How thick will your lenses actually be?

What actually makes a lens thick

Three things, and the one people focus on is only the first of them.

Power. A lens bends light by being curved, and a stronger prescription needs more curve. On a minus lens that curve is ground into the back surface, hollowing out the middle and leaving the rim standing proud. On a plus lens it works the other way: the middle bulges and the rim thins to nothing. This is why short-sighted people complain about thick edges and long-sighted people complain about weight and magnified eyes.

Distance from the optical centre. The depth of that curve at any point follows the sagitta, and the sagitta grows with the square of the distance from the centre. This is the single most important fact on this page and the one least often said out loud. Doubling how far the edge sits from the optical centre does not double the thickness there, it roughly quadruples the part that comes from the curve. That is why frame size matters so much, and why a millimetre or two of decentration is not the trivial detail it sounds like.

Refractive index. A material that bends light more strongly needs less curve to achieve the same power. The index number — 1.50, 1.60, 1.67, 1.74 — is the ratio of the speed of light in a vacuum to its speed in that material. Going from 1.50 to 1.74 reduces the required curve depth by roughly half, because the sagitta is inversely proportional to (n − 1), and 0.74 is about 1.48 times 0.50.

The arithmetic, worked through

The approximation labs and calculators use is:

s = d² × |F| ÷ (8000 × (n − 1))

where s is the sagitta in millimetres, d is the lens diameter in millimetres, F is the power in dioptres, and n is the refractive index. The 8000 carries the unit conversions.

Take a −4.00 lens in a 52 mm frame, with the optical centre on the frame centre, made in CR-39 at 1.50. The diameter is 52, so d² is 2704. Multiply by 4 and you get 10,816. Divide by 8000 × 0.50 = 4000, and the sagitta is 2.70 mm. Add the 2.0 mm minimum centre thickness a lab will work to and the edge lands at about 4.70 mm.

Now change one thing at a time. Move to 1.67: the denominator becomes 8000 × 0.67 = 5360, the sagitta drops to 2.02 mm, and the edge becomes 4.02 mm — with the thinner 1.3 mm centre these materials allow, about 3.32 mm. A saving of well over a millimetre, which is genuinely visible.

Instead, keep CR-39 and drop the frame to 46 mm. Now d² is 2116, the sagitta is 2.12 mm, and the edge is 4.12 mm. Most of the same saving, from a decision that costs nothing.

Decentration: the millimetre nobody mentions

A frame has its own pupillary distance, and it is not a secret number — it is the lens width plus the bridge. A 52-18 frame has a frame PD of 70 mm. If your PD is 62 mm, each optical centre has to sit 4 mm inward of where the lens blank's centre would naturally fall.

Consider what that does. The lens is 52 mm wide, so its temporal edge was 26 mm from centre. After decentring it is 30 mm. Squared, that is 900 against 676 — a 33 per cent increase in the curve-derived part of the thickness. On our −4.00 in CR-39, the sagitta at the temporal edge rises from 2.70 mm to 3.60 mm. The edge has gained nearly a millimetre, from nothing but frame choice.

The uncomfortable implication is that people with narrow PDs are systematically disadvantaged by frames designed around an average face, and a frame that fits the face properly can save more thickness than an expensive material does. It is worth running the calculator twice, once with the frame you like and once with a frame 4 mm narrower, before spending on index.

If you do not know your PD, measure it here — the figure carries over to this calculator automatically.

Which meridian is thickest

A prescription with a cylinder describes two powers at right angles to each other: the sphere on its own, and the sphere plus the cylinder. Written in minus cylinder form, which is what most prescriptions use, −4.00 −1.25 means one meridian at −4.00 and the other at −5.25.

The lens is thickest at its rim along the meridian with the most minus power, so this prescription behaves like a −5.25 where it is thickest and a −4.00 where it is thinnest. The edge is not a uniform ring; it varies as you travel around it, which is why a lens can look thin from one angle and thick from another.

Many free calculators use the sphere alone, or the spherical equivalent — the sphere plus half the cylinder. Both under-report. This tool uses the governing meridian and tells you which figure it used, so the number it gives you is the worst case rather than an average that will never appear on the actual lens.

Edge thickness at a glance

Temporal edge thickness in millimetres, for a 52 mm lens with the optical centre on the frame centre and no decentration. Real lenses in real frames will be thicker than this, which is what the calculator above is for.

Read along a row and watch how little happens at the top and how much at the bottom. At −1.00 the whole range spans a few tenths of a millimetre. At −10.00 it spans several millimetres. The material upgrade is not a fixed benefit you buy; it is a benefit that scales with your prescription, and at low powers there is close to nothing there to buy.

Frame size against material

The comparison optical shops rarely set up. Edge thickness for a −5.00 in CR-39, the cheapest material there is, purely as a function of how wide the frame is.

Going from a 56 mm frame to a 48 mm frame saves about as much as going from CR-39 to a mid-index material, and costs nothing. Doing both is how people with strong prescriptions end up with glasses that look ordinary.

The trade you make going thinner

High index is not free even after you have paid for it.

Chromatic aberration. A lens bends different wavelengths by slightly different amounts, splitting white light the way a prism does. The Abbe number measures how badly: higher is better. CR-39 sits at 58, which is excellent. Polycarbonate sits at 30, which is poor. At the centre of your field of view you will never see it. Toward the edge, on a high-contrast boundary like a street light against a night sky, it shows as a coloured fringe. People sensitive to it describe it as the edges of their vision looking slightly wrong, and it is the commonest reason someone dislikes new lenses they cannot otherwise fault.

