What is interest?

Interest is the amount paid or earned for the use of money over a period. It is one component of financing costs for borrowers and returns for savers. An interest rate alone does not reveal the full cost of a transaction or its gain in purchasing-power terms.

Main types of interest and calculations

1. Principal, rate and time Interest calculations combine the initial amount, rate and duration. At 10 percent annual simple interest, borrowing 1,000 lira for one year costs 100 lira in interest if there are no additional charges. Monthly and annual rates are different, so the period must be specified. 2. Simple and compound interest Simple interest is calculated on the initial principal. With compounding, interest earned becomes part of the amount used in subsequent calculations. At 10 percent annual compound interest, 1,000 lira grows to 1,210 lira over two years. This assumes an unchanged rate and no deductions. 3. Nominal and real interest Nominal interest is the monetary rate; real interest reflects the change after inflation. The exact calculation is (1 + nominal rate) / (1 + inflation rate) − 1, using decimal rates. Subtracting inflation gives an approximation that becomes less accurate at high rates.

Why do interest rates change?

4. Policy rates and market rates A central bank’s policy rate influences financial conditions but is not identical to every bank’s lending or deposit rate. Risk, maturity, collateral, competition and funding costs create differences. Transmission to the economy can take time. 5. Fixed and variable rates A fixed rate remains unchanged for the period specified in the agreement. A variable rate can reset according to a defined benchmark, changing instalments or total payments. Late-payment terms, reset dates and early-repayment provisions require separate examination. 6. Total cost and comparisons Fees, taxes, insurance and other charges may affect total financing costs. Compare total repayment information for equivalent maturities and payment schedules. Judging an offer only by a low advertised rate can obscure its actual cost.