Michael Roxas

Contact Lens Conversion Table (VD: 12 mm)

31 Jul, 2026

Sometimes I don’t like to compute and input a bunch of buttons on a calculator or a web app. So I made this post instead for me to convert dioptric powers and make mental calculations simple.

Contents

Contact Lens Formula
Minus Power Table
Plus Power Table

Variables

$F_{c}$ : Effective power of the contact lens (in Diopters, $D$ )
$F_{s}$ : Power of the spectacle lens (in Diopters, $D$ )
$d$ : Vertex distance from the cornea to the spectacle lens (in meters, e.g., $12 mm = 0.012 m$ )

$F_{c} = \frac{F_{s}}{1 - d F_{s}}$

Minus Power Step Adjustments

Spectacle Power Range	Step Adjustment	Example Spectacle $\to$ CL Power
-0.25 D to -3.75 D	0 steps (No change)	-3.00 D $\to$ -3.00 D
-4.00 D to -5.75 D	1 step lower (-0.25 D)	-4.00 D $\to$ -3.75 D
-6.00 D to -7.75 D	2 steps lower (-0.50 D)	-6.00 D $\to$ -5.50 D
-8.00 D to -9.75 D	3 steps lower (-0.75 D)	-8.00 D $\to$ -7.25 D
-10.00 D to -11.75 D	4 steps lower (-1.00 D)	-10.00 D $\to$ -9.00 D
-12.00 D to -12.75 D	6 steps lower (-1.50 D)	-12.00 D $\to$ -10.50 D
-13.00 D to -13.75 D	7 steps lower (-1.75 D)	-13.00 D $\to$ -11.25 D
-14.00 D to -14.75 D	8 steps lower (-2.00 D)	-14.00 D $\to$ -12.00 D
-15.00 D to -15.75 D	9 steps lower (-2.25 D)	-15.00 D $\to$ -12.75 D
-16.00 D to -16.75 D	10 steps lower (-2.50 D)	-16.00 D $\to$ -13.50 D
-17.00 D to -17.75 D	11 steps lower (-2.75 D)	-17.00 D $\to$ -14.25 D
-18.00 D to -18.75 D	13 steps lower (-3.25 D)	-18.00 D $\to$ -14.75 D
-19.00 D to -19.75 D	14 steps lower (-3.50 D)	-19.00 D $\to$ -15.50 D
-20.00 D	16 steps lower (-4.00 D)	-20.00 D $\to$ -16.00 D

Plus Power Step Adjustments

Spectacle Power Range ( $F_{s p e c}$ )	Step Adjustment	Example Spectacle $\to$ CL Power
+0.25 D to +3.75 D	0 steps (No change)	+3.00 D $\to$ +3.00 D
+4.00 D to +4.75 D	1 step higher (+0.25 D)	+4.00 D $\to$ +4.25 D
+5.00 D to +5.75 D	1 to 2 steps higher (+0.25 D to +0.50 D)	+5.00 D $\to$ +5.25 D / +5.50 D
+6.00 D to +6.75 D	2 steps higher (+0.50 D)	+6.00 D $\to$ +6.50 D
+7.00 D to +7.75 D	3 steps higher (+0.75 D)	+7.00 D $\to$ +7.75 D
+8.00 D to +8.75 D	3 to 4 steps higher (+0.75 D to +1.00 D)	+8.00 D $\to$ +8.75 D / +9.00 D
+9.00 D to +9.75 D	4 to 5 steps higher (+1.00 D to +1.25 D)	+9.00 D $\to$ +10.00 D
+10.00 D to +10.75 D	5 to 6 steps higher (+1.25 D to +1.50 D)	+10.00 D $\to$ +11.25 D / +11.50 D
+11.00 D to +11.75 D	7 steps higher (+1.75 D)	+11.00 D $\to$ +12.75 D
+12.00 D to +12.75 D	8 steps higher (+2.00 D)	+12.00 D $\to$ +14.00 D
+13.00 D to +13.75 D	10 steps higher (+2.50 D)	+13.00 D $\to$ +15.50 D
+14.00 D to +14.75 D	11 to 12 steps higher (+2.75 D to +3.00 D)	+14.00 D $\to$ +16.75 D / +17.00 D
+15.00 D to +15.75 D	13 to 14 steps higher (+3.25 D to +3.50 D)	+15.00 D $\to$ +18.25 D / +18.50 D
+16.00 D to +16.75 D	15 to 16 steps higher (+3.75 D to +4.00 D)	+16.00 D $\to$ +19.75 D / +20.00 D
+17.00 D to +17.75 D	17 to 18 steps higher (+4.25 D to +4.50 D)	+17.00 D $\to$ +21.25 D / +21.50 D
+18.00 D to +18.75 D	20 steps higher (+5.00 D)	+18.00 D $\to$ +23.00 D
+19.00 D to +19.75 D	22 to 23 steps higher (+5.50 D to +5.75 D)	+19.00 D $\to$ +24.50 D / +24.75 D
+20.00 D	25 to 26 steps higher (+6.25 D to +6.50 D)	+20.00 D $\to$ +26.25 D / +26.50 D

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#calculations #contact lens #conversion #optometry #tables