Compound Interest Calculator

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (calculated only on the principal), compound interest grows exponentially — your money earns interest on interest, which is why Einstein reportedly called it 'the eighth wonder of the world.'

Each period, interest is calculated on your total balance (principal + previously earned interest). For example, $10,000 at 7% annual return becomes $10,700 after year 1, then $11,449 after year 2 (7% of $10,700, not $10,000).

Over 30 years, this compounding effect turns $10,000 into $76,123 — even without adding any more money.

The Rule of 72 is a quick way to estimate how long it takes to double your money. Divide 72 by your annual return rate.

At 7% returns: 72 ÷ 7 ≈ 10.3 years to double. At 10%: 72 ÷ 10 = 7.2 years.

At 4%: 72 ÷ 4 = 18 years. It's a useful mental shortcut for evaluating investments.

Starting 10 years earlier can more than double your final balance. Someone investing $500/month from age 25 to 65 at 7% would have ~$1.2M.

Starting at 35 with the same contributions would yield ~$567K — less than half. The extra decade of compounding is worth more than 20 years of additional contributions.

The S&P 500 has historically returned ~10% annually before inflation, or ~7% after inflation. For conservative planning, financial planners often use 6-7% (inflation-adjusted).

Bond-heavy portfolios might assume 3-5%. Use a rate that matches your risk tolerance and time horizon.

Yes, but the difference is smaller than most people think. $10,000 at 7% compounded annually = $10,700 after one year.

Compounded monthly = $10,722. Compounded daily = $10,725.

The difference matters more for debt (credit cards compound daily) than for S&P 500 investments (which compound continuously via market price changes).