Compound Interest Calculator — See How Money Grows
Our free compound interest calculator shows exactly how your money grows over time through the power of compounding. Enter your initial investment, interest rate, time period, and optional regular contributions. See the year-by-year growth breakdown, compare compounding frequencies, and model real purchasing power with inflation adjustment.
This calculator is for educational and illustrative purposes only. Past returns do not guarantee future results. Consult a qualified financial advisor for investment decisions.
What Is Compound Interest?
Compound interest is often called the eighth wonder of the world — a phrase frequently attributed to Albert Einstein, though its exact origin is disputed. The power of compounding comes from earning interest not just on your original principal, but on all the interest you have already earned.
With simple interest, $10,000 at 7% earns $700 per year, every year — always calculated on the original $10,000. With compound interest, the first year earns $700, making the balance $10,700. The second year earns 7% of $10,700 = $749. The third year earns 7% of $11,449 = $801. Each year the interest amount grows because the balance grows. Over decades, this difference becomes enormous.
Compound Interest Formula
The standard compound interest formula is:
A = P × (1 + r/n)^(n×t)
Where A is the final amount, P is the principal, r is the annual interest rate as a decimal, n is the number of compounding periods per year, and t is the time in years.
Simple Interest vs Compound Interest
| Year | Simple Interest (7%) | Compound Interest (7% annual) | Difference |
|---|---|---|---|
| 1 | $10,700 | $10,700 | $0 |
| 2 | $11,400 | $11,449 | $49 |
| 5 | $13,500 | $14,026 | $526 |
| 10 | $17,000 | $19,672 | $2,672 |
| 20 | $24,000 | $38,697 | $14,697 |
| 30 | $31,000 | $76,123 | $45,123 |
| 40 | $38,000 | $149,745 | $111,745 |
Starting principal: $10,000 at 7% interest rate.
Effect of Compounding Frequency
How often interest compounds significantly affects your final balance, especially over long periods.
| Compounding | Formula | $10,000 at 7% after 30 years | Effective Annual Rate |
|---|---|---|---|
| Annually | (1 + 0.07)^30 | $76,123 | 7.000% |
| Semi-Annually | (1 + 0.035)^60 | $77,641 | 7.123% |
| Quarterly | (1 + 0.0175)^120 | $78,402 | 7.186% |
| Monthly | (1 + 0.00583)^360 | $78,915 | 7.229% |
| Weekly | (1 + 0.001346)^1560 | $79,085 | 7.246% |
| Daily | (1 + 0.000192)^10950 | $79,141 | 7.250% |
| Continuously | e^(0.07×30) | $79,163 | 7.251% |
The Rule of 72
The Rule of 72 gives a quick estimate of how long it takes to double your money at a given interest rate:
Years to double ≈ 72 ÷ Annual Interest Rate
| Annual Return | Years to Double | $10,000 becomes $20,000 by Year |
|---|---|---|
| 2% | 36.0 years | Year 2058 (from 2024) |
| 4% | 18.0 years | Year 2042 |
| 6% | 12.0 years | Year 2036 |
| 7% | 10.3 years | Year 2034 |
| 8% | 9.0 years | Year 2033 |
| 10% | 7.2 years | Year 2031 |
| 12% | 6.0 years | Year 2030 |
| 15% | 4.8 years | Year 2029 |
Impact of Regular Contributions
Adding regular contributions dramatically accelerates wealth building. The combination of compound interest and consistent contributions creates an exponential growth curve. Here is how a monthly contribution of $500 affects a $10,000 initial investment at 7%:
| Years | No Contributions | With $500/month | Extra from Contributions |
|---|---|---|---|
| 5 | $14,026 | $55,348 | +$41,322 |
| 10 | $19,672 | $114,928 | +$95,256 |
| 15 | $27,590 | $201,294 | +$173,704 |
| 20 | $38,697 | $326,753 | +$288,056 |
| 25 | $54,274 | $506,865 | +$452,591 |
| 30 | $76,123 | $764,523 | +$688,400 |
All values at 7% annual return, compounded monthly (illustrative).
Inflation and Real Returns
Compound interest grows your nominal wealth, but inflation erodes purchasing power over time. To understand real wealth growth, you need to account for inflation:
Real Return ≈ Nominal Return − Inflation Rate (simplified — the precise formula is (1 + nominal) / (1 + inflation) - 1)
| Scenario | Nominal Return | Inflation | Real Return | $10,000 real value in 30yr |
|---|---|---|---|---|
| Savings account (low rate) | 2% | 3% | -1% | $7,374 |
| Bonds (moderate) | 4% | 3% | 1% | $13,478 |
| Diversified portfolio | 7% | 3% | 4% | $32,434 |
| S&P 500 historical avg | 10% | 3% | 7% | $76,123 |
| Growth stocks (high risk) | 12% | 3% | 9% | $132,677 |
Common Investment Benchmarks
| Investment Type | Historical Annual Return | Risk Level | Notes |
|---|---|---|---|
| US Savings Account | 0.5-5% (varies) | Very Low | FDIC insured, no market risk |
| US Treasury Bonds | 2-5% | Very Low | Government backed |
| Corporate Bonds | 3-7% | Low-Medium | Depends on credit rating |
| S&P 500 Index Fund | ~10% (nominal), ~7% real | Medium | Historical 90-year average |
| Small Cap Stocks | ~11-12% | High | Higher volatility |
| International Stocks | ~8-9% | Medium-High | Currency and country risk |
| Real Estate (REITs) | ~8-10% | Medium | Includes dividends |
Past performance does not guarantee future results.
How to use this compound interest calculator
- Enter starting principal — cash already invested or saved today.
- Set annual rate and years — use realistic long-run stock/bond blends or a conservative stress test.
- Add monthly contributions if you invest regularly — end-of-month deposits match most retirement models.
- Pick compounding frequency — daily or monthly compounding matters more over decades than over a few years.
- Toggle inflation to see purchasing power, not just nominal dollars.
$1,000 growth at 5%, 7%, and 10% (annual compounding)
Illustrative balances with no additional contributions — actual markets vary year to year.
| Years | 5% rate | 7% rate | 10% rate |
|---|---|---|---|
| 10 | $1,629 | $1,967 | $2,594 |
| 20 | $2,653 | $3,870 | $6,728 |
| 30 | $4,322 | $7,612 | $17,449 |
How to calculate compound interest manually
Use A = P(1 + r/n)nt where P is principal, r is annual rate as a decimal, n is compounds per year, and t is years.
Example: $5,000 at 6% compounded monthly for 10 years: r = 0.06, n = 12, t = 10 → A ≈ $9,096(more than simple interest because each month's interest earns interest).
Tips and common mistakes
- Confusing APR with APY — APY already reflects compounding; do not compound twice.
- Ignoring fees and taxes — expense ratios and capital-gains tax drag reduce net growth.
- Assuming linear returns — real portfolios have down years; run multiple rate scenarios.
- Forgetting contributions — monthly deposits often matter more than a slightly higher rate on small principals.
More Q&A lives in the Frequently Asked Questions section below (matches FAQPage structured data).