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🇺🇸United States

Compound Interest Calculator

Calculate compound interest with different compounding frequencies.

$
%
years
$

Results

Future Value$300.9K
Total Contributions$130.0K
Total Interest Earned$170.9K
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What is Compound Interest?

Compound interest is the interest earned on both your initial investment and the accumulated interest from previous periods. Albert Einstein reportedly called it the 'eighth wonder of the world,' and for good reason — compounding turns modest regular investments into substantial wealth over time.

This calculator is especially useful for Americans planning retirement savings through 401(k), IRA, Roth IRA, or regular brokerage accounts. The S&P 500 has historically returned about 10% annually (7% after inflation), making compound interest the primary driver of long-term wealth in the US.

The key insight: time matters more than amount. $500/month invested at 7% for 30 years becomes $566,765. But waiting 10 years and then investing $1,000/month for 20 years gives only $520,927 — even though you invested $60,000 more. Starting early is the single most powerful financial decision you can make.

The Rule of 72 provides a quick estimate: divide 72 by the interest rate to find how many years it takes to double your money. At 7%, your money doubles approximately every 10.3 years.

Formula

Future Value = P(1+r)^n + PMT × [((1+r)^n − 1) / r]

Where: - P = Principal (initial investment) - r = Monthly interest rate - n = Total months - PMT = Monthly contribution

Example — $10,000 initial + $500/month at 7% for 20 years: Principal growth: $10,000 × (1.00583)^240 = $40,387 Monthly growth: $500 × [((1.00583)^240 − 1) / 0.00583] = $260,464 Future Value = $300,851 Total contributions = $10,000 + $120,000 = $130,000 Interest earned = $170,851 (more than your contributions!)

Growth of $500/month at 7%: - 10 years: $86,541 - 20 years: $260,464 - 30 years: $566,765 - 40 years: $1,197,811

How to use this Compound Interest Calculator?

1. Enter your initial investment amount. 2. Set the annual interest rate (7% is the historical inflation-adjusted S&P 500 average, 10% is nominal). 3. Choose time period in years. 4. Enter monthly additions (your regular investment amount). 5. See the future value, total contributions, and total interest earned.

Frequently asked questions

What is a good rate of return to assume?
Conservative: 5-6% (bonds/balanced). Moderate: 7% (S&P 500 inflation-adjusted). Aggressive: 10% (S&P 500 nominal). For retirement planning, most advisors use 6-7% real return.
What is the Rule of 72?
Divide 72 by the annual interest rate to estimate years to double. At 6%: 72/6 = 12 years. At 8%: 72/8 = 9 years. At 10%: 72/10 = 7.2 years. At 12%: 72/12 = 6 years.
How does compound interest work in a 401(k)?
Your 401(k) contributions and employer match are invested in mutual funds that compound over time. With a $23,500 annual contribution and 7% return, a 401(k) grows to about $2.4 million in 30 years. Employer match is essentially free money that also compounds.
Simple interest vs compound interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on principal plus accumulated interest. $10,000 at 7% for 20 years: simple interest = $24,000 total; compound interest = $40,387 total — that's $16,387 more from compounding.
How often should interest be compounded?
More frequent compounding yields slightly higher returns. $10,000 at 7% for 10 years: annually = $19,672; monthly = $20,097; daily = $20,137. The difference between monthly and daily compounding is negligible.
What is the impact of fees on compound returns?
Fees dramatically erode compound growth. A 1% annual fee on a $500K portfolio costs $5,000/year but $400,000+ over 30 years due to lost compounding. Choose low-cost index funds (0.03-0.10% expense ratio) over actively managed funds (0.5-1.5%).
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