Compound Interest Calculator
Compound interest runs your whole financial life — either for you or against you, no in-between. Drop in your numbers and watch what steady contributions actually turn into, one doubling at a time.
The long-run US stock market average is ~10% before inflation, ~7–8% after.
Value in 25 years
$322,009
You put in
$95,000
Growth (money your money made)
$227,009
🐻 Rule of 72: at 8%, your money doubles roughly every 9.0 years — that's about 2 doublings in this plan, and the last one is bigger than all the others combined.
Growth by year — ■ contributed ■ growth
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How this is calculated
Your starting amount grows by FV = P(1 + r/n)ⁿᵗ, and each contribution grows from the moment it's added: FV = PMT × ((1 + r/n)ⁿᵗ − 1) ÷ (r/n), where r is your annual return, n the compounding frequency, and t the years.
The Rule of 72 is the shortcut: 72 ÷ your return rate ≈ years for money to double. At 8%, that's about every 9 years — so 36 years holds four doublings, and the last doubling adds more than the first three combined.
The flat early years are the design, not a failure. Compounding back-loads its payoff, which is why starting now beats starting bigger later — every year of delay costs you your final (largest) doubling.
Common questions
What return rate should I use?
The US stock market's long-run average is roughly 10% per year before inflation — most planners model 7–8% in real terms for broad index funds. Savings accounts, bonds, and individual choices differ; the calculator lets you test any assumption.
How is compound interest different from simple interest?
Simple interest pays only on your original amount. Compound interest pays on your original amount plus every bit of interest it already earned — growth on growth. Over decades that difference isn't small; it's most of the final number.
How often does compounding frequency matter?
Less than people think. Monthly versus daily compounding changes the outcome by a fraction of a percent — the rate, the time, and the consistency of contributions do nearly all the work.
Go deeper
Investing fundamentals — make compounding work for you
This math is taught step-by-step in Module 5 of the Bear Bankroll Wealth System — with interactive worksheets that run the numbers on YOUR situation.
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Educational tool only — estimates, not financial advice. Your numbers stay in your browser and are never sent to our servers.