An approximate formula established by Sanders, Retzlaff and Kraft to determine the power of an intraocular lens implant P, in aqueous, to render the eye emmetropic (and ignoring the lens thickness)
P = A − 2.5X − 0.9Kwhere A is a numerical term specific to the implant and to the manufacturer. It is equal, on average, to 115 for anterior chamber implants and 116.8 for posterior chamber implants. X is the axial length of the eye (in mm) and K is the average keratometer reading (in dioptres). The formula gives satisfactory results for eyes of average length. Example: A = 116.8, X = 23mm and K = 43D, P = 116.8 − 57.5 − 38.7 = 20.6D.The power of the intraocular lens implant differs in air and is given by the following formula
where nI is the index of refraction of the implant and P its power in aqueous. Example: assuming nI is equal to 1.523 and the power is 20.6D, Pair = (1.523 − 1.0)/(1.523 −1.333) ✕ 20.6 = 56.70D.There are several other formulae used to predict the power of an intraocular lens implant (e.g. Holladay, Hoffer Q). See biometry of the eye; intraocular lens.