You have $5,000 to invest in a savings account or certificate of deposit (CD), and the bank offers a 4% interest rate compounded monthly. Or perhaps you're borrowing $10,000 for a car at 7% interest over 5 years. Your question is the same in both cases: How much will this cost me (or earn me) in the end? Interest isn't just about the rate—it's about how that rate is applied. Is it simple interest? Is it compounded daily, monthly, or annually? These small details can change the final number by hundreds or even thousands of dollars. You could try to calculate it manually using complex formulas like $A = P(1 + r/n)^{nt}$. But that requires understanding exponents, compounding frequencies, and careful math. Or you could use an interest calculator to instantly see that your $5,000 investment will grow to $6,105 after 5 years, earning you $1,105 in pure interest. An interest calculator computes how much interest you will earn on savings or pay on a loan based on your principal a...