Compound Interest Calculator

How Compound Interest Works

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest — which only applies to the original principal — compound interest causes your balance to grow exponentially over time. Einstein reportedly called it the "eighth wonder of the world," and the math backs that up.

The formula is: A = P(1 + r/n)^(nt), where P is principal, r is annual rate (decimal), n is compounding periods per year, and t is years. The more frequently interest compounds, the faster your balance grows.

Compound vs. Simple Interest

On a $10,000 investment at 7% for 10 years:

• Simple interest: $10,000 + ($10,000 × 7% × 10) = $17,000
• Compound interest (monthly): $10,000 × (1 + 0.07/12)^(120) ≈ $20,097

That's a $3,097 difference from compounding alone — no extra deposits required.

The Rule of 72

A quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 6%, your money doubles in roughly 12 years (72 ÷ 6). At 9%, about 8 years. At 4%, roughly 18 years. It's not exact, but it's accurate enough for quick planning.

Compounding Frequency Comparison

Starting with $10,000 at 7% for 10 years, the impact of frequency:

• Annually: ~$19,672
• Quarterly: ~$20,016
• Monthly: ~$20,097
• Daily: ~$20,137

The difference between monthly and daily compounding is only $40 over 10 years — so frequency matters less than rate and time. Focus on maximizing your rate, not chasing daily compounding.

$10,000 at 7% Over Time

• 10 years: ~$20,097 (doubles)
• 20 years: ~$40,388 (4×)
• 30 years: ~$81,165 (8×)
• 40 years: ~$163,100 (16×)

Notice the pattern — it roughly doubles every 10 years at 7%. The later decades add far more in dollar terms because the base is so much larger.

Starting Early vs. Starting Late

This is where compound interest gets life-changing. Consider two investors, both targeting retirement at 65:

• Alex (starts at 25): Invests $10,000 once at 7% for 40 years → ~$149,745
• Jordan (starts at 35): Invests $10,000 once at 7% for 30 years → ~$76,123

That 10-year head start nearly doubles the ending balance — even though both invested the same $10,000. Time is the most powerful variable in the compound interest formula.

Best Accounts for Compound Interest

• High-Yield Savings Accounts (HYSA): 4–5% APY in 2025, FDIC insured, fully liquid
• Certificates of Deposit (CDs): Fixed rates up to 5%, locked for a term
• Money Market Accounts: Similar to HYSA, often with check-writing privileges
• Index Funds / ETFs: Average ~7–10% long-term, not guaranteed, reinvest dividends for compounding
• Roth IRA / 401(k): Tax-advantaged accounts where compound growth is sheltered from taxes

Frequently Asked Questions

What's a good compound interest rate? For savings accounts, 4–5% is excellent in 2025. For long-term investing, the S&P 500 has historically returned about 10% annually (7% after inflation).

What's the difference between APY and APR? APR is the base annual interest rate. APY (Annual Percentage Yield) accounts for compounding frequency, so it's always equal to or higher than APR. When comparing savings accounts, always compare APY.

Does compound interest work against me too? Yes — on debt. Credit cards, student loans, and mortgages all use compound interest against you. Paying off high-interest debt first is mathematically equivalent to earning that interest rate risk-free.

How often does interest compound in real life? Most savings accounts compound daily but credit APY monthly. Most loans compound monthly. Bonds typically pay simple interest. Always check the account terms.