Estimate the edge or center thickness of your prescription lenses based on material and frame size.
CR-39 / Standard (1.50)
Trivex (1.53)
Polycarbonate (1.59)
High Index (1.61)
High Index (1.67)
Ultra High Index (1.74)
Understanding Lens Thickness
The thickness of an optical lens is determined by a combination of the prescription (power), the physical size of the lens (diameter), and the material used (refractive index). This calculator uses the Sagitta (Sag) formula to approximate how thick a lens will be at its thickest point.
1. Refractive Index and Thickness
The refractive index represents how efficiently a material bends light. A higher index means the material is more "powerful" at bending light, allowing the lens to be thinner for the same prescription.
1.50 (Standard): Thickest, best for low prescriptions.
1.59 (Polycarbonate): Impact resistant and thinner than standard.
1.67 / 1.74 (High Index): Recommended for strong prescriptions to reduce "coke bottle" effect.
2. Plus vs. Minus Lenses
Minus Lenses (Nearsightedness): These lenses are thin in the center and thick at the edges. The larger the frame (diameter), the thicker the edges will be.
Plus Lenses (Farsightedness): These lenses are thick in the center and thin at the edges. A larger frame requires a larger center thickness to maintain structural integrity.
3. The Calculation Formula
The calculator uses the Sagitta formula: Sag = R - sqrt(R² - r²), where:
R is the radius of curvature: (Index - 1) / (Power / 1000)
r is half of the lens diameter.
For a minus lens, the Edge Thickness = Center Thickness + Sagitta. For a plus lens, the Center Thickness = Edge Thickness + Sagitta.
Example Calculation
If you have a -5.00 SPH prescription with a 1.67 High Index lens and a 50mm frame diameter:
Total Power: -5.00 Diopters
Radius of half-diameter (r): 25mm
Calculated Sagitta: ~2.35mm
Min Center Thickness: 1.5mm
Final Edge Thickness: 3.85mm
function calculateLensThickness() {
var sph = parseFloat(document.getElementById('sphPower').value) || 0;
var cyl = parseFloat(document.getElementById('cylPower').value) || 0;
var index = parseFloat(document.getElementById('lensIndex').value);
var diameter = parseFloat(document.getElementById('lensDiameter').value);
var minThick = parseFloat(document.getElementById('minThickness').value);
// Calculate maximum power in any meridian
// Usually, thickness is dictated by the total combined power
var totalPower = sph + (cyl 0 && cyl > 0) {
totalPower = sph + cyl;
} else if (sph < 0 && cyl Math.abs(sph + cyl) ? sph : (sph + cyl);
}
var absPower = Math.abs(totalPower);
var radius = (diameter / 2);
// Sagitta formula: s = r^2 * P / (2000 * (n – 1))
// This is a standard paraxial approximation used in optics for quick estimates
var sagitta = (Math.pow(radius, 2) * absPower) / (2000 * (index – 1));
var finalThickness = 0;
var resultLabel = "";
if (totalPower < 0) {
// Minus Lens: Thick at edge, min thickness is at center
finalThickness = minThick + sagitta;
resultLabel = "Estimated Edge Thickness:";
document.getElementById('resType').innerHTML = "Lens Type: Minus (Concave / Nearsighted)";
} else if (totalPower > 0) {
// Plus Lens: Thick at center, min thickness is at edge
finalThickness = minThick + sagitta;
resultLabel = "Estimated Center Thickness:";
document.getElementById('resType').innerHTML = "Lens Type: Plus (Convex / Farsighted)";
} else {
finalThickness = minThick;
resultLabel = "Estimated Uniform Thickness:";
document.getElementById('resType').innerHTML = "Lens Type: Plano (No Power)";
}
document.getElementById('resTotalPower').innerHTML = "Calculation Power: " + totalPower.toFixed(2) + " D";
document.getElementById('resSagitta').innerHTML = "Calculated Sagitta: " + sagitta.toFixed(2) + " mm";
document.getElementById('resFinalThickness').innerHTML = resultLabel + " " + finalThickness.toFixed(2) + " mm";
document.getElementById('lensResult').style.display = 'block';
}