*Results are rounded to the nearest standard 0.25D step. Consult your optometrist for a final prescription.
Understanding Vertex Distance Conversion
If you wear glasses and are looking to switch to contact lenses, you might notice that the power on your contact lens box doesn't always match your glasses prescription. This is due to a physics principle called Vertex Distance.
What is Vertex Distance?
Vertex distance is the space between the back surface of your glasses lens and the front of your cornea (the surface of your eye). For most people, this distance is approximately 12 to 14 millimeters. When you move a lens from your frame (12mm away) directly onto your eye (0mm away), the effective power of that lens changes.
The Vertex Conversion Formula
To calculate the effective power needed for a contact lens, we use the following formula:
Fc = Fs / (1 – (d * Fs))
Fc = Power of the contact lens
Fs = Power of the spectacle (glasses) lens
d = Vertex distance in meters (e.g., 12mm = 0.012m)
Why Power Changes for High Prescriptions
The vertex adjustment usually only becomes clinically significant for prescriptions stronger than +/- 4.00 Diopters.
For Nearsightedness (Minus Lenses): As the lens moves closer to the eye (from glasses to contacts), it becomes effectively "stronger." Therefore, the contact lens power needs to be weaker (less minus) than the glasses.
For Farsightedness (Plus Lenses): As the lens moves closer to the eye, it becomes effectively "weaker." Therefore, the contact lens power needs to be stronger (more plus) than the glasses.
Example Calculations
Glasses Power
Vertex Distance
Calculated Contact Power
Standard Lens Step
-6.00 D
12 mm
-5.59 D
-5.50 D
+5.00 D
12 mm
+5.32 D
+5.25 D
-8.00 D
12 mm
-7.30 D
-7.25 D
Important Considerations
This calculator specifically addresses the Sphere component of your prescription. If you have astigmatism (Cylinder), the conversion must be applied to both the sphere and the cylinder (or the resultant principal meridians). Furthermore, contact lenses require a proper fitting by a licensed eye care professional to ensure eye health, comfort, and oxygen permeability.
function calculateContactLensPower() {
var fs = parseFloat(document.getElementById("spectaclePower").value);
var d_mm = parseFloat(document.getElementById("vertexDistance").value);
var resultDiv = document.getElementById("clResultArea");
var outputDiv = document.getElementById("clFinalPower");
if (isNaN(fs)) {
alert("Please enter a valid spectacle power.");
return;
}
if (isNaN(d_mm)) {
d_mm = 12; // Default to standard 12mm
}
// Convert vertex distance from mm to meters
var d_m = d_mm / 1000;
// Formula: Fc = Fs / (1 – (d * Fs))
var fc = fs / (1 – (d_m * fs));
// Round to nearest 0.25 Diopter step
// 0.25 step logic: multiply by 4, round to nearest integer, divide by 4
var roundedPower = (Math.round(fc * 4) / 4).toFixed(2);
// Add '+' sign for positive numbers
var displayPower = roundedPower > 0 ? "+" + roundedPower : roundedPower;
outputDiv.innerHTML = displayPower + " D";
resultDiv.style.display = "block";
// Scroll smoothly to results on mobile
resultDiv.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}