Compound Interest Calculator

Why Compound Interest Rewards Starting Early

Compound interest means you earn returns not just on your original savings, but on all the returns you have already earned. The formula is simple — Future Value = P(1+r)ⁿ — but the effect is dramatic over time. $10,000 growing at 7% a year becomes about $38,700 in 20 years without adding a single dollar. Add $500 a month and you are looking at roughly $265,000, of which more than half is pure growth.

The Rule of 72: doubling time in your head

Divide 72 by your annual return to estimate how many years it takes your money to double. At 7%, money doubles roughly every 10.3 years. At 10%, every 7.2 years. This is why a 25-year-old's dollar is worth so much more than a 45-year-old's: the younger dollar gets two extra doublings.

Three levers that matter more than picking stocks

• Time is the biggest lever. Starting 10 years earlier beats earning 2% more per year in almost every realistic scenario.
• Contributions beat returns early on. In your first decade, how much you save matters more than your rate of return. Automate it.
• Fees compound too — against you. A 1% annual fee on a 7% return does not cost you 1% of the outcome; over 30 years it eats roughly a quarter of your final balance.

Realistic return assumptions

The long-run U.S. stock market return is about 10% nominal, ~7% after inflation — which is why 7% is the default here and a sensible planning number for a stock-heavy portfolio. Bonds historically return ~4–5%, high-yield savings accounts currently ~4–5% (variable). Be skeptical of any projection promising sustained double-digit returns with no risk.

Compound interest FAQ

What is the difference between compound and simple interest? Simple interest pays only on the original principal. Compound interest pays on principal plus accumulated interest — the gap between them explodes over decades.

Does compounding frequency matter? Monthly vs. annual compounding changes the result slightly (more frequent is marginally better). What matters far more is the rate and the time horizon.

Should I pay debt or invest? As a rule of thumb: if the debt's interest rate is higher than your expected investment return (credit cards at 20%+ almost always win), kill the debt first. Employer 401(k) match is the exception — that is a 50–100% instant return, take it before extra debt payments.

Disclaimer: projections use a fixed assumed return and ignore taxes, fees, and inflation. Investing involves risk, including loss of principal — this is educational, not financial advice.