Compound Interest Calculator: Formula, Tables and the Rule of 72
Short answer: a compound interest calculator grows a balance using the formula A = P × (1 + r/n)^(n×t), where P is the starting amount, r the yearly rate, n the compounding periods a year and t the years. $10,000 at 7% a year becomes $19,672 in 10 years, $38,697 in 20 and $76,123 in 30. Adding $300 a month on top makes the end result far larger, and the Rule of 72 estimates doubling time as 72 divided by the rate.
Key takeaways
- Compound interest is interest earning interest. Time is the biggest lever, bigger than the rate in most cases.
- $10,000 at 7% roughly doubles every 10 years: $19,672 at 10, $38,697 at 20, $76,123 at 30.
- How often interest compounds matters little: $10,000 at 6% for 10 years is $17,908 yearly and $18,220 daily.
- Inflation shrinks the real result. At 3% inflation, $76,123 in 30 years buys about what $31,361 buys today.
What is the formula?
A = P × (1 + r/n)n×t
- P is the starting amount, for example $10,000.
- r is the yearly rate as a decimal, so 7% is 0.07.
- n is how many times a year interest compounds (1, 12 or 365).
- t is the number of years.
With yearly compounding it simplifies to P × (1 + r)t. For $10,000 at 7% for 10 years: 10,000 × 1.0710 = $19,672.
What does $10,000 grow to?
| Years | At 5% | At 7% | At 9% |
|---|---|---|---|
| 10 | $16,289 | $19,672 | $23,674 |
| 20 | $26,533 | $38,697 | $56,044 |
| 30 | $43,219 | $76,123 | $132,677 |
After 30 years, 9% grows the same $10,000 to more than three times the 5% result. See how to invest $10,000 for where such money could go.
What about regular deposits?
A calculator with monthly deposits adds the future value of a stream of payments. At 7% for 20 years:
| Monthly deposit | You put in | Value after 20 years |
|---|---|---|
| $100 | $24,000 | $52,093 |
| $300 | $72,000 | $156,278 |
| $500 | $120,000 | $260,463 |
See how much $500 a month is worth in 10, 20 and 30 years for longer horizons, and try the Compound Clash game.
Does compounding frequency matter?
| Compounding | $10,000 at 6% for 10 years |
|---|---|
| Yearly | $17,908 |
| Monthly | $18,194 |
| Daily | $18,220 |
Going from yearly to monthly adds $286, and from monthly to daily only $26 more. The rate and the time dominate. When comparing accounts, compare the annual percentage yield (APY), which already includes the compounding frequency.
What is the Rule of 72?
Divide 72 by the yearly rate to estimate years to double. It is a quick mental shortcut:
| Rate | Rule of 72 | Exact |
|---|---|---|
| 4% | 18.0 years | 17.7 years |
| 6% | 12.0 years | 11.9 years |
| 8% | 9.0 years | 9.0 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6.0 years | 6.1 years |
It works in reverse for debt. Money at 24% doubles in about three years, which is why high-interest balances grow so fast. See compound interest and the Rule of 72.
How does inflation change the result?
Prices rise, so a future sum buys less. A 7% return with 3% inflation is a real return of about 3.9%. $10,000 grows to $76,123 in 30 years in nominal terms, but in today's purchasing power it is about $31,361. Use a real rate when you plan for goals measured in today's prices. See how inflation erodes your savings and the savings goal calculator guide.
Why does starting early matter so much?
Compare two savers who each invest $200 a month at an assumed 7% until age 65. Saver A starts at 25 and stops after 10 years, putting in $24,000. Saver B starts at 35 and keeps going for 30 years, putting in $72,000.
| Saver | Years investing | You put in | Value at 65 |
|---|---|---|---|
| A: starts at 25, stops at 35 | 10 | $24,000 | $263,513 |
| B: starts at 35, continues to 65 | 30 | $72,000 | $243,994 |
Saver A ends up with more while putting in a third as much, because the first 10 years of money have 30 more years to compound. The lesson is not to stop after 10 years, but that early dollars are the most valuable ones. Try the Start Early Race to see it.
How does compounding work against you in debt?
The same maths grows debt. A $5,000 card balance at 22% with interest added monthly and no payments becomes about $14,872 after five years. That is why paying more than the minimum matters so much. See the debt payoff calculator guide and how to pay off credit card debt fast.
How do you use a compound interest calculator well?
- Use a realistic rate. For a diversified stock portfolio, 5% to 7% after fees is a common planning range. Savings accounts pay less.
- Include regular deposits, since they usually matter more than the starting sum.
- Adjust for inflation. Use a real rate to see purchasing power.
- Test a lower rate. If the plan only works at a high rate, add margin.
- Account for fees and taxes. A 1% yearly fee cuts a 7% return to 6% and takes a large share of the result. See how fund fees add up.
Sources and further reading: SEC Investor.gov has a free compound interest calculator.
General education, not investment advice. Reviewed October 2026. Figures assume a constant rate, no taxes or fees and deposits at the end of each month. Returns are not guaranteed, and investing involves risk, including loss of principal.
Frequently asked questions
What is the compound interest formula?
A = P × (1 + r/n)^(n×t), where P is the starting amount, r the yearly rate, n the compounding periods a year and t the years. With yearly compounding it is P × (1 + r)^t.
How much will $10,000 be worth in 20 years?
At 7% a year, about $38,697. At 5% it would be $26,533, and at 9% it would be $56,044. These are illustrations at a steady rate, and real returns vary.
What is the Rule of 72?
A shortcut that divides 72 by the yearly rate to estimate how many years it takes to double. At 8% it is 9 years, and at 6% it is 12 years.
Does daily compounding make a big difference?
Not much. $10,000 at 6% for 10 years is $17,908 compounded yearly and $18,220 daily. The rate and the years matter far more. Compare accounts by their annual percentage yield.
How does inflation affect compound interest?
It reduces the real value of the result. A 7% return with 3% inflation is a real return of about 3.9%, so $76,123 after 30 years buys about what $31,361 buys today.