Convert your glasses prescription to contact lens power using the vertex distance formula. Essential for prescriptions stronger than ±4.00D. Instant results for both eyes.
✓ Verified: American Academy of Ophthalmology — April 2026
Right Eye (OD)
Enter OD sphere power (e.g. -6.00)
Left Eye (OS)
Enter OS sphere power (e.g. -5.50)
Standard vertex distance is 12mm. Your optometrist measures this.
Contact Lens Power
—
⚠️ Disclaimer: This calculator is for educational purposes only. Always consult a licensed optometrist or ophthalmologist for your actual contact lens prescription. Contact lenses require a valid prescription and professional fitting.
Was this calculator helpful?
✓ Thanks for your feedback!
Sources & Methodology
✓ Vertex distance formula verified against American Academy of Ophthalmology and College of Optometrists clinical guidelines.
👓
American Academy of Ophthalmology — Clinical Optics
Vertex distance compensation methodology for contact lens prescriptions. aao.org
📖
College of Optometrists — Vertex Distance
Clinical guidance on vertex distance measurement and prescription conversion. college-optometrists.org
🧪
Optics of Contact Lens Fitting — Bennett & Weissman
Standard textbook formula: F_CL = F_spec / (1 − d × F_spec). Rounded to nearest 0.25D increment.
Formula: F_CL = F_spec ÷ (1 − d × F_spec)
Where F_spec = spectacle lens power (diopters), d = vertex distance (meters, e.g. 0.012 for 12mm) Cylinder: Convert sphere+cyl to find corrected total power, subtract converted sphere. Rounding: All results rounded to nearest 0.25D (standard contact lens increment).
Last reviewed: April 2026
How the Vertex Distance Formula Works
When you wear glasses, the lenses sit approximately 12mm in front of your cornea. Contact lenses sit directly on the eye — effectively at zero vertex distance. For prescriptions stronger than ±4.00 diopters, this change in distance significantly alters the effective power of the lens reaching your eye. The vertex distance formula corrects for this difference.
The formula F_CL = F_spec ÷ (1 − d × F_spec) calculates the precise power needed at the corneal plane. Minus lenses become less minus in contacts; plus lenses become less plus. The effect grows larger as prescription power increases.
For prescriptions under ±4.00D, the vertex conversion changes the power by less than 0.25D — the minimum manufacturing increment for soft contact lenses. Below this threshold, the glasses and contact lens powers are typically identical. Above ±4.00D, the conversion becomes clinically meaningful and must be applied.
Glasses Rx
CL Power (12mm VD)
Difference
Conversion needed?
−2.00D
−2.00D
0.00D
No
−4.00D
−3.75D
0.25D
Borderline
−6.00D
−5.75D
0.25D
Yes
−8.00D
−7.25D
0.75D
Yes
−10.00D
−9.00D
1.00D
Yes
+6.00D
+5.75D
0.25D
Yes
+10.00D
+9.00D
1.00D
Yes
Understanding OD, OS, Sphere, and Cylinder
OD (oculus dexter) refers to the right eye; OS (oculus sinister) to the left. The sphere (Sph) is the main lens power for distance correction. The cylinder (Cyl) corrects astigmatism. Both sphere and cylinder require vertex conversion at high powers. The axis does not change with vertex distance conversion.
Vertex Distance and Different Lens Types
Lens Type
Typical Vertex Distance
Note
Standard glasses frame
12–14mm
Most common; varies by frame
Sports/wrap frames
10–11mm
Closer to eye
High-power glasses
May be measured precisely
Rx sheet may specify VD
Soft contact lenses
0mm (on cornea)
Vertex conversion required >±4D
RGP contact lenses
0mm (on cornea)
Same conversion formula applies
💡 Pro Tip: If your glasses prescription sheet specifies a vertex distance (often written as “VD 13” or similar), use that exact value in the calculator for the most accurate conversion. If not specified, 12mm is the standard assumption used by most opticians.
Frequently Asked Questions
Vertex distance is the distance in mm between the back of your glasses lens and the front of your cornea. Standard is 12mm. Contacts sit at 0mm, so powers above ±4.00D need converting.
Use the formula F_CL = F_spec ÷ (1 − d × F_spec). Enter your glasses sphere and cylinder for each eye along with your vertex distance. The calculator does this automatically and rounds to 0.25D.
Generally no. Prescriptions under ±4.00D have less than 0.25D difference after vertex conversion, which is below the rounding threshold. Above ±4.00D the conversion becomes clinically significant.
Only for prescriptions under ±4.00D. For stronger prescriptions the contact lens power will differ. A −8.00D glasses prescription converts to approximately −7.25D in contacts at 12mm.
The standard assumed vertex distance is 12mm (0.012 metres). Actual distance ranges from 10–16mm depending on frame fit. Your optometrist measures this during refraction for high prescriptions.
OD = oculus dexter = right eye. OS = oculus sinister = left eye. OU = both eyes. These Latin abbreviations are universal across all optical and medical prescriptions.
At high powers, moving a lens closer to the eye changes its effective power significantly. A −10D lens moved from 12mm to 0mm changes effective power by about 1.00D — enough to cause blurred vision if ignored.
Contact lens powers are rounded to the nearest 0.25D because that is the standard manufacturing increment for soft lenses. Some specialty lenses use 0.12D steps. This calculator rounds to 0.25D as standard.
Yes when significant. The cylinder is calculated as the vertex-corrected sphere+cylinder total, minus the vertex-corrected sphere alone. This calculator performs both conversions automatically.
−8.00D at 12mm converts to approximately −7.25D in contacts: −8 ÷ (1 − 0.012 × −8) = −8 ÷ 1.096 = −7.30D, rounded to −7.25D. This calculator does it instantly for both eyes.