Compound Interest Calculator
See exactly how your savings or investments grow over time with the power of compound interest — and why starting earlier makes such a dramatic difference.
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Why Compound Interest Is Called the Eighth Wonder
Compound interest means you earn returns on your returns — not just on your original principal. A $10,000 investment at 7% grows to $10,700 after year one. In year two, you earn 7% on $10,700 (not $10,000), giving you $749 instead of $700. This snowball effect accelerates dramatically over time.
The Rule of 72: Divide 72 by your interest rate to find roughly how many years it takes to double your money. At 7%, your money doubles every ~10 years. At 10%, every ~7 years.
Time is the key variable. Investing $10,000 at age 25 at 7% grows to $149,745 by age 65. Waiting until age 35 to invest the same amount leaves you with only $76,122 — less than half, just for waiting 10 years.
Compound Interest: Real-World Growth Examples
Example 1: $10,000 Investment at 7% for 30 Years
A single $10,000 investment at 7% annual return (historical S&P 500 inflation-adjusted average) grows to $76,122 in 30 years — with no additional contributions. Of that $76,122, only $10,000 is your original investment. The remaining $66,122 is pure compound growth. This is why starting early is so powerful: the last 10 years of a 30-year investment generate more growth than the first 20 years combined.
Example 2: $500/Month Contribution at 7% for 20 Years
Contributing $500/month to a retirement account or index fund at 7% annual return for 20 years grows to $260,463. You contributed $120,000 of your own money ($500 x 240 months). The remaining $140,463 is compound interest — more than you contributed. At 30 years the total reaches $567,765 with only $180,000 contributed, and $387,765 from compounding.
Example 3: High-Yield Savings — 0.01% vs. 4.5% APY
Keeping $20,000 in a traditional savings account at 0.01% APY earns just $2/year. Moving that same $20,000 to a high-yield savings account at 4.5% APY earns $900 in the first year. Over 5 years with monthly compounding, the HYSA balance grows to $24,870 while the traditional account sits at $20,010. That is a difference of $4,860 for doing nothing different except choosing the right account.
Example 4: The Cost of Waiting — Starting at 25 vs. 35
Investing $400/month starting at age 25 at 7% gives you $1,054,358 by age 65 (40 years). Starting the same $400/month investment at age 35 gives you only $491,428 by age 65 (30 years). 10 years of delay costs you $562,930 — even though you only contributed $48,000 more by starting earlier. Compound interest rewards patience and punishes delay more than almost any other financial concept.