Compound Interest Calculator
Enter a starting amount, monthly contribution, interest rate, and time. Watch the compounding effect: the calculator separates your own deposits from the growth they earn.
| Total you deposited | — |
| Interest earned | — |
| Growth multiple | — |
The compound interest formula
For a lump sum, compound growth follows:
A = P × (1 + r/n)n·t
where P is the principal, r the annual rate, n compounding periods per year, and t years. Monthly contributions are added each month and then compound as well — that's what the calculator simulates month by month.
Why time matters more than amount
$300/month at 7% becomes about $156,000 in 20 years — but about $367,000 in 30 years. The last 10 years contribute more growth than the first 20 combined, because compounding accelerates: interest earns interest.
The rule of 72
Divide 72 by your interest rate to estimate how many years money takes to double. At 7%, money doubles roughly every 10 years; at 10%, about every 7 years.
What rate should you assume?
| Where the money sits | Reasonable planning rate |
|---|---|
| High-yield savings account | 3–5% (varies with Fed rates) |
| Bonds / CDs | 3–5% |
| Diversified stock index funds | 7% (inflation-adjusted, long-run) |
Frequently asked questions
How much will $300 a month be worth in 20 years?
At a 7% annual return compounded monthly, $300/month grows to roughly $156,000 in 20 years — about $72,000 of deposits and $84,000 of compound growth.
What is the difference between simple and compound interest?
Simple interest is earned only on the original principal. Compound interest is earned on principal plus previously earned interest, so growth accelerates over time.
How often should interest compound?
More frequent compounding helps slightly, but the difference between monthly and daily is small. At 7%, $10,000 over 10 years yields $20,097 monthly vs $20,136 daily.
Does this account for inflation or taxes?
No. To think in terms of current buying power, use an inflation-adjusted return (e.g. 7% instead of 10% for stocks). Taxes depend on the account type — retirement accounts defer or avoid them.
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Last updated: 2026-07-07