for interest rates "as stated" without adjustment for the full effect of compounding. An interest rate is called nominal if the frequency of compounding is not identical to the basic time unit in which the nominal rate is quoted.
The concept of real interest rate is useful to account for the impact of inflation. In the case of a loan, it is this real interest that the lender effectively receives. For example, if the lender is receiving 8 percent from a loan and the inflation rate is also 8 percent, then the real rate of interest is zero: despite the increased nominal amount of currency received, the lender would have no monetary value benefit from such a loan because each unit of currency would get devaluated due to inflation by the same factor as the nominal amount gets increased. The relationship between the real interest value, the nominal interest rate value, and the inflation rate value is given by
When the inflation rate is low, the real interest rate is approximately given by the nominal interest rate minus the inflation rate, i.e.,
In this analysis, the nominal rate is the stated rate, and the real interest rate is the interest after the expected losses due to inflation. Since the future inflation rate can only be estimated, the ex ante and ex post real interest rates may be different; the premium paid to actual inflation.
The nominal interest rate, also known as an Annualised Percentage Rate or APR, is the periodic interest rate multiplied by the number of periods per year. For example, a nominal annual interest rate of 12% based on monthly compounding means a 1% interest rate per month. A nominal interest rate for compounding periods less than a year is always lower than the equivalent rate with annual compounding. Note that a nominal rate without the compounding frequency is not fully defined: for any interest rate, the effective interest rate cannot be specified without knowing the compounding frequency and the rate. Although some conventions are used where the compounding frequency is understood, consumers in particular may fail to understand the importance of knowing the effective rate. Nominal interest rates are not comparable unless their compounding periods are the same; effective interest rates correct for this by "converting" nominal rates into annual compound interest. In many cases, depending on local regulations, interest rates as quoted by lenders and in advertisements are based on nominal, not effective interest rates, and hence may understate the interest rate compared to the equivalent effective annual rate. Confusingly, in the context of inflation, 'nominal' has a different meaning. A nominal rate can mean a rate before adjusting for inflation, and a real rate is a constant-prices rate. The Fisher equation is used to convert between real and nominal rates. To avoid confusion about the term nominal which has these different meanings, some finance textbooks use the term 'Annualised Percentage Rate' or APR rather than 'nominal rate' when they are discussing the difference between effective rates and APR's. The term should not be confused with simple interest which is not compounded. The effective interest rate is always calculated as if compounded annually. The effective rate is calculated in the following way, where r is the effective rate, i the nominal rate, and n the number of compounding periods per year :
Examples
Monthly compounding
Example 1: A nominal interest rate of 6% compounded monthly is equivalent to an effective interest rate of 6.17%. Example 2: 6% annually is credited as 6%/12 = 0.5% every month. After one year, the initial capital is increased by the factor 12 ≈ 1.0617.
Daily compounding
A loan with daily comp have a substantially higher rate in effective annual terms. For a loan with a 10% nominal annual rate and daily compounding, the effective annual rate is 10.516%. For a loan of $10,000, the borrower would pay $51.56 more than one who was charged 10% interest, compounded annually.
