Vertex Distance Calculator — Glasses to Contact Lens Power
Convert glasses power to contact lens power (or back) using the vertex distance formula, plus the ±4.00 D clinical significance threshold.
Bikash Roy
Online Research Specialist · SEO Strategist · Web Developer
Bikash personally verifies every calculator against official standards (WHO, NIH, government data). Online research specialist, SEO strategist, and web developer with 10+ years building accurate digital tools.
Full bio →Corrected Power = F ÷ (1 − d × F), where F is the original lens power and d is the vertex distance in meters. Converting glasses power to contact lens power uses a positive d (moving the correction closer to the eye); converting contact lens power back to glasses power uses the algebraic inverse (moving the correction away from the eye). This is a vertex-distance power conversion only — it does not replace an eye doctor's full contact lens fitting exam, which also accounts for corneal curvature, tear film, and lens material.
Reference Values
Last verified:| Category | Range | What It Means | Status |
|---|---|---|---|
| Vertex Distance Formula ★ | Corrected Power = F ÷ (1 − d × F) | F = original lens power in diopters, d = vertex distance in meters. Used to convert a spectacle (glasses) power to the equivalent contact lens power, or a contact lens power back to the equivalent spectacle power. | ★ Best |
| Standard Vertex Distance ★ | 12 mm (0.012 m) | The most common default vertex distance used for glasses-to-contact-lens power conversions in everyday prescribing. | ★ Best |
| Alternate Vertex Distance | 13.5 mm (0.0135 m) | A slightly longer vertex distance some practices use as their default, closer to a typical phoropter measurement (about 13.75 mm) or a frame that sits further from the eye. | Good |
| Clinically Significant Threshold | ±4.00 D or higher (spectacle power) | Above this power, the difference between spectacle and contact lens power is large enough to matter — typically 0.25 D or more, which is the smallest step contact lenses are manufactured in. | Okay |
| Below Threshold | Under ±4.00 D (spectacle power) | Vertex compensation is usually skipped in practice — the calculated difference is well under 0.25 D, smaller than the smallest available contact lens power step. | Good |
Source: ODReference.com — Vertex Distance Calculator and Power Chart (Fc = Fs ÷ (1 − d × Fs) formula, standard 12 mm/13.5 mm vertex distances); ODReference.com — Contact Lens Conversion Calculator (±4.00 D clinical significance threshold)
Worked Examples
High Myopia: Glasses → Contacts
- Spectacle Power
- -10.00 D
- Direction
- Glasses to contacts
- Vertex Distance
- 12 mm
-10.00 ÷ (1 - 0.012×-10.00) = -10.00 ÷ 1.12 = -8.93 D, rounded to the nearest available 0.25 D step = -9.00 D. Minus powers get weaker (less negative) moving from glasses to contacts.
Moderate Hyperopia: Glasses → Contacts
- Spectacle Power
- +8.00 D
- Direction
- Glasses to contacts
- Vertex Distance
- 12 mm
+8.00 ÷ (1 - 0.012×8.00) = +8.00 ÷ 0.904 = +8.85 D, rounded to +8.75 D. Plus powers get stronger (more positive) moving from glasses to contacts — the opposite direction from minus powers.
High Myopia: Contacts → Glasses
- Contact Lens Power
- -6.00 D
- Direction
- Contacts to glasses
- Vertex Distance
- 12 mm
-6.00 ÷ (1 + 0.012×-6.00) = -6.00 ÷ 0.928 = -6.47 D, rounded to -6.50 D. Converting the other direction reverses the sign of the vertex distance change, so minus powers get stronger instead of weaker.
Contacts → Glasses at 13.5 mm
- Contact Lens Power
- +5.00 D
- Direction
- Contacts to glasses
- Vertex Distance
- 13.5 mm (alternate)
+5.00 ÷ (1 + 0.0135×5.00) = +5.00 ÷ 1.0675 = +4.68 D, rounded to +4.75 D. Using the 13.5 mm alternate vertex distance instead of the 12 mm standard shifts the result slightly — always confirm which vertex distance your prescribing doctor measured.
Low Myopia — Below the Clinical Threshold
- Spectacle Power
- -2.00 D
- Direction
- Glasses to contacts
- Vertex Distance
- 12 mm
-2.00 ÷ (1 - 0.012×-2.00) = -2.00 ÷ 1.024 = -1.95 D. The change is only 0.05 D — well under the 0.25 D step contact lenses are manufactured in, so vertex correction is not clinically necessary at this power.
How to Use This Calculator
- 1
Enter your lens power
Type the spectacle power (for glasses-to-contacts) or the contact lens power (for contacts-to-glasses) in diopters — negative for myopia, positive for hyperopia.
- 2
Choose the conversion direction
Select "Glasses power → Contact lens power" or "Contact lens power → Glasses power" depending on which number you're starting from.
- 3
Select a vertex distance
Use the 12 mm standard, the 13.5 mm alternate, or enter a custom distance if your eye doctor measured and recorded a specific vertex distance for your frame.
- 4
Read the corrected power and significance note
The result shows the exact and rounded corrected power, plus a note on whether the difference is large enough to matter clinically at your prescription strength.
What Each Value Means
- Spectacle Power (diopters (D))
- The lens power prescribed for glasses, measured at the spectacle plane — typically 12 to 13.5 mm in front of the eye.
- Contact Lens Power (diopters (D))
- The lens power needed at the corneal plane (directly on the eye) to produce the same focusing effect as the spectacle prescription, once vertex distance is accounted for.
- Vertex Distance (millimeters (mm))
- The distance between the back surface of a lens and the front of the cornea. This value is the 'd' in the vertex distance formula and directly controls how much the power shifts between glasses and contacts.