Compound interest calculator
Four questions and you will see what time does with your money. Enter your own rate, or start from what a real market delivered over the last 30 years, with your exact plan replayed on its prices.
Step 1 : How much are you investing today?
The amount you put in once, at the start.
€Step 2 : Will you add money on a schedule?
Step 3 : What return do you expect?
Step 4 : For how many years?
At a glance
| Tool | Compound Interest Calculator |
|---|---|
| What it does | Starting capital and scheduled deposits: monthly, quarterly, half-yearly or yearly. Interest compounded monthly, quarterly, half-yearly or yearly. Historical mode: your plan replayed on real monthly prices of ten country and world ETFs. Animated 3D towers and year-by-year chart, exports to PDF, Excel and Google Sheets. |
| Universe | Ten equity markets through their ETFs: S&P 500, developed Europe, France, Germany, the United Kingdom, Spain, Canada, China, South Korea and MSCI World, from 1993 for the oldest to August 2026. |
| Data source | Monthly closes of each ETF, adjusted for share splits and excluding dividends, so historical returns are conservative. The projection itself runs entirely in the browser. |
| Price | Free. No account and no sign-in. |
Four questions, in the order a saver asks them
How much do you invest today, how much will you add and how often, what return do you expect, and for how long. The calculator asks them one at a time, folds each answer into a line you can reopen, and shows the result once all four are set. From then on every change updates the figures, the chart and the 3D view immediately.
The projection runs month by month. Interest accrues each month on the balance and is credited at the compounding frequency you choose, which gives exactly one plus the rate divided by the number of periods over a full period. Deposits land at the end of each deposit period, after interest, the most common and the most prudent convention: a deposit earns nothing during the period it arrives in.
Start from a real market instead of a guess
Instead of typing a rate, you can pick one of ten markets and a window from five years up to its full history (33 years for the S&P 500). The page then shows three figures for that window: the cumulative return, the simple average (cumulative return divided by the number of years), and the annualized return, the constant yearly rate that compounds to the same cumulative result.
The projection uses the annualized return, not the simple average, and the page says why. Compounded, a simple average counts the gains twice: the S&P 500 returned about 1,600 % from 1993 to 2026, which is 48 % a year as a simple average but 8.9 % a year annualized. Only the second one, compounded, gives back what the market actually did.
The same window also replays your exact plan on the real monthly prices: your starting capital buys units at the first close, each deposit buys units at the close of its month, and the final value is the number of units times the last price. Nothing is smoothed, so the crashes of 2000, 2008 and 2020 are in the result.
What the figures do not include
Historical prices are those of US-listed ETFs, in dollars, adjusted for share splits but not for dividends. Returns are therefore price returns, below what a reinvested portfolio would have earned by roughly the dividend yield of each market. Currency movements against the euro are included, since the prices are in dollars.
The projection assumes a constant rate, taxes and fees are not modelled, and real markets never grow in a straight line. Past returns do not predict future ones, and investing carries a risk of capital loss. This is an educational tool, not investment advice.
Questions
- What is compound interest?
- Interest earned on previous interest. Each time interest is credited it joins the balance, and the next interest is computed on that larger balance, so growth accelerates over time. Time matters more than the rate: the longer the horizon, the larger the share of the final value that is interest.
- Does compounding monthly instead of yearly make a big difference?
- Less than people expect. At 7 %, 10,000 grows to 19,672 in ten years compounded yearly and to 20,097 compounded monthly. The rate and the duration matter far more than the frequency.
- Why does the historical mode use the annualized return and not the average?
- Because a simple average compounded over many years grossly overstates the result. The annualized return is the only constant rate that, compounded, reproduces the real cumulative return of the period.
- Can I export the results?
- Yes: to PDF and Excel directly, and to Google Sheets through a CSV file that Sheets imports in one step.
EPTA5 publishes data and research tools. Nothing on this page is investment advice, a recommendation, or a solicitation to buy or sell any financial instrument. Past performance does not predict future returns.