Savings & Investing
Compound Interest Calculator
Compound interest pays you on interest you already earned, which is why the last few years of a long term do most of the work. Enter a starting balance, a rate, and how long you will leave it alone — add regular deposits if you make them — and see how the ending balance splits between your own money and growth.
Estimates only. Verify important figures against your own records before acting on them. See our disclaimer.
What this calculator includes
What you enter
- Starting balance
- Annual interest rate
- Time invested
- Compounding frequency
- Regular contribution (optional)
- Contribution frequency
What you get back
- Ending balance
- Interest earned
- Total you put in
- Effective annual yield
- Growth multiple
- Contributions alone
How this calculation works
How this calculation works
With no contributions this is the classic compound interest formula. Once you add deposits, the balance is stepped forward one contribution period at a time so each deposit only earns from the moment it lands.
- Rate per contribution period = (1 + annual rate ÷ 100 ÷ compounds per year)^(compounds per year ÷ contributions per year) − 1
- Each period: balance = balance × (1 + period rate) + contribution
- Repeat for contributions per year × years periods
- Interest earned = ending balance − starting balance − total contributions
- Effective annual yield = (1 + annual rate ÷ 100 ÷ compounds)^compounds − 1
What the result means
The ending balance splits into money you deposited and interest earned. Interest earned is the growth alone, and the effective annual yield converts your nominal rate at its compounding frequency into a single yearly figure.
Common mistakes and assumptions
- Mixing up compounding frequency and contribution frequency — they are separate inputs here.
- Expecting a real-world investment return to be as smooth as a fixed rate.
- Ignoring tax and inflation, neither of which this calculator applies.
Worked example
$10,000 to start, 7% compounded monthly for 20 years, with $300 added every month.
- 1Monthly rate = 7 ÷ 100 ÷ 12 = 0.00583333, and there are 240 months.
- 2The $10,000 alone grows to $10,000 × 1.00583333^240 = $40,387.
- 3The $300 monthly deposits total $72,000 and grow to about $156,278.
- 4Ending balance is roughly $196,665.
- 5Of that, $82,000 is money you put in and about $114,665 is interest.
Frequently asked questions
- Does compounding frequency matter much?
- Less than people expect. At 7%, monthly compounding yields 7.229% effective versus 7.250% daily — a rounding difference over a year. Time in the account and the rate itself matter far more.
- Are contributions added at the start or end of the period?
- The end, which is the conservative assumption and matches most automatic transfers. Deposits at the start of each period would earn one extra period of interest each.
- Is this adjusted for inflation?
- No. To see purchasing power, enter a real rate instead — roughly your nominal rate minus expected inflation.
- Can I use this for an investment account?
- Yes, treating the rate as an average annual return. Real markets do not deliver a steady rate, so use it for planning ranges rather than as a prediction.
Disclaimer
This calculator returns estimates based only on the values you enter. It does not account for taxes, financing terms, local regulations, or conditions specific to your operation, and it is not accounting, legal, tax, or investment advice. Confirm any figure that carries real cost before you rely on it.
Calculations run entirely in your browser and the numbers you type are never sent to us or stored. Read how Answerivo calculators are built.
Formulas and worked example last reviewed .