Internal rate of return (IRR) is a method of calculating an investment's rate of return. The term internal refers to the fact that the calculation excludes external factors, such as the risk-free rate, inflation, the cost of capital, or financial risk.
The method may be applied either ex-post or ex-ante. Applied ex-ante, the IRR is an estimate of a future annual rate of return. Applied ex-post, it measures the actual achieved investment return of a historical investment.
An interest rate is the amount of interest due per period, as a proportion of the amount lent, deposited, or borrowed. Interest rate periods are ordinarily a year and are often annualized when not. Alongside interest rates, three other variables determine total interest: principal sum, compounding frequency, and length of time.
Interest rates reflect a borrower's willingness to pay for money now over money in the future. In debt financing, companies borrow capital from a bank, in the expectation that the borrowed capital may be used to generate a return on investment greater than the interest rates. Failure of a borrower to continue paying interest is an example of default, which may be followed by bankruptcy proceedings. Collateral is sometimes given in the event of default.