Compound Interest Calculator
What this compound interest calculator does: it projects how a starting amount and regular deposits grow when interest is added to the balance and then earns interest itself. It has two modes:
1. Grow My Money: the final balance, how much of it is your deposits vs. interest, its value after inflation, and a best/worst range
2. Reach a Target: the regular deposit needed to reach a goal by a date
3. APY: the yearly yield including compounding, the figure US banks must disclose on deposit accounts
4. A growth chart and a year-by-year (or month-by-month) schedule
Free, no sign-up. Your numbers stay in your browser. Results are educational projections at a steady rate, not financial advice or a promised return.
See how your money grows — with APY by law, contribution step-ups, inflation & a goal planner
Under the Truth in Savings Act (Regulation DD, 12 CFR Part 1030), U.S. banks must disclose the Annual Percentage Yield (APY) — the yearly yield including compounding, so deposit accounts can be compared. For investments, the rate is an assumption, not a disclosed yield. Source: CFPB — Reg DD · The SEC's Investor.gov also has a compound interest calculator.
| Date | Invested | Interest | Balance | Real Value |
|---|
How to use the compound interest calculator
1. Choose Grow My Money, then pick Lump Sum, SIP / Recurring (regular deposits only), or Lump Sum + SIP.
2. Set the currency and start month, then enter the Initial Investment, Monthly Contribution, Interest / Growth Rate, and Time Period (years or months).
3. Under Advanced Settings, choose the Compounding Frequency, Contribution Frequency, and Contribution Timing (start or end of each period). Optionally add an Annual Step-Up (deposits rise each year), a Rate Variance for a best/worst range, and an Inflation Rate.
4. Results update as you type. Switch to Reach a Target to solve for the deposit instead. Save with Copy Summary, Export CSV, Export PDF, or Copy Shareable Link.
How to read your results
| Result | What it means |
|---|---|
| Total Maturity Value | The projected balance at the end of the period. |
| Total Principal / Total Interest | What you put in, and what the balance earned on top. |
| Purchasing Power | The final balance in today’s money, after the inflation rate you entered. |
| Wealth Multiplier | Final balance ÷ total principal. |
| APY | The rate you entered, converted to a yearly yield that includes compounding. |
| Best/worst band | Final balances at your rate minus and plus the variance you set, to show how much the rate assumption matters. |
Worked example: $10,000 plus $200 a month for 20 years
Here’s one example run through the calculator: $10,000 to start, $200 at the end of each month, 5% a year compounded monthly, for 20 years, with 2.5% inflation.
| Result | Figure |
|---|---|
| Final balance | $109,333 |
| Total principal / total interest | $58,000 / $51,333 |
| Wealth multiplier | 1.89x |
| In today’s money (2.5% inflation) | $66,723 |
| APY at 5% compounded monthly | 5.12% |
| Range at 3% / 7% | $83,868 / $144,573 |
| Deposits rising 3% a year instead | $132,326 (with $74,489 deposited) |
| Monthly deposit to reach $100,000 instead | $178 |
🧮 How this was calculated
- Monthly rate: 5% ÷ 12 = 0.4167%. Month 1: $10,000 × 0.4167% = $41.67 of interest, then the $200 deposit: $10,241.67
- The starting amount grows to $10,000 × (1 + 0.004167)240 = $27,126
- The deposits grow to $200 × ((1 + 0.004167)240 − 1) ÷ 0.004167 = $82,207. Total: $27,126 + $82,207 = $109,333
- Principal: $10,000 + 240 × $200 = $58,000. Interest: $109,333 − $58,000 = $51,333
- APY: (1 + 0.05 ÷ 12)12 − 1 = 5.12%. Today’s money: $109,333 ÷ 1.02520 = $66,723
Assumptions: the same rate every month · deposits at the end of each month · no withdrawals, taxes, or fees.
Example figures are illustrative, not averages or statistics.
Methodology and sources
1. Growth: the annual rate is converted to an equivalent monthly rate for your compounding frequency, (1 + rate ÷ n)n ÷ 12 − 1, then interest and deposits are added month by month.
2. APY: (1 + rate ÷ n)n − 1. Under the Truth in Savings Act (Regulation DD), US banks and credit unions must state the APY on deposit accounts so they can be compared. For investments, the rate you enter is an assumption, not a disclosed yield.
3. Reach a Target: finds the steady deposit that reaches the goal, rounded up to the next dollar. The SEC’s Investor.gov offers similar free calculators.
4. Inflation: balance ÷ (1 + inflation)years.
5. Rule of 72: 72 ÷ the rate is a quick estimate of the years to double; at 5%, 72 ÷ 5 = 14.4 years.
6. Not included: taxes, fees, and the ups and downs of real investment returns.
Frequently Asked Questions
1. What is compound interest?
Ans: Interest that’s added to your balance and then earns interest itself. In the example, month 1 earns $41.67 on $10,000; month 2 earns interest on $10,241.67. Over 20 years, interest made up $51,333 of the $109,333 total.
2. How often should interest compound for the best returns?
Ans: The more frequently your interest compounds, the faster your money grows. Daily compounding will yield slightly higher returns than monthly compounding, and monthly yields more than annual.
3. What is the difference between simple and compound interest?
Ans: Simple interest is paid only on the original amount: $10,000 at 5% earns $500 every year. Compound interest is paid on the growing balance, so each year’s interest is a little larger than the last.
4. Does compounding daily instead of monthly make a big difference?
Ans: Usually a small one. At 5%, monthly compounding gives an APY of 5.12% and daily about 5.13%. In the worked example, daily compounding adds about $150 over 20 years. The rate and the time involved matter much more.
5. What is APY?
Ans: Annual percentage yield: the yearly return including compounding. US banks and credit unions must disclose it on savings accounts and certificates of deposit under the Truth in Savings Act, so accounts with different compounding can be compared on one number.
6. Did Einstein call compound interest the eighth wonder of the world?
Ans: The quote is often attributed to him, but there’s no reliable record that he said it. The math of compounding stands on its own without the attribution.
7. What rate should I enter?
Ans: For a savings account or CD, the APY or rate the bank quotes. For investments, there’s no fixed rate; use the Rate Variance setting to see a range, since actual returns rise and fall from year to year.
Kevin Brown researches personal finance topics using official sources; he is not a licensed financial advisor or mortgage lender. Last reviewed: October 2026.
