Compound Interest Calculator - Savings Growth Projection

Compound interest is interest calculated on both the original principal and the interest already credited. The compound interest calculator turns that idea into a savings projection you can compare across rates, terms, and deposit habits. It is useful when you are choosing a high-yield savings account, a certificate of deposit, a money-market fund, or a simple investment plan that reinvests earnings. Unlike a quote from a bank, the result is not a contractual yield. It is the mathematical future value implied by the principal, nominal annual rate, compounding frequency, years, and optional end-of-period contributions you provide. The lump-sum piece is the standard compound-interest formula FV = P(1 + r/n)^(n t). Here P is the starting principal, r is the annual nominal rate as a decimal, n is how many times interest is compounded each year, and t is time in years. Recurring deposits are modeled as an ordinary annuity added at the end of each compounding period: PMT × [((1 + r/n)^(n t) − 1) / (r/n)]. If the rate is zero, deposits add as PMT × n × t with no earnings. Interest earned equals future value minus principal minus the sum of contributions, which makes it easy to see how much of the ending balance is growth rather than new money. Frequency is not cosmetic. Annual compounding at 6% is not identical to monthly compounding at a 6% nominal rate, because monthly credits begin earning interest sooner. The difference is the gap between nominal rate and effective annual yield. Contributions must use the same period as compounding: a $200 entry with monthly compounding is $200 a month, while the same entry with annual compounding is $200 a year. Mixing a monthly savings habit with annual compounding without converting the deposit will understate the plan. People typically use the compound interest calculator to answer three planning questions. First, what will today’s balance become if it is left alone? Second, how much extra interest do regular deposits add? Third, is a slightly higher rate worth switching accounts once fees and minimums are considered? Because the exponent grows with time, a one-percentage-point change over a decade is more important than the same change over a year. Compare a conservative rate from current offers with a stretch rate, and keep the term realistic for money you may need to withdraw. The projection does not subtract taxes, early-withdrawal penalties, account fees, or inflation. Bond coupon reinvestment, stock dividend drag, and market volatility are also outside the formula. If a bank advertises APY, that figure already includes compounding; entering APY as the nominal rate and compounding again double-counts. For taxable interest, a rough after-tax rate is the nominal rate times one minus your marginal tax rate. Use the compound interest calculator as a transparent worksheet: save the inputs, test one change at a time, and confirm large decisions against the account disclosure or a licensed adviser. The arithmetic should be boring and checkable so the judgment about risk and liquidity can stay in the foreground.