How compound interest works

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods — in other words, you earn interest on your interest. The standard formula is A = P(1 + r/n)^nt, where P is your starting principal, r is the annual interest rate, n is the number of times interest compounds per year, and t is the number of years. This calculator lets you solve for any one of those variables: leave Principal blank to find the starting amount needed, Rate blank to find the required annual return, Years blank to find how long it takes to reach a target, or Final Amount blank to project the end value of your investment.

Regular contributions and increasing deposits

Most real-world savings plans involve ongoing contributions, not just a single lump sum. This calculator lets you add monthly or yearly contributions on top of your starting principal to model how consistent saving accelerates growth. Each deposit earns compound interest from the moment it's added, so even modest regular contributions can make a dramatic difference over long time horizons.

You can also set contributions to increase over time by a fixed dollar amount or a percentage — useful for modeling raises, annual savings goals, or inflation adjustments. For example, you might start with $500 per month and increase that by $50 every year, or raise it by 3% annually to keep pace with salary growth. The calculator simulates month by month, applying increases at the interval you choose (monthly or yearly), and shows a year-by-year breakdown of your total balance, cumulative contributions, and interest earned so you can see exactly how your money grows.

Common use cases

This compound interest calculator is useful for a wide range of financial planning scenarios: estimating how much a retirement savings account like a 401(k) or IRA will grow over decades, projecting returns on a brokerage or index fund investment, comparing the effect of different compounding frequencies (monthly vs. daily), figuring out how long it will take to reach a savings goal, or simply understanding the power of compound growth. Everything runs in your browser — no data is sent anywhere, no account required.