Reflections. A denser material reflects a greater proportion of incident light at each surface. Uncoated CR-39 loses about 8 per cent to reflection; 1.74 loses closer to 14 per cent. That light comes back as ghost images and as the glare other people see on your lenses in photographs. Anti-reflective coating handles it, but on a high index lens the coating stops being an upsell and becomes a requirement.

Brittleness. Generally, the higher the index, the less impact the material tolerates. This is why polycarbonate and Trivex — neither of which is especially thin — remain the standard for children, sports and safety eyewear. If you break glasses regularly, the thinnest material is the wrong answer regardless of prescription.

Weight is a separate question from thickness

The two get conflated, and they do not track each other. Density varies independently of index. Trivex at 1.53 has a density of 1.11 g/cm³, the lowest in common use; 1.74 has a density of 1.46. A 1.74 lens is thinner but made of heavier material, so the weight saving at moderate powers is smaller than people expect, and at low powers a 1.74 lens can actually weigh more than a Trivex one.

The calculator reports approximate weight per lens alongside the thickness for this reason. If your complaint about your glasses is that they slide down your nose or leave marks, weight is the number to optimise, and it points at a different material than thickness does.

What to do with the result

If the saving is under about 0.6 mm, buy the cheapest material that suits your lifestyle and spend the difference on a good anti-reflective coating. You will notice the coating every day and the thickness never.

If it is between roughly 0.6 and 1.5 mm, look at mid-index around 1.60. It usually captures most of the available saving at a fraction of the premium, and its Abbe number is better than polycarbonate's.

If it is over 1.5 mm, the upgrade is doing real work. Before paying for it, run the calculator once more with a frame 4 to 6 mm narrower and see how much of that saving you can get for nothing. Then consider asking about an aspheric or lenticular design, which this calculator does not model and which can help further at high powers.

In all cases, take the numbers to the dispenser rather than treating them as a verdict. A good dispenser will be glad you arrived informed; the figures here give you something to ask about rather than something to argue with.

This calculator estimates lens geometry from standard optical formulae. It is not a prescription, not a quotation, and not a substitute for advice from a qualified dispensing optician, who can account for base curve, lens design, frame shape and the specific lab's tolerances. Figures assume a best-form single vision lens and will differ for progressive, aspheric, lenticular or high-base-curve wrap designs.

Related: measure your pupillary distance, decode the numbers on your frame, read your prescription, or find the frame size that fits your face.

Questions people ask

How thick will my glasses be?

It depends on three things, not one. The power of the prescription sets how fast the lens thickens away from its optical centre; the size of the frame sets how far it has to travel before the edge; and the material's refractive index sets how much curve is needed to bend light by that amount. A -6.00 in a small 46 mm frame can finish thinner than a -4.00 in a wide 56 mm one. The calculator above works all three at once rather than giving you a figure for the power alone.

Is high index lens worth it?

Below about 2.50 dioptres, almost never — the whole range of materials is within half a millimetre of itself, and you would be paying a large premium for a difference no one can see while accepting worse optics. Between about 2.50 and 4.00 a mid-index around 1.60 captures most of the available saving cheaply. Above about 5.00 the upgrade is doing real work. The calculator states the saving in millimetres so you can judge it rather than take the shop's word.

Why does a smaller frame make lenses thinner?

Edge thickness grows with the square of the distance from the optical centre. Cut 6 mm off the lens width and the temporal edge does not get 6 mm thinner, it gets thinner in proportion to the change in that squared term — often a third of the total thickness. This is the single cheapest thickness reduction available, and it costs nothing, which is presumably why it is mentioned less often than the material upgrade that costs a great deal.

What is decentration and why does it make my lenses thicker?

A lens is ground so its optical centre sits in front of your pupil, not in the middle of the frame. If the frame's own PD — lens width plus bridge — is wider than yours, each optical centre has to move inward, which pushes the temporal edge of the lens further from that centre. Since thickness follows the square of that distance, a badly matched frame quietly adds a millimetre or more. It is one of the reasons two people with the same prescription can end up with visibly different glasses.

Does astigmatism make lenses thicker?

It makes one meridian thicker than the other. A prescription contains two powers: the sphere, and the sphere plus the cylinder. The lens edge is thickest along whichever of those is most minus, and thinnest along the other, so the rim varies in thickness as you go around it. A calculator that uses the sphere alone flatters every astigmatic prescription; this one uses the governing meridian and says which it used.

Are thinner lenses always better?

No, and this is the part optical retail tends not to raise. Higher index materials almost always have a lower Abbe number, which means more chromatic dispersion — colour fringing visible toward the edge of your field of view, worst on high-contrast edges. Polycarbonate at 1.59 has an Abbe number of 30, against CR-39's 58. They also reflect more light, so anti-reflective coating stops being optional. Below about four dioptres you would be trading real optical quality for a thickness difference nobody will notice.

Can I estimate my lens height if it is not marked on the frame?

Frames are marked with lens width, bridge and temple length, but not height, because height was historically less variable. Measure the vertical opening of the frame's eye wire at its tallest point with a ruler. Most adult frames fall between 32 and 45 mm; a deep rectangular or aviator shape may exceed that. The height only affects the corner thickness figure, so an estimate within a few millimetres is fine for judging the material decision.