Free calculator

Compound interest calculator

See how a starting amount and monthly savings grow over the years, in any currency, with the answer also shown in today's money. Same method as the SEC's calculator on investor.gov, plus the parts it leaves out.

No sign-up, nothing saved, works in any currency.
years
% a year
Long-run averages, each with its period and source further down the page. Past averages are not forecasts.
% points
% a year
In 30 years you will have$1,338,572

In today's money, at 3% inflation: $551,474

Between $895,582 and $2,020,269 if returns are 2 points lower or higher.

You put in$190,000
Growth$1,148,572
Monthly amount used$500
  • From year 7, your money earns more each year than you pay in.
  • You cross $1,000,000 in year 28.
  • At the end, 86% of your balance is growth you never paid in.
The link carries every input, nothing else.

StockFly tracks what your real investments actually did, in any currency.

How it is calculated

Each compounding period, the balance earns the period's share of the yearly return, and the contributions made during that period are added at its end. That is the rule the SEC's calculator on investor.gov uses, so the two agree to the cent. With annual compounding a year's twelve deposits earn nothing until the year ends; with daily compounding the monthly amount is spread over 365 days.

balance after k years = P x (1 + r/n)^(k x n) + c x ((1 + r/n)^(k x n) - 1) / (r/n)
c = monthly contribution x 12 / n
n = 1 (annually), 2 (semiannually), 4 (quarterly), 12 (monthly) or 365 (daily)
low and high = the same formula at r minus and plus the return range
today's money = balance / (1 + inflation)^k

Goal mode runs the same formula backwards: the balance is linear in the monthly amount, so the amount that lands exactly on the target is one division away. Today's money divides a future amount by inflation over the same years, so a target set decades out can be compared with prices you know.

The presets are long-run averages, each with its period and source. Past averages are not forecasts.

PresetRatePeriodSource
S&P 500, long-run average10.7% a yearJanuary 1957 to August 2026, dividends reinvestedRobert Shiller's dataset, compiled by officialdata.org
S&P 500, after inflation6.8% a year, after inflationJanuary 1957 to August 2026, dividends reinvested, adjusted with US CPIRobert Shiller's dataset, compiled by officialdata.org
Global stocks (MSCI World)9.1% a yearDecember 1987 to August 2026, gross return in US dollarsMSCI World Index factsheet
US bonds (Bloomberg US Aggregate)6.6% a year1976 to 2023, total returnBogleheads wiki, citing the index publisher
High-yield savings, typical in 20264% a yeara typical top online savings rate in September 2026, not a long-run averageFDIC national rates and published bank rate tables

Questions, answered

What is compound interest?
Growth on top of growth. Each period, the return is added to the balance, and the next period's return is calculated on the bigger balance. Over decades this is why the growth line bends upward instead of rising in a straight line.
How is this different from a simple interest calculator?
Simple interest pays the same amount every period, on the original sum only. Compound interest pays on the original sum plus everything it has already earned. The gap is small in year one and enormous in year thirty.
Which return should I use?
Nobody knows future returns, so use a long-run average and a range. The presets give sourced averages: the S&P 500 returned about 10.7% a year from 1957 to 2026 with dividends reinvested, about 6.8% after inflation. A broad global stock index sits a little lower, bonds lower still. The return range field shows what happens if reality lands a couple of points either side.
Why is the result also shown in today's money?
A million in thirty years buys far less than a million today. Dividing the future balance by inflation over the same years turns it into an amount you can compare with prices you know. At 3% inflation, money loses about 59% of its buying power over thirty years.
How do monthly contributions and the compounding setting work together?
The same way as the SEC's calculator on investor.gov, so the two agree to the cent. Contributions are pooled for each compounding period and added at the end of it. With annual compounding, a year's twelve deposits earn nothing until the year ends. With monthly compounding each deposit starts earning the next month. With daily compounding the monthly amount is spread evenly over 365 days.
Is this a forecast?
No. It is arithmetic on the assumptions you enter. Real markets do not return the same percentage every year, and a long-run average says nothing about the next five years. StockFly shows the projection so you can compare choices, and tracks what your real investments actually did so you can see the difference.

That was a projection. To see what your real investments actually did, open the live demo, try the Real Return Calculator or read the guides. No account needed for any of them.