It stands for since-inception internal rate of return; in relation to a fund/ a portfolio (also, private equity investments), it is the discount rate that makes the net present value of all the cash flows since the date of its inception (formation), equal to zero. This measure of internal rate of return (IRR) is used for evaluating the performance of a fund (or a portfolio) rather than a project, over a specific period (or periods) since inception and up to a date marking the end of a period. The cash flows used for the calculation in each period is the aggregate cash flow of all holdings of the fund/ portfolio.

The since-inception IRR (SI-IRR) is a variation of the IRR in which the measurement period of the money-weighted return (MWR) spans the entire investment period since formation of a fund/ a portfolio.