Compound Interest Calculator
See how savings can grow with compounding and monthly contributions.
Educational estimate only. This is not financial, investment, tax, legal or lending advice, a quote or a guarantee. Read the disclaimer.
Your inputs
Illustrative growth by year.
How this calculator works
a is the annual percentage ÷ 100, f is compounds per year, y is years, P is starting balance and C is the monthly contribution. At a = 0, FV = P + 12 × y × C. In this calculator the selected compounding frequency applies only to the starting balance; monthly contributions always use monthly compounding and arrive at month-end. Thus frequencies other than 12 use a mixed convention, not a single account compounding schedule. Total contributed is P + 12 × y × C. Taxes, fees, inflation and variable returns are excluded. Fractional years use fractional exponents; charts show completed years.
Dollar results are in USD and generally rounded to the nearest whole dollar; salary equivalents and percentage outputs use two decimal places. Calculations retain JavaScript floating-point precision between steps rather than rounding every payment to cents. Displayed components may not sum exactly to displayed totals. Payoff times use whole monthly payment periods. Defaults are examples, not current rates or recommendations.
Full methodology · About the publisher and corrections
Content and calculation checks: October 12, 2026. Educational estimates; no independent professional certification is claimed.
Worked example: $10,000 plus $300 per month
Starting with $10,000, adding $300 per month and using a 7% annual rate for 20 years produces an illustrative ending balance of about $196,665. Total contributions are $82,000, so the model attributes about $114,665 to compounded growth.
This is a constant-rate illustration, not a forecast. Actual savings or investment returns can be higher, lower or negative.
Practical guide
Use compound-interest scenarios carefully
- Compare several return assumptions rather than relying on one optimistic rate.
- Change contribution amount and time horizon separately to see which has more impact.
- Remember that fees, taxes and inflation reduce real-world outcomes.
- For a simpler bank-savings view, compare the Savings Calculator.
How to use the result
Change one assumption at a time to compare scenarios. For example, test a lower rate, larger contribution, different term or extra payment. This makes the calculator most useful as a decision-support tool rather than a single-number answer.
Important financial decisions can depend on taxes, fees, eligibility rules, lender policies, market returns and personal circumstances that this simplified calculator does not capture.
Frequently asked questions
Is this compound interest calculator free?
Yes. SumStride calculators are free to use and do not require an account.
Are the results exact?
No. Results are estimates based on the values you enter and the assumptions described on this page. Real-world fees, taxes, rates, policies and timing can differ.
Does SumStride save my inputs?
No account is required. The calculator runs in your browser; SumStride does not need your entered values to perform the calculation.