Vertex Distance Adjustment Calculator

Adjust lens power when changing the vertex distance (e.g., from glasses to contacts or between different frame positions).

Original Lens Power

Original Vertex Distance

New Vertex Distance

Tab to move between fields

Adjusted Power

-4.75 D

Change Magnitude0.25 D

Effective Power at Cornea-4.72 D

How to Use This Calculator

Enter the Original Lens Power in diopters (from the current glasses prescription or trial frame).
Enter the Original Vertex Distance in mm — typically 12 mm for standard eyeglass frames.
Enter the New Vertex Distance in mm — use 0 mm for contact lenses or the new frame's measured vertex distance.
The formula P_new = P_effective / (1 + d₂ × P_effective) calculates the adjusted power.
Review the Adjusted Power (rounded to the nearest 0.25 D) and Change Magnitude to determine whether a prescription update is clinically significant.

How the result changes with Original Lens Power

Original Lens Power: Current lens power in diopters at the original vertex distance.
Original Vertex Distance: Original vertex distance in millimeters (glasses typically 12 mm).
New Vertex Distance: New vertex distance in millimeters (contacts = 0 mm, trial frame may vary).

New Power = P / (1 - d × P)

Identify Input Parameters
Original Lens Power = -5, Original Vertex Distance = 12, New Vertex Distance = 0 = 3 input(s) provided
Calculate Adjusted Power
-4.75 = -4.75
Calculate Change Magnitude
Change Magnitude
0.25 = 0.25
Calculate Effective Power at Cornea
-4.72 = -4.72