The Power of Compounding
Compound Interest Calculator 2026
A
Written by the USAFinCalc Team
Enter a starting balance, monthly contribution, and interest rate. The calculator shows year-by-year growth with daily, monthly, or annual compounding — and includes an inflation-adjusted view of what that balance is actually worth.
Quick answer: $10,000 invested at 7% for 30 years = $76,123 — with no extra contributions. Add just $200/month and it becomes $283,382. That's the power of compounding.
Investment Details
$
$
yrs
%
%
Enter your investment details to see compound growth.
How Compound Interest Works
A = P(1 + r/n)^(nt)
A = Final Amount · P = Principal · r = Annual Rate · n = Compounds/Year · t = Years
Compounding = Interest on Interest
Simple interest only earns on your principal. Compound interest earns on your principal plus all previously earned interest — creating exponential, not linear, growth.
Frequency Matters
Daily compounding earns slightly more than monthly, which earns more than annual. On $100K at 7% for 10 years: daily = $100,179 more than annual compounding.
Time is the X-Factor
Starting at 25 vs 35 with the same contributions can mean 2× the final balance. The first 10 years have the highest compounding impact — every year of delay costs you dearly.
Rule of 72
Divide 72 by your annual rate to estimate doubling time. At 7%: 72 ÷ 7 = ~10.3 years to double. At 10%: 7.2 years. At 4%: 18 years. Simple and powerful.
Compound Interest vs Simple Interest
| Scenario | Simple Interest | Compound (Monthly) | Difference |
|---|---|---|---|
| $10,000 @ 7% for 10 years | $17,000 | $20,097 | +$3,097 |
| $10,000 @ 7% for 20 years | $24,000 | $40,388 | +$16,388 |
| $10,000 @ 7% for 30 years | $31,000 | $81,165 | +$50,165 |
| $10,000 @ 10% for 30 years | $40,000 | $198,374 | +$158,374 |
Assumes no additional contributions. Monthly compounding used for compound column. Numbers rounded to nearest dollar.
Frequently Asked Questions
What is the difference between APY and APR? ⌄
APR (Annual Percentage Rate) is the stated interest rate without compounding. APY (Annual Percentage Yield) accounts for compounding — it's the actual return you earn in a year. A savings account with 5% APR compounded daily has an APY of about 5.13%. Always compare APY when shopping for savings accounts or CDs.
What is a realistic interest rate to use? ⌄
It depends on where you're investing: HYSA/CDs (2026): 4–5%. Bonds: 3–5%. S&P 500 (historical avg): 7% after inflation, 10% nominal. Real estate: 5–8%. For long-term retirement projections, most financial planners use 6–8% as a conservative to moderate estimate.
How does inflation affect compound interest? ⌄
Inflation erodes your purchasing power over time. If you earn 7% but inflation is 3%, your real return is about 4%. A million dollars in 30 years buys far less than a million today. The inflation-adjusted value in this calculator shows you what your final balance is worth in today's dollars.
Where can I actually earn compound interest? ⌄
Common compound interest vehicles: High-Yield Savings Accounts (4–5% in 2026), CDs (4–5.5%), Money Market Accounts (3–5%), Treasury bonds, 401(k) and Roth IRA invested in index funds (7–10% historically), and dividend reinvestment in stocks (DRIP). The key is to reinvest all earnings — that's what creates compounding.
What is the Rule of 72? ⌄
The Rule of 72 is a quick mental math trick: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 6% → 12 years. At 8% → 9 years. At 12% → 6 years. It works because ln(2) ≈ 0.693, and 72 is a convenient approximation that divides evenly by many common rates.