Lens thickness calculator
Power, blank size and material in; the rim or the center thickness of the glasses lens out.
For general information only — not medical advice. This page gives an approximate educational estimate; it is not a prescription or a lab quote. A prescription and lens choice must come from an eye-care professional.
Use the sphere power, or the strongest meridian if you have astigmatism. Choose “center” for a minus lens to see the rim, and “edge” for a plus lens to see how thick it must be in the middle. The example values are only examples.
Why a lens gets thick
A spectacle lens is a piece of glass or plastic with two curved faces. A minus lens is thin in the middle and thick at the rim; a plus lens is the reverse. The thickness difference between center and rim is the sum of how much each face curves away over the half-width of the lens, and the amount of curve needed for a given power depends on the material’s refractive index. That is the whole reason for “high-index” lenses.
The method, variable by variable
- Surface radius: r = (n − 1) ÷ F, with the surface power F in diopters, r in meters and n the index. This is the one-surface form of the lensmaker’s equation (OpenStax University Physics, Volume 3, section 2.4).
- Sag: a surface of radius r drops by s = r − √(r² − y²) at distance y from the axis, where y is half the blank diameter. This is geometry (Pythagoras).
- Surface powers: the front surface has the front-curve power F₁; the back surface supplies the rest, F₂ = P − F₁. This thin-lens split is our simplification.
- Thickness: t(y) = center − s₁ − s₂, with sags counted positive for plus surfaces and negative for minus surfaces. Given the center, this yields the rim; given the rim, it yields the center.
Worked example
Take a −6.00 D lens, a 60 mm blank (y = 30 mm), a +2.00 D front curve and a 2 mm center in CR-39 plastic (n = 1.498). The back surface must supply −8.00 D, so its radius is 0.498 ÷ 8 meters, or 62.3 mm, and its sag over 30 mm is 7.71 mm. The front surface (radius 249.0 mm) drops 1.81 mm. The rim is 2 + 7.71 − 1.81 = 7.9 mm. In index 1.60 the same lens has a 6.8 mm rim, and in 1.74 it has 5.8 mm. For a plus lens, a +4.00 D lens with a 1 mm rim on the same blank needs a 4.8 mm center in CR-39 and 4.1 mm in 1.60, with a +6.00 D front curve.
Rim thickness by power and material
| Power | n = 1.498 | n = 1.586 | n = 1.6 | n = 1.67 | n = 1.74 |
|---|---|---|---|---|---|
| −2.00 D | 3.9 mm | 3.6 mm | 3.5 mm | 3.4 mm | 3.2 mm |
| −4.00 D | 5.8 mm | 5.2 mm | 5.1 mm | 4.8 mm | 4.5 mm |
| −6.00 D | 7.9 mm | 6.9 mm | 6.8 mm | 6.2 mm | 5.8 mm |
| −8.00 D | 10.2 mm | 8.7 mm | 8.5 mm | 7.7 mm | 7.1 mm |
| −10.00 D | 13.0 mm | 10.8 mm | 10.5 mm | 9.4 mm | 8.6 mm |
Computed with a 60 mm blank, a 2 mm center and a +2.00 D front curve for every row. The indexes are the usual nominal values for plastic CR-39, polycarbonate and the high-index plastics; makers quote slightly different figures, and consumer sources disagree by about 0.01 on polycarbonate (1.586 or 1.59).
Putting the numbers to use
The estimate is best used to compare, not to quote. Run the same prescription through each material to see how much rim you save per step of index; the gain from 1.498 to 1.60 is 1.1 mm in the example, while the step from 1.60 to 1.74 saves a further 1.0 mm. Then try a smaller blank: the example loses 1.9 mm at the rim when the blank shrinks from 60 to 50 mm. Ten millimeters of blank saves nearly as much here as the whole step from 1.498 to 1.74, so frame choice can matter as much as a pricier material. Bring the numbers to your optician, who knows the real blank size, lens design and minimum thickness the lab will make.
Limits and common mistakes
- Blank diameter, not frame size. A lens is cut from a larger blank. If the frame is wide or your pupils sit far from the frame center, the blank must be larger, and the rim thickens fast.
- Thin-lens split. Real lenses take their back vertex power from the thickness too, and many are aspheric, so a lab’s numbers will differ by a fraction of a millimeter or more.
- Astigmatism. Cylinder makes the rim thickest along the most minus meridian. Run the calculator with that meridian’s power (sphere plus cylinder in minus-cylinder form; see the spherical equivalent calculator).
- Choosing by index alone. A higher index trades thickness for dispersion: the Abbe value is usually lower. Impact resistance, weight and coatings matter too, and the choice belongs to your optician.
Common questions
Why is my minus lens thick at the edge?
A minus lens is a concave lens, thinnest in the middle and thickest at the rim. For −6.00 D in ordinary plastic, a 2 mm center on a 60 mm blank comes to about 7.9 mm at the rim in this model. The stronger the minus and the wider the blank, the bigger the gap.
Does a higher index make the lens thinner?
Yes, for the same power and blank. In the worked example the rim shrinks from 7.9 mm at index 1.498 to 6.8 mm at 1.60 and 5.8 mm at 1.74. A higher index means each surface needs less curvature to produce the same power (the radius is r = (n − 1) ÷ F), and flatter surfaces have a smaller sag. The gain shrinks as the index rises, and higher-index materials usually disperse color more.
What diameter should I enter?
The width of the uncut lens needed for your frame. A common estimate is the frame’s longest effective diameter plus twice the distance by which the lens center must be shifted to sit in front of your pupil. A smaller frame or a smaller shift gives a thinner edge: dropping the blank from 60 mm to 50 mm cuts the rim of the example lens from 7.9 mm to 6.0 mm. Your optician can give the numbers.
What is the front curve and why does it matter?
The front curve (base curve) is the power of the front surface. The back surface supplies the rest, so a steeper front curve means a deeper back curve. The model keeps both surfaces spherical and adds their sags, so the front curve changes the thickness a little. Leave the field at its default if you do not know the number; opticians choose it from the lens design.
How thick is a plus lens at the center?
A plus lens is thick in the middle. With a 1 mm rim, a +4.00 D lens on a 60 mm blank needs a 4.8 mm center in ordinary plastic and 4.1 mm in index 1.60 in this model, using a +6.00 D front curve.
How accurate is the estimate?
It is an approximation. It treats the lens as thin and uses the sphere power only, so the result is the thickness along one meridian of a round blank. Real lenses are cut to a frame shape, may be aspheric, and have bevels, tints, coatings and a minimum thickness set by the lab. For a stronger astigmatism use the power of the strongest meridian. Use it to compare materials, not to quote a final lens.
Related pages: the thin lens calculator for focal length and image distance, the vertex distance calculator and the prism diopter calculator, where an off-center lens creates unwanted prism.
For general information only — not medical advice. This is an approximate educational estimate, not a prescription or a lab quote; lens choices must come from an eye-care professional. Sources: lensmaker’s equation, OpenStax University Physics Volume 3, section 2.4 “Thin Lenses” (CC BY); sag from circle geometry; nominal refractive indexes from consumer and trade eyewear references (not standards). The thin-lens thickness model is our own approximation. How we check our calculators.