Home › Compound Interest Calculator
Compound Interest Calculator: Start Amount, Monthly Savings, Years
Compound interest means your interest starts earning interest. Enter a start amount, what you add each month, an annual rate and the horizon — you get the future balance, how much of it you contributed yourself, and how much came from compounding. Runs in your browser.
Compound interest calculator
Calculates in your browser, stores nothing. Assumes a constant rate — real results fluctuate. A model, not a promise.
How the calculation works
Without contributions: balance = start × (1 + rate/periods)periods × years
Monthly contributions are compounded month by month on top. The calculator runs the actual monthly loop, so timing matches a real savings plan.
The rate you enter does the heavy lifting: at 5 % over 20 years, money roughly doubles about every 14 years; at 8 %, every 9. A handy shortcut is the rule of 72: divide 72 by the return in percent to get the doubling time.
What the result doesn't tell you
- Real returns are not constant — years with −20 % happen, the order matters for people withdrawing money
- Inflation shrinks what the final number can buy
- Taxes on interest, dividends and capital gains are not included
- Costs (fund fees, broker fees) reduce the effective rate — 1 % of fees can cost a fifth of the result over 30 years
Risk notice: general information, not investment advice. Investments can lose value, up to total loss.
Where the return comes from
If your plan invests in stocks rather than a savings account, StockPilot AI screens around 1,800 stocks nightly and summarises the market in plain text. Free during beta, general information only.
- Market traffic light instead of ticker stress
- Key metrics per stock, explained understandably
- Daily briefing on trading days, unsubscribe anytime
- Server in Germany
Note: general information for all users alike — no individual investment advice and no buy or sell recommendations.
FAQ
What is compound interest?
Interest that itself earns interest — each period's interest is added to the balance, so growth accelerates over time.
How do you calculate compound interest?
Balance = start × (1 + rate/periods)periods × years, plus contributions compounded the same way. This calculator compounds monthly, matching a real savings plan.
What is the rule of 72?
Divide 72 by the annual return in percent to estimate the doubling time. At 6 %, roughly 12 years.
Is the result guaranteed?
No. It assumes a constant rate. Real investments fluctuate and bank interest changes. Treat it as a model.
Updated: November 10, 2026 · Author: Kay Conrad, Gemivo (Lübeck, Germany)