Will My Prescription Glasses Be Thick?

It’s one of the first questions directed at optometrists when it comes time to settle on a pair of prescription lenses: “How thick will they be?”

How Thick Will My Prescription Glasses Be?

Will My Prescription Glasses Be ThickThe answer depends on two things: you and your prescription.

You, because “thick” is subjective. Some people consider any lenses that are not completely hidden within a frame to be thick. Others notice thickness only when it reaches the level of “Coke Bottle” glasses.

Your prescription, in that a stronger one will produce a thicker lens. Diopters are the units used to measure refractive errors and the lenses used to correct them. The higher your corrective diopter rating grows the more curve your lens will require to compensate…and more physical material will be required to house those longer, deeper curves. Any prescription with a diopter rating in excess of +/- 4.0 or so is generally considered to be entering the high end of the spectrum. The higher that diopter rating creeps, the thicker the lenses grow. Some people require prescriptions as high as +/- 20.00 diopters or more, producing lenses that are extremely thick, heavy, and distortive.

Additionally, the type of prescription you require plays a factor in thickness. A prescription for farsightedness, measured in “+” diopters, requires lens curves that are thinnest at the center and grow progressively thicker as they fan out. The edges of a farsighted prescription are the thickest part of the lens…so even a relatively mild prescription can result in lenses that jut out from the frames, distort the look of the eyes, or preclude you from selecting certain types of frames due to the thickness and weight of the lenses.

High index lenses, which are made of a plastic or glass material that is specially formulated and compressed to produce thinner lenses, is a common way to combat the effects of thick lenses. Standard-material lenses – such as CR39 plastic, Trivex, or polycarbonate plastic – fall within the 1.50-1.56 index. For a lower prescription, these materials produce relatively thin lenses, but as the prescription rises in strength a standard material lens can become prohibitively thick. Here’s a look at the higher index ratings and their ability to reduce lens thickness:

  • 1.61-1.67 index – the first upgrade level considered “high index.” Lenses produced from material in this range can reduce thickness by up to 30% in comparison to 1.50 lenses.
  • 1.74 index – the flattest and thinnest lens available in plastic. Lenses produced from 1.74 material can reduce thickness by up to 45% in comparison to 1.50 lenses.
  • Above the 1.74 index rating, only glass lenses are available, up to an index rating of 1.9. Because of the changeover from glass to plastic, lens thickness is not comparable. However, high index glass lenses can be significantly thinner when compared to low index glass lenses.

Shop High Index GlassesIf the diopter rating on your prescription is high enough to require lenses that are too thick for your wants or needs, consider making the switch to high index lenses. The higher your prescription is, the more benefit you will reap if you upgrade from standard material to high index.

Guide to High Index Lenses 8 Responses to Will My Prescription Glasses Be Thick?
  1. Jeff Wan
    August 14, 2014 | 7:09 am

    Just ordered new 1.74 high index glass, length wise the lens is v. Similar but the edge is thicker than 1.67. I thought it is supposed to be thinner. What is your take, I refuse to accept the glass.

    • Kieran Hunt
      August 25, 2014 | 2:12 am

      Hi Jeff,

      I definitely understand your frustration, but there’s a lot more that goes into the determination of the thickness of a lens than just its index. The lens may only be a few millimeters wider, but that would be enough to increase the thickness of the edge substantially. Also, if your prescription has gotten stronger, that will have an effect. And, if the lens is more curved than your original pair (which might be the case if your new frame is slightly less flat than your last frame), that will have a big impact on your lens thickness. Bottom line, unless they’ve lied about the material they’ve used, the 1.74 would be thinner in that frame than the 1.67.

      Let me know if you have any more questions!

      Kieran Hunt

  2. Eric Mann
    February 2, 2015 | 5:56 pm

    I just received a pair of new glasses that are “supposed” to be high index, I was never told the refractive index that they planned to use. The new glasses are roughly the same size and slightly weaker (by .25 diopter in each eye) than my last pair of glasses (which were rimless, the new are full rim). These new lenses are nearly twice the thickness on the sides and bottom than the old pair. My Rx is -8.00 in right eye and -7.75 in left eye. Shouldn’t a full rimmed, High Index lens be thinner than a rimless one?

    • elizabeth
      March 21, 2015 | 12:20 pm

      What amount of high index did you get?

  3. Jessica
    February 4, 2015 | 3:25 pm

    Did you get the glasses from us?

  4. Lee
    February 9, 2015 | 12:13 pm


    Its a shame they didn’t tell you what refractive index they gave you… sounds like it could be a few things:

    Either the frame is bigger (this is in relation to eye size, nose bridge , even if its a few mm bigger then your prev specs, and in optics 1-2mm will make a huge difference cosmetically

    Shape of frame plays a part

    Or they could of used a lower than prev hi index lenses

    Or combination of all above. Like what ive said to all my patients: you will only ever get a good outcome if you choose your frames wisely and choosing the right hi-index. Even if your visions improved: select a bigger frame and the wrong hi index you will be disappointed.

    All the best!

  5. jj
    July 27, 2016 | 1:42 pm

    My prescription is -4.75 will the glass stick out of my glassses

    • Ryan Phillips
      August 9, 2016 | 3:31 pm

      It will depend on the type of frame you select and the material. To get the thinnest lens, you want to pick a frame that is small and circular. This would require less material to be used. For material, you would be fine going with the high index plastic 1.67 or 1.70 to help cut down on some of the thickness.

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