Compound Interest Calculator

Enter a starting amount, a monthly contribution, and a return rate — see the future value, how much of it is interest, and a year-by-year table of the balance growing on itself.

Your numbers

Future value
Total contributed
Interest earned
YearBalanceContributedGrowth

Assumes monthly compounding and end-of-month contributions at a constant rate. Markets don't return a constant rate — use a long-term average (e.g., ~7% real for broad stock indexes) and treat results as a planning estimate.

Watch it compound in your own accounts

The SheetWell Net Worth Tracker ($4.99) gives compounding a scoreboard: 24 months of assets vs liabilities, automatic monthly change, and a trend chart — five minutes a month to see the curve bend.

See the Net Worth Tracker →

The mechanics, in one paragraph

Compound interest means you earn returns on your past returns. In year one the growth comes almost entirely from your contributions; by the time the balance is large, the growth line outweighs the contribution line — your money out-earns your deposits. Run 20+ years above and look at the table's last column: that crossover is the entire argument for starting early.

The three inputs, ranked by what you control

  1. Time — the most powerful and least controllable after the fact. Ten extra years routinely doubles the outcome. The best moment to start was years ago; the second best is this month.
  2. Contribution — fully in your control. Doubling the monthly contribution roughly doubles the contributed portion and nearly doubles long-run interest too.
  3. Rate — the one people obsess over and control least. Chasing an extra 2% with exotic bets usually adds more risk than return; most long-term investors capture the market average with broad index funds and win on the other two inputs instead.

A realistic rate to use

Broad stock-market indexes have averaged around 9–10% nominal, or roughly 7% after inflation, over long periods. Savings accounts compound too — just slower; check the high-yield rate you actually earn and try it in the savings goal calculator for short-term money. For mixed portfolios, blend accordingly. Whatever you choose, consistency of contribution beats precision of forecast.

To watch compounding happen in your own accounts — not in a hypothetical — track your balances monthly. Our Net Worth Tracker turns that into a 5-minute monthly habit with an automatic trend chart.

Frequently asked questions

How often should interest compound for this to apply?
The calculator compounds monthly, which matches most savings accounts and is a close approximation for investment portfolios. Daily vs monthly compounding changes results by well under 1% — frequency matters far less than rate and time.
What rate should I enter for stock investments?
A common planning choice is 7% (the rough long-run inflation-adjusted return of broad US stock indexes) if you want answers in today's purchasing power, or 9–10% nominal if not. Conservative planners use 5–6%.
Is compound interest really 'the eighth wonder of the world'?
The quote is (probably apocryphally) attributed to Einstein, but the math is real: at 7%, money doubles roughly every 10.3 years — the Rule of 72 (72 ÷ rate ≈ doubling years) is a handy mental shortcut.
Does this work against me with debt?
Exactly — unpaid interest on a 22% APR credit card compounds the same way, just in the wrong direction. That's why paying off high-APR debt is mathematically a guaranteed 22% return. See the debt snowball calculator.

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