PiggyCompound Interest Calculator
See how an initial deposit plus monthly contributions grow over time — and how much of the final balance is pure interest.
How this is calculated
Interest compounds monthly: each month the balance grows by
annual rate ÷ 12, then your contribution is added.
Future value is FV = P(1+r)ⁿ + PMT × ((1+r)ⁿ − 1) ÷ r.
Worked example: $10,000 to start plus $500 a month at 7% for 20 years grows to $300,851. You contribute $130,000 of that — the remaining $170,851 is interest.
Your plan
Growth over time
The math behind compound growth
This calculator compounds monthly: each month your balance earns annual rate ÷ 12, then your contribution is added. Over long horizons the interest-on-interest effect dominates — which is why the gap between the two lines above widens every year.
Where the growth actually comes from
Look at the two lines on the chart. The lower one is money you deposited; the upper one is your balance. In the early years they run close together, because a small balance cannot generate much return and your contributions do nearly all the work. Somewhere around year ten to twelve at typical rates, the crossover happens: annual investment returns begin to exceed annual contributions, and from then on the portfolio grows faster than you can fund it. On the default example — $10,000 to start, $500 a month, 7% for twenty years — you contribute $130,000 and finish with $300,851. More than half the final balance is growth you did not deposit.
This is why starting early beats saving more later, and by a wider margin than intuition suggests. The last decade of any long compounding period produces more growth than the first two combined, so every year you delay removes a year from the most productive end of the curve, not the least.
The returns you should actually model
The S&P 500 has averaged roughly 10% a year over the long run before inflation, but three deductions stand between that headline and your outcome. Inflation takes about three points, leaving roughly 7% in real terms. Fund fees take whatever your expense ratio is — the difference between a 0.03% index fund and a 1% actively managed one is around $60,000 on a portfolio like the example above. And unless the money sits in a tax-advantaged account, tax applies to dividends and to gains when you sell.
Average is also not the same as reliable. A 7% average conceals years of +25% and −20%, and the order in which those arrive matters enormously if you are withdrawing rather than accumulating. For a long accumulation phase the average is a fair guide; for anything within a few years of when you will need the money, it is not.
Assumptions worth knowing
The model compounds monthly, adds your contribution at the end of each month, and assumes the rate never changes and the contribution never stops. Real portfolios do none of these things. It also ignores inflation entirely, so a projected $300,851 in twenty years buys what roughly $166,000 buys today at 3% inflation. If you want a figure in today's money, enter your expected return minus inflation — around 4% rather than 7% — and read the result as real purchasing power.
Practical implications
Three things follow from the arithmetic. Automate the contribution, because consistency matters more than the amount and a monthly transfer you never think about survives busy periods. Increase it whenever your income does, since a contribution that stays flat for a decade quietly shrinks in real terms. And leave it alone: the projections here assume you never withdraw, and an early withdrawal costs not only the money taken but every year of compounding it would have produced. For retirement-specific projections including the 4% withdrawal rule, use our retirement calculator.