Lens thickness.

The short answer

Thickness grows with power and with distance from the lens centre, and shrinks with a higher index. For -4.00 D in a 52 mm lens at 64 mm PD the estimate is about 5.42 mm at the edge in CR-39 and 4.29 mm in 1.74 high-index.

Worked example

Prescription -4.00 DS, frame 52-18, PD 64 mm, centre thickness 2 mm. Frame PD is 52 + 18 = 70 mm, so each lens is decentred (70 - 64) / 2 = 3 mm and the farthest edge is 26 + 3 = 29 mm from the optical centre. For polycarbonate (1.586) the back surface radius is 1000 x 0.586 / 4 = 146.5 mm, and the sag at 29 mm is 146.5 - sqrt(146.5² - 29²) = 2.90 mm, so the estimated edge is 2 + 2.90 = 4.90 mm. All six materials: CR-39 plastic (1.498) 5.42 mm; Trivex (1.532) 5.20 mm; Polycarbonate (1.586) 4.90 mm; High-index 1.60 (1.6) 4.83 mm; High-index 1.67 (1.67) 4.53 mm; High-index 1.74 (1.74) 4.29 mm.

How it works

The estimate treats the lens as a plano-front lens whose back surface supplies all of the power. The surface radius is R = 1000 x (n - 1) / |P| millimetres (the lensmaker's relation: surface power = (n - 1) / R), and the thickness added at a distance r from the centre is the sag S = R - sqrt(R² - r²), which is about r² / (2R) for a gentle curve. For a minus lens edge = centre thickness + S; for a plus lens centre = edge thickness + S.

What r is. r is half the lens width plus the decentration, where decentration = (lens width + bridge - PD) / 2. This treats the frame shape as a circle of the lens width, so a wide, oblong or aviator shape will really be thicker than shown. For astigmatism, the stronger of the two meridians (sphere, or sphere + cylinder) sets the number.

Assumptions that are not from a standard. The centre and edge thickness defaults, the circular frame shape and the plano-front shape are simplifications. Real lenses have a curved front, coatings, bevels and lab minimums, so the lab's figure will differ. Indices used: CR-39 1.498, Trivex 1.532, polycarbonate 1.586, and 1.60, 1.67 and 1.74 high-index (Wikipedia lists 1.60 to 1.74 as the high-index range).

Sources (opened 2026-10-08): Wikipedia, Sagitta (optics); Wikipedia, Lens (lensmaker's equation); Wikipedia, Corrective lens (refractive index classes).

FAQ

How thick will my glasses lenses be?

It depends on power, lens index and frame size. For -4.00 D in a 52-18 frame at 64 mm PD the estimate here is CR-39 plastic (1.498) 5.42 mm; Trivex (1.532) 5.20 mm; Polycarbonate (1.586) 4.90 mm; High-index 1.60 (1.6) 4.83 mm; High-index 1.67 (1.67) 4.53 mm; High-index 1.74 (1.74) 4.29 mm. Your lab's figure will differ.

Does a higher index make lenses thinner?

Yes. At the same power and distance from the centre, the sag is roughly proportional to 1/(n - 1). Index 1.74 (n - 1 = 0.74) has about 21% less sag than polycarbonate (0.586) and about 33% less than CR-39 (0.498).

Why does frame size matter?

The sag grows with the square of the distance from the lens centre. A smaller or better-centred frame cuts the farthest edge distance and cuts thickness quickly.

Why does my PD change the answer?

If your PD is smaller than the frame PD (lens width plus bridge), each lens is decentred and its far edge is further from the optical centre, so it gets thicker there.

Is plus power thicker at the centre?

Yes. For plus lenses the thickest part is the centre. At +4.00 D with the same frame the estimate is CR-39 plastic 4.92 mm; Trivex 4.70 mm; Polycarbonate 4.40 mm; High-index 1.60 4.33 mm; High-index 1.67 4.03 mm; High-index 1.74 3.79 mm, with a 1.5 mm edge assumption.

Is this a quote from a lab?

No. It is a geometric estimate with stated assumptions. Ask your optician for the actual lens thickness for your frame and prescription.