Interest rate.

In plain English

An interest rate is what borrowing or lending costs over a year. If you take a $10,000 loan at 5%, you owe roughly $500 in interest the first year. If you put $10,000 in a savings account paying 4%, the bank pays you roughly $400 the first year. The rates banks charge for loans are always higher than what they pay savers, that gap is how banks make money.

01Why it matters

Interest rates set the price of nearly everything in finance, mortgages, car loans, credit cards, savings accounts, bonds, and stock valuations. They move together: when the Federal Reserve raises its policy rate, almost every other rate adjusts within months. Knowing what's a 'normal' rate for each kind of debt or savings keeps you from accepting a bad deal.

02The math, step by step

Same $300,000 mortgage, 30 years: at a 6% rate, the monthly payment is $1,799 and total interest paid is $347,514. At a 7.5% rate, the payment is $2,098 and total interest paid is $455,219. A 1.5% rate difference adds $107,705 in interest over the life of the loan, same house, same buyer, just a different rate window.

Illustrative example. The amounts here are hypothetical, chosen to show how the math works, not real quoted rates or figures.

03What this is NOT