Lens Thickness Calculator

For minus (myopic) prescriptions, the sag (thickness-building) term in the approximation formula divides by Lens Index minus 1, so a higher index directly shrinks the calculated sag for the same power and frame size. For plus (hyperopic) prescriptions, the thin edge itself uses a lower practical minimum at higher indices, reflecting that more impact-resistant high-index materials are commonly edged thinner in real lab practice. Either way, a higher-index material bends light more per unit of surface curvature, so less curvature -- and less resulting thickness -- is needed to deliver the same prescription power.

Estimated Weight is calculated from the lens volume, which is modeled as a cylinder using the frame's radius -- so a wider Frame Lens Diameter increases that radius, and volume grows with the square of radius. A larger frame opening therefore increases estimated weight noticeably faster than a proportional increase in thickness alone would suggest.

Sagittal depth (or sag) is how far a curved lens surface bows away from a flat plane across its diameter -- the deeper the curve, the more the lens has to taper or thicken to accommodate it. This calculator uses sag as the core thickness-building term because it is the standard approximation opticians use to estimate uncut lens blank thickness before a lens is actually surfaced.

A minus lens diverges light and is thinnest at its optical center, growing thicker toward the edge, while a plus lens converges light and does the opposite -- thickest at center, thinnest at the edge. This calculator applies the sag term to Edge Thickness for minus powers and to Center Thickness for plus powers, matching how each lens shape actually tapers.

Decentration -- how far the lens blank must shift to align its optical center with your eye -- depends on Frame PD (Frame Lens Diameter plus Bridge Width, the true center-to-center distance the lenses are cut around), not on Frame Lens Diameter alone. Without Bridge Width, Frame PD is understated enough that Pupillary Distance rarely produces any decentration at typical values, so this calculator adds Bridge Width in to keep Pupillary Distance meaningful.