Lens Thickness Estimator
See how power, material, and frame size drive lens thickness — and how much a higher index or a smaller frame actually slims a strong prescription.
Estimated thickness
Minus lenses are thin in the centre and thickest at the edge — the number that shows in the frame rim. A higher index thins that edge.
Where lens thickness comes from
A lens is a curved piece of material, and the depth of that curve — the sag — is what you see as thickness. The stronger the prescription, the steeper the curve; the wider the lens, the deeper that curve reaches; and the lower the material's index, the more curve is needed for the same power. Multiply those together and you can see why a strong myope in a big, low-index lens ends up with thick, heavy edges.
Two levers slim things down: a higher-index material, which reaches the power with a flatter surface, and a smaller frame, which cuts the radius the curve has to span. This estimator lets you try both and compare the trade-offs before you order, so you know whether it's worth paying up for 1.67 or 1.74 glass.
Frequently Asked Questions
Why are stronger prescriptions thicker?
Lens power comes from surface curvature, and a curved surface has depth — the sagitta or 'sag'. The stronger the power and the wider the lens, the more sag, and the more the edge (minus lenses) or centre (plus lenses) has to grow to accommodate it. The tool estimates that extra thickness on top of the lab's minimum.
What does the lens index change?
The refractive index is how strongly the material bends light. A higher index bends light more for a given curve, so it reaches the same power with a flatter, shallower surface — less sag and a thinner lens. Moving from 1.50 CR-39 to 1.67 or 1.74 can noticeably slim a strong prescription, which is why high-index lenses are sold for high powers.
Which is thicker, a minus or a plus lens?
They're thick in different places. A minus (near-sighted) lens is thin in the centre and thickest at the edge — the rim you see in the frame. A plus (far-sighted) lens is thin at the edge and thickest in the centre, giving a magnified, bulging look. This tool reports whichever is the meaningful thickness for your power.
How accurate is the estimate?
It's a physics approximation using the sag of a single power surface. Real finished thickness also depends on the frame shape, how far the lens is decentred, the base curve chosen, and the lab's minimum thickness rules. Treat the number as a good guide for comparing materials and frame sizes, not an exact spec.
Does a smaller frame really help?
Yes. Thickness grows with the square of the lens radius, so a smaller, rounder lens can be dramatically thinner than a large one at the same power. For a strong prescription, choosing a smaller frame is often as effective as bumping up the index — and the two together work best.
Educational estimate from the sag of a single power surface. Real thickness also depends on frame shape, decentration, base curve, and lab minimums — confirm with your optician.