact lenses →Glasses & contacts here →

Measuring vertex distance at the phoropter — d is measured, not guessed, at high powers

Common questions

Vertex distance, answered.

Anything else, call the office at 310-828-2010.

The distance from the back surface of a spectacle lens to the front of the cornea — commonly assumed to be about 12 mm. It is part of how a glasses prescription is defined, which is why the same eye needs a different number at the cornea.
A lens moved closer to the eye changes its effective power. Converting between the spectacle plane and the cornea uses vertex compensation, the calculation this tool performs.
As a rule of thumb, above about ±4.00 D. Below that, the compensation is smaller than a 0.25 D trial-lens step and is usually ignored clinically.
Fc = F ÷ (1 − d·F) going from glasses to the cornea, and F ÷ (1 + d·F) going the other way, with d as the vertex distance in meters. Cylinder is compensated by converting each principal meridian separately.
It starts at the common 12 mm assumption and lets you change it — preset chips for 10, 12, 13.75 and 14 mm, or any value from 5 to 20 mm.
No. Vertex math is one ingredient; a contact lens prescription also depends on corneal measurements, lens design, material and an on-eye evaluation. We fit all of that in office, including scleral and specialty designs for high prescriptions.

Go deeper

From conversion to a fitted lens.

The calculator shows why the numbers move; a fitting with a Los Angeles optometrist measures your eye and designs the lens — soft, toric, multifocal, or fully custom scleral designs for the prescriptions vertex math matters most for.

Book a Contact Lens Fitting Or call 310-828-2010