Compound Interest Calculator.
Plug in your starting amount, monthly contribution, expected return, and time horizon — see what your portfolio compounds to over the years that matter.
Before you run the numbers.
Compound interest is the lesson at the heart of almost every financial book worth reading. Bogle covered it. Graham covered it. Housel covered it. They all say the same thing: time + consistency does more than picking the right stock.
This calculator lets you stress-test that claim with your own numbers. Start with what you have today, decide how much you can add monthly, pick a realistic return (8% is the long-run S&P average; 10% if you're optimistic), and see what 30 years of doing the boring thing looks like.
The surprise for most people: starting earlier matters more than saving more. $200/month from age 25 compounded at 8% beats $400/month from age 35 by a wide margin. The math is brutal — and it's the whole point of why the books on this site exist.
Run your scenario.
| Year | Balance |
|---|---|
| 5 | $49,973 |
| 10 | $106,639 |
| 15 | $186,971 |
| 20 | $300,851 |
See a worked example
Start with $10,000, add $500/month, assume a 7% annual return compounded monthly, for 20 years.
FV = P(1 + r/n)nt + PMT · [((1 + r/n)nt − 1) / (r/n)]
(1 + 0.07/12)12·20 = 1.0058333240 = 4.038739
FV = 10,000 × 4.038739 + 500 × [(4.038739 − 1) / 0.0058333]
FV = 40,387 + 260,463 = $300,851
Of that $300,851, only $130,000 came out of your pocket — the other $170,851 is growth. That gap is the whole argument these books make.
Educational tool, not financial advice. Results are hypothetical projections based on the numbers you enter and a constant assumed return. Real investment returns vary year to year and are never guaranteed — markets fall as well as rise. This calculator ignores taxes, fees, and inflation. Consult a licensed financial professional before making investment decisions.
Taught in these books.
Common questions.
What return should I assume?
8% is a reasonable long-run assumption for a diversified equity portfolio (matching the historical S&P 500 average after inflation roughly). 10% if you want to be optimistic; 6% if you want to be conservative. Don't use 12%+ — that's a sales pitch number, not a real-world planning number.
Why does starting earlier matter so much?
Compound interest is exponential, not linear. Each year of growth gets applied to a bigger base. Starting 10 years earlier doesn't add 10 years — it adds 10 years of compounding ON TOP of all the years that come after. Run the numbers above to see it.