Compound Interest Calculator

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (which only applies to the principal), compound interest grows exponentially — often called “earning interest on interest.”

More frequent compounding means faster growth. Daily compounding yields slightly more than monthly, which yields more than quarterly or annually. The difference becomes more pronounced at higher interest rates and over longer time periods. The Effective Annual Rate (EAR) captures this difference.

The Rule of 72 is a quick way to estimate how long it takes to double your money. Divide 72 by the annual interest rate: at 7%, your money doubles in roughly 72 ÷ 7 ≈ 10.3 years. This calculator shows the exact figure year by year.

Regular monthly contributions dramatically increase your final balance, especially over long periods, because each contribution also earns compound interest. Even a small monthly addition (e.g. $100/month) can double or triple your final balance over 20–30 years compared to a lump-sum investment alone.