Compound Interest Calculator
See how your money grows over time with compound interest. Add regular contributions and compare scenarios.
Last updated: January 2026
Compound Interest Calculator
What is compound interest?
Compound interest is the interest you earn on both your original principal and the interest that accumulates over time. Albert Einstein reportedly called it the "eighth wonder of the world." When you earn interest on your interest, your money grows exponentially rather than linearly.
The secret to compound interest is time. The earlier you start investing, the more powerful the compounding effect. A small amount invested in your 20s can grow to more than a much larger amount invested in your 40s.
How to use this calculator
Enter your starting principal, expected annual rate of return, how often interest compounds, the investment timeline in years, and any monthly contributions. The calculator will show your final balance, total contributions, total interest earned, and a year-by-year breakdown.
Try adjusting the monthly contribution to see how even small additional amounts make a big difference over time. This is especially powerful for retirement planning.
This is an estimate. Actual returns vary.
More guidance for the Compound Interest Calculator
The formula behind compound interest
Compound interest uses the formula A = P(1 + r/n)^(nt), where P is your starting principal, r is the annual rate, n is the number of compounding periods per year, and t is time in years. Monthly contributions are added as a growing annuity, so every deposit has its own smaller compounding window. A $10,000 starting balance at 7% for 10 years grows to about $19,672 before contributions; adding $500 per month can push the ending balance above $105,000 because each deposit earns its own return.
Rule of 72
The Rule of 72 is a quick mental shortcut: divide 72 by the annual return to estimate how many years it takes money to double. At 6%, money doubles in about 12 years. At 8%, it doubles in about 9 years. This is only an estimate, but it helps you understand why starting earlier matters more than trying to find a perfect investment. Use the retirement calculator to see the same effect across a full working life.
Compounding frequency and real returns
Daily, monthly, quarterly, and annual compounding produce slightly different results. The higher the frequency, the more often interest is added to the balance. The difference is usually smaller than the effect of contribution size, time horizon, or rate of return. Historically, the S&P 500 has averaged around 10% nominal return before inflation and roughly 7% real return after inflation, but any future year can be much higher or lower. Compare nominal growth with purchasing power using the inflation calculator.
When to consult a professional
Talk to a financial planner when the result affects retirement timing, college savings, tax strategy, or investment allocation. This calculator shows the math of compounding, but it does not decide which account type, asset mix, or tax treatment is right for you.
Practical example
The safest way to use the Compound Interest Calculator is to run one realistic case, then change one assumption at a time. Start with your current numbers, save or write down the result, then test a conservative scenario and an optimistic scenario. This makes the tool more useful than a single answer because you can see which input actually drives the outcome. For money, tax, legal, or health-adjacent decisions, the range is often more important than the exact midpoint.
Decision checklist
Before relying on any calculator result, check whether the inputs match your real situation, whether rates or rules have changed this year, whether the result excludes fees or local rules, and whether a professional review would be cheaper than a mistake. Use the result as a planning estimate, then verify critical numbers against official documents, lender quotes, payroll records, contracts, or professional advice.