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Compound Interest Calculator
By: Conor Keenan, AWMA®
Editorial Independence: Our recommendations and reviews are our own. Partner commissions may help keep our site free and do not influence our editorial content.
Simple Compound Interest Calculator
Enter your info below. Monthly contributions are assumed at the end of each month.
Estimate how an initial deposit and regular monthly contributions could grow with compound interest. Enter an annual interest rate, choose how often interest compounds, and compare the estimated ending balance with the money you put in.
This calculator uses a nominal annual interest rate, not annual percentage yield (APY). It assumes a constant rate and month-end contributions. Results are illustrations, not a promised account balance or investment return.
Enter the stated annual interest rate, not APY. APY already reflects compounding; entering it here with a daily, monthly or quarterly frequency can overstate the result.
In This Article
Key Takeaways
- Compounding adds interest to your balance so that later interest can earn interest too.
- Your starting deposit, contribution amount, time and interest rate all affect the result.
- APY already includes compounding. Entering an APY as a nominal rate can overstate growth.
- Taxes, fees, inflation, withdrawals and changing rates are excluded.
How to use the compound interest calculator
- Enter your initial deposit. Use the amount you have today. You can start at $0 if you plan to contribute later.
- Enter the annual interest rate. Use a nominal rate for the compounding frequency you select. For an account advertising APY, check its interest-rate disclosure before entering a number.
- Choose the number of years. Longer periods give interest more time to compound, provided the assumed rate continues.
- Select a compounding frequency. The choices are annually, quarterly, monthly and daily. Daily uses 365 periods per year.
- Add an optional monthly contribution. The calculator treats each contribution as arriving at the end of a month. Leave it blank or enter $0 to model the starting deposit alone.
The ending balance includes your initial deposit, contributions and estimated interest. Interest earned excludes the money you contribute. For contribution scenarios, whole-month time periods are easiest to interpret; fractional months use a mathematical approximation.
Example: $10,000 with a $100 monthly contribution
Suppose you deposit $10,000, add $100 at each month-end and earn a hypothetical 5.00% nominal annual rate compounded monthly. These figures are examples, not current offers.
| Time | Monthly contribution | Total money deposited | Estimated interest | Ending balance |
|---|---|---|---|---|
| 10 years | $0 | $10,000.00 | $6,470.09 | $16,470.09 |
| 10 years | $100 | $22,000.00 | $9,998.32 | $31,998.32 |
| 30 years | $0 | $10,000.00 | $34,677.44 | $44,677.44 |
| 30 years | $100 | $46,000.00 | $81,903.31 | $127,903.31 |
The contribution rows show why it helps to separate deposits from earnings: a larger balance comes partly from saving more and partly from interest. At a 0% rate, the result is simply the initial deposit plus your contributions.
How compound interest is calculated
For a starting deposit with no additional contributions:
Ending balance = P × (1 + r ÷ n)^(n × t)
Here, P is the initial deposit, r is the nominal annual rate written as a decimal, n is compounding periods per year and t is years. A 5% rate is entered as 5 in the tool and used as 0.05 in the formula.
For monthly contributions, the calculator converts the selected compounding rate into an equivalent monthly growth rate, then adds the future value of the month-end deposits. This is a constant-rate model; an institution’s actual crediting schedule and day-count rules can differ.
The Consumer Financial Protection Bureau explains compounding as earning interest on both principal and accumulated interest. Its deposit-account definitions distinguish APY from the interest rate: APY includes the effect of compounding.
Choose assumptions that fit your goal
Use this tool to compare contribution amounts or time horizons under the same assumptions. A daily compounding option does not establish that one account pays more than another: when comparing deposit accounts, consider their APYs, fees, access and qualification requirements.
For account shopping, compare savings account rates. For a deposit held for a specific term, use the CD calculator, which estimates earnings from each displayed offer’s APY and term. Other tools are available in our financial calculator library.
Frequently asked questions
Is compound interest safe?
Compounding does not prevent losses. Eligible savings accounts and bank CDs at FDIC-insured banks are protected if the bank fails, generally up to $250,000 per depositor, per insured bank, per ownership category. Account fees and CD early-withdrawal penalties can reduce what you receive. Investments are not FDIC-insured and can lose value. See the FDIC’s deposit-insurance guidance.
Can I use APY in this calculator?
Use the nominal annual interest rate that corresponds to the selected compounding frequency. APY already includes compounding, so treating it as a nominal rate can add a compounding effect twice. An advertised APY alone does not identify every detail of an account’s interest-crediting method.
Are contributions added at the beginning or end of the month?
At the end. A beginning-of-month deposit would have an extra month to grow, so a calculator using that convention can produce a different result.
Does the estimate include taxes or inflation?
No. The displayed balance is before taxes and fees and is not adjusted for inflation. Changing account rates, investment losses or withdrawals can also make actual results different.
*National Savings Rate Average as published by FDIC
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