Compound Interest Calculator

For a lump sum: A = P × (1 + r/n)^(n×t) where P = principal, r = annual rate (decimal), n = compounding periods per year, t = years. For example: $10,000 at 7% compounded monthly for 10 years: A = $10,000 × (1 + 0.07/12)^(12×10) = $20,097. The interest earned is A − P = $10,097 — more than doubling the original investment.

The Rule of 72 is a quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 6% → 72 ÷ 6 = 12 years to double. At 9% → 8 years. At 12% → 6 years. It's an approximation but accurate within 1–2 years for rates between 4% and 15%.

Enormous. $10,000 invested at 7% for 30 years grows to ~$76,000. The same amount invested for 40 years grows to ~$150,000 — nearly twice as much for just 10 extra years. Starting a $200/month contribution at 25 vs 35 (at 7%) produces a difference of over $200,000 by age 65. Time in the market is the most powerful compounding variable.