Compound Interest Calculator
What Is Compound Interest?
Compound interest is interest calculated on both your initial principal and the accumulated interest from previous periods. Unlike simple interest, which only grows linearly, compound interest grows exponentially — your money earns interest on top of interest, creating a snowball effect over time.
Albert Einstein reportedly called compound interest "the eighth wonder of the world. He who understands it, earns it; he who doesn't, pays it." Whether or not Einstein said this, the math is undeniable: a single $5,000 investment at 7% annual return for 30 years becomes approximately $38,061 through compound growth — with no additional contributions.
The Compound Interest Formula
The basic formula for compound interest is: A = P(1 + r/n)^(nt)
Where: A = final amount, P = principal (initial investment), r = annual interest rate (as decimal), n = compounding frequency per year, t = time in years.
Compounding Frequency Matters
The frequency at which interest is compounded has a real impact on your final balance, even at the same stated interest rate. Consider $10,000 at 5% for 10 years:
- Annually: $16,288.95
- Quarterly: $16,436.19
- Monthly: $16,470.09
- Daily: $16,486.65
The difference between annual and daily compounding is $197.70 on a $10,000 investment — not dramatic over 10 years, but the gap widens significantly with larger principals and longer time horizons.
The Rule of 72
The Rule of 72 is a quick mental math shortcut to estimate how long it takes to double your money at a given interest rate: divide 72 by the annual interest rate.
- At 4% return: 72 ÷ 4 = 18 years to double
- At 6% return: 72 ÷ 6 = 12 years to double
- At 8% return: 72 ÷ 8 = 9 years to double
- At 10% return: 72 ÷ 10 = 7.2 years to double
The historical average annual return of the S&P 500 (US stock market) is approximately 10% before inflation, or about 7% after inflation — meaning inflation-adjusted stock investments have doubled every ~10 years historically.
Compound Interest vs Simple Interest
Simple interest is calculated only on the original principal: I = P × r × t. A $5,000 loan at 5% simple interest for 3 years yields $750 in interest. With compound interest at the same rate (monthly), the interest is approximately $808 — 7.7% more. The difference grows dramatically over longer periods and at higher rates, which is why compound interest is the standard for long-term investments.
How to Maximize Compound Growth
- Start early: Time is the most powerful variable. Starting at 25 vs 35 can nearly double your retirement balance.
- Make regular contributions: Monthly contributions dramatically accelerate growth (use the "Monthly Contribution" field).
- Reinvest dividends: In investment accounts, always enable dividend reinvestment (DRIP).
- Minimize fees: A 1% management fee reduces a 7% return to 6%, which cuts your ending balance by about 20% over 30 years.