Compound Interest Calculator

See exactly how your money grows over time. Add a starting amount and a monthly contribution — the projected balance, total contributions, and compound growth update instantly. The formula is shown, and the chart separates what you put in from what compounding earned you. No sign-up.

$
$
%
Projected balance
$0
after the period above
  • Total you contribute$0
  • Compound interest earned$0

Contributions vs. compound growth

Grey = what you put in. Green = total value. The widening gap is compounding.

The exact formula — with your numbers

Future value = starting amount compounded, plus the future value of your regular contributions:

FV = P(1 + i)N + PMT · (1 + i)N − 1i

P = starting amount · i = annual rate ÷ compounding frequency · N = years × frequency · PMT = contribution per period. Returns are assumed constant; real markets fluctuate.

How to use this calculator

  1. Starting amount & monthly contribution — what you begin with and add each month.
  2. Annual return rate — a long-run stock index average is often assumed around 7% after inflation, but use whatever rate fits your plan.
  3. Years & frequency — time is the biggest lever. Try doubling the years and watch the balance more than double.

What your result means

The compound interest earned line is the part you didn't pay in — it's growth on growth. Early contributions matter far more than late ones because they compound for longer, which is why starting sooner beats contributing more later.

Frequently asked questions

What is compound interest?

Interest earned on both your principal and the interest already added, creating exponential growth over time.

Does compounding frequency change much?

More frequent compounding raises the final balance slightly, but time and contribution size matter far more.

Is 7% a safe assumption?

It reflects a long-run, inflation-adjusted stock-market average — not a guarantee. Markets rise and fall; this is an estimate, not advice.

Method & sources

  • Calculation: Standard future-value formula (shown above) with contributions added each compounding period. Assumes a constant rate and ignores taxes and fees.
  • Reviewed: · Assumptions reviewed quarterly.

Educational estimate, not financial advice. Investment returns are not guaranteed.