In case of a loan n = number of periods (eg months) i = % intrest per period (ie 3%/year = .03 becomes 3/12% or .25% or .0025 /month if month = period) pmt = payment per period pv = initial amount or capital fv = 0 at the end type = 0, we pay at the end of each period then pmt = pv * i --------------------- 1 1 - ---------- (1 + i) ^ n There's no way - that I know of - to get the i isolated to either side of the equation (in order to get i = ....)