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Compound Interest Calculator — See How Your Money Grows Over Time

Reviewed by FinRatePro Editorial Team Last reviewed: June 2026

Albert Einstein is often credited with calling compound interest "the eighth wonder of the world" and stating that "he who understands it, earns it; he who does not, pays it." Whether or not Einstein actually said this, the sentiment captures a profound truth: compound interest is the most powerful force in personal finance. It is the mechanism by which a modest investment, given enough time, can grow into substantial wealth.

Our compound interest calculator shows you exactly how an initial investment (or debt) grows over time when interest is earned (or charged) on both the principal and the accumulated interest. You can adjust the initial amount, interest rate, compounding frequency, time horizon, and regular contributions to model any savings, investment, or debt scenario.

Compound Interest Calculator
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Your Investment Growth
Final Value
$1,020,842
Initial Investment$10,000
Total Contributions$220,000
Total Interest Earned$800,842
Interest as % of Final Value
78%
Year-by-Year Growth
YearBalanceContributionsInterest Earned
1$16,955$16,000$955
2$24,413$22,000$2,413
3$32,411$28,000$4,411
4$40,986$34,000$6,986
5$50,182$40,000$10,182
6$60,042$46,000$14,042
7$70,614$52,000$18,614
8$81,952$58,000$23,952
9$94,108$64,000$30,108
10$107,144$70,000$37,144
11$121,122$76,000$45,122
12$136,110$82,000$54,110
13$152,182$88,000$64,182
14$169,416$94,000$75,416
15$187,895$100,000$87,895
16$207,710$106,000$101,710
17$228,958$112,000$116,958
18$251,742$118,000$133,742
19$276,173$124,000$152,173
20$302,370$130,000$172,370
21$330,461$136,000$194,461
22$360,582$142,000$218,582
23$392,881$148,000$244,881
24$427,515$154,000$273,515
25$464,653$160,000$304,653
26$504,475$166,000$338,475
27$547,176$172,000$375,176
28$592,964$178,000$414,964
29$642,062$184,000$458,062
30$694,709$190,000$504,709
31$751,162$196,000$555,162
32$811,696$202,000$609,696
33$876,606$208,000$668,606
34$946,208$214,000$732,208
35$1,020,842$220,000$800,842

How to Use This Calculator

  • Initial Investment (Principal) — Enter the starting amount of your investment or debt. This is the amount on which interest will be calculated.
  • Monthly Contribution — Enter the amount you plan to add (or pay down) each month. For investments, this is your regular savings contribution. For debt, enter a negative number to model debt paydown.
  • Annual Interest Rate — Enter the annual interest rate as a percentage. For investments, use the expected annual return (e.g., 7% for a stock market index fund). For debt, use the APR.
  • Compounding Frequency — Select how often interest is calculated and added to the principal. Common frequencies: daily (most savings accounts), monthly (most loans and credit cards), quarterly, annually. More frequent compounding produces slightly higher returns for investments and higher costs for debt.
  • Time Horizon (Years) — Enter the number of years the money will grow. The longer the time horizon, the more dramatic the compounding effect. Try entering 10, 20, and 30 years to see how the final value changes.

Formula & Methodology

The compound interest formula calculates the future value of an investment (or debt) when interest is calculated on both the principal and the accumulated interest. The basic formula for compound interest without additional contributions is:

A = P × (1 + r/n)n×t

Where: A = final amount; P = principal (initial investment); r = annual interest rate (as a decimal); n = number of times interest is compounded per year; t = time in years.

When regular contributions are added, the formula becomes more complex. The future value of a series of equal monthly contributions (an annuity) is calculated as: FV = PMT × [((1 + r/n)^(n×t) - 1) / (r/n)] × (1 + r/n), where PMT is the monthly contribution amount. The total future value is the sum of the compound interest on the principal plus the future value of the contributions.

The compounding frequency (n) significantly affects the result. Daily compounding (n=365) produces more growth than annual compounding (n=1) because interest is calculated and added to the principal more frequently. At 20% (typical credit card rate), the difference between annual and daily compounding on $10,000 over 10 years grows to $1,469 ($61,917 vs $63,386). This is why credit card companies use daily compounding.

Worked Example: Retirement Savings

Let us model a realistic retirement savings scenario. A 30-year-old investor starts with $10,000 in an S&P 500 index fund and contributes $500 per month for 35 years (until age 65). We will assume a 7% average annual return (the historical inflation-adjusted return of the S&P 500 from 1957 to 2023, per data from NYU Stern).

Step-by-Step Calculation
Step 1: Future value of initial $10,000

A = $10,000 × (1 + 0.07/12)^(12×35) = $10,000 × 11.348 = $113,480

Step 2: Future value of monthly contributions

FV = $500 × [((1.005833)^420 - 1) / 0.005833] × 1.005833 = $892,310

Step 3: Combine final values

Final Balance = $113,480 + $892,310 = $1,005,790

Note: This models monthly compounding. Total contributed is $220,000 ($10,000 principal + $500/month × 420 months). Total interest earned is $785,790. Real purchasing power will be lower due to inflation; adjusted for 3% historical inflation, the final value would be approximately $425,000 in today's dollars.

Frequently Asked Questions

What is the difference between simple interest and compound interest?
Simple interest is calculated only on the principal. If you invest $10,000 at 7% simple interest for 10 years, you earn $700 per year ($7,000 total), and your final balance is $17,000. Compound interest is calculated on the principal plus accumulated interest. With annual compounding, you earn $700 in year 1, $749 in year 2 (on $10,700), and so on. After 10 years, your balance is $19,672 — $2,672 more than with simple interest. Over 35 years, the difference becomes enormous.
What is a realistic return rate for stock market investments?
The S&P 500 has returned an average of approximately 10% annually from 1957 to 2023 (not adjusted for inflation). Adjusted for inflation, the real return is approximately 7%. However, past performance does not guarantee future results, and returns are highly variable from year to year. For planning purposes, financial advisors typically recommend using 6-8% for diversified stock portfolios and 3-5% for bond portfolios.
How does inflation affect my investment returns?
Inflation erodes the purchasing power of money over time. If your investment earns 7% nominal return but inflation is 3%, your real return (purchasing power) is only 4%. To maintain purchasing power, your investments must earn at least the inflation rate. The calculator shows nominal values; to see real values, subtract the inflation rate from your expected return.
Should I use this calculator for debt paydown scenarios?
Yes. For credit card debt at 24% APR with daily compounding, enter the current balance as the principal, the interest rate as 24%, compounding as daily, and enter your monthly payment as a negative contribution. The calculator will show how the balance decreases over time and how much total interest you will pay. A $10,000 credit card balance at 24% with $200 monthly payments takes 79 months to pay off and costs $5,766 in interest.
What is the Rule of 72?
The Rule of 72 is a mental math shortcut for estimating how long it takes for an investment to double at a given interest rate. Divide 72 by the interest rate to get the doubling time in years. At 7% return, money doubles every 10.3 years (72/7). At 10%, it doubles every 7.2 years. At 24% (credit card rate), debt doubles every 3 years. This rule is approximate but accurate within 1% for interest rates between 6% and 10%.
How are taxes handled in this calculator?
The calculator shows pre-tax values. If your investments are in a tax-advantaged account (401k, IRA, Roth IRA), the shown values are accurate. If your investments are in a taxable brokerage account, you will owe taxes on dividends and capital gains, which will reduce your effective return. For taxable accounts, reduce your expected return by approximately 1-2% to account for taxes.