Compound interest calculator
$10,000 invested at 8% for 30 years becomes $100,627 with compound interest — but only $34,000 with simple interest. The difference ($66,627) is interest earning interest. Einstein allegedly called compound interest the "eighth wonder of the world." Whether he said it or not, the math is undeniable: time is the most powerful factor in wealth building.
Good to know
The Rule of 72 gives quick estimates. Divide 72 by your interest rate to find years to double. At 8%, your money doubles in roughly 9 years (72 ÷ 8 = 9). At 6%, about 12 years. At 10%, about 7.2 years. Works for any rate between 6-10% with decent accuracy.
Starting early beats investing more. Someone who invests $5,000/year from age 25-35 (10 years, $50,000 total) then stops will have more at 65 than someone who invests $5,000/year from age 35-65 (30 years, $150,000 total). The early starter's money compounds for 30 extra years.
Credit card debt works the same way — against you. A $5,000 balance at 20% APR, paying only minimums, takes 27 years to pay off and costs $8,549 in interest. Compound interest is devastating when you're on the paying side. This is why the first financial priority is escaping high-interest debt.
Disclaimers & sources
For reference only. Not financial or investment advice. Returns are not guaranteed.