The Box constant/lens constant is a value that the surgeon can use for IOL calculation to start with before he/she has a significant experience with the IOL to work on his/her own personalized constant. If you are using a two variable formula like the SRK/T, Holladay I or the Hoffer Q you will need to input the IOL constant to get the IOL power. The three constants that are used are the A-constant for SRK/T, Surgeon Factor (SF) for Holladay I, and ACD for Hoffer Q. Each of them represent the position of the IOL in the eye from the corneal vertex (SRK T) for an average patient with a lens power of 22.0D, with an average Keratometry of 43.86D, axial length of 23.55 mm, horizontal white to white 11.77, phakic ACD of 3.16 mm, phakic lens thickness of 4.72mm, pre operative refraction of -.60 D and age 72 years reports Jack Holladay*.
The A-constant, however, is in diopters and represents to the position of the lens in an average eye; eg, an A-constant of 118.5 D is equal to an ACD of 5.26 mm (distance from the corneal vertex) and an SF of 1.51 mm (distance from the iris plane) as per Jack Holladay*.
To understand the conversion of Aconstant to ACD or A constant to SF, the equations are listed here:
* International Intraocular Lens & Implant Registry 2003 Jack T. Holladay,J Cataract Refract Surg 2003; 29:176–197 © 2003 ASCRS and ESCRS