About
A plain explanation of who builds this, how the math works, and what the projections can and cannot tell you.
This is a free compound interest and portfolio projection calculator. It exists because most compounding calculators give you a single number and no sense of the range around it, no way to compare two plans side by side, and no way to work backward from a goal. This one does all three.
You can model a plain savings rate, compare two different scenarios on one chart, solve for the monthly contribution or rate of return a goal would require, and build a portfolio of up to five ETFs weighted the way you actually hold them.
NowCompound is built and maintained by a single person based in Virginia. I am not a financial professional. I built this because I wanted a clearer way to think about my own long-term saving, and every calculator I found gave me one confident-looking number with no sense of the range around it.
The goal here is honesty about uncertainty. A projection that shows a band of outcomes, tells you where its data came from, and admits what it cannot model is more useful than one that hands you a single figure and lets you assume it is a promise.
Edit this section to add your own name and background if you would like the page to be more personal.
This site is independently operated. It is not affiliated with, endorsed by, or sponsored by Vanguard, Fidelity, Invesco, Schwab, State Street, iShares, or any fund provider or brokerage named on the site. Fund names and ticker symbols are used only to identify the actual products being modeled.
The core projection runs month by month rather than using a single closed-form formula. Your chosen compounding frequency is converted into an equivalent monthly rate, and each month the balance grows by that rate and then your contribution is added. Running it this way means contributions, withdrawals, and compounding frequency all interact correctly instead of being approximated.
The standard formula behind the concept is A = P(1 + r/n)^(nt), where P is the starting principal, r is the annual rate, n is the number of compounding periods per year, and t is time in years. The site extends this with recurring contributions, which the basic formula does not account for.
Because a 6% rate compounded monthly is not the same as 6% compounded annually, the calculator also shows the effective annual rate, which is what you actually earn once compounding is included.
The portfolio section uses trailing 10-year annualized total returns with dividends reinvested, gathered from public fund performance data as of mid-2026, along with each fund's published expense ratio and an approximate annualized standard deviation.
Three things are worth being direct about:
It assumes a constant rate of return. Real markets deliver their average through a sequence of good and bad years, and the order those years arrive in matters a great deal, particularly if you are withdrawing money. It also does not model taxes, inflation, fund turnover, bid-ask spreads, or changes in your own contributions over time.
Treat every projection here as an illustration of how compounding behaves, not as a prediction of what your account will be worth.
Nothing on this site is investment, tax, or legal advice, and nothing here is a recommendation to buy or sell any security. This is a general-purpose educational tool. It does not know your situation, and it does not tailor anything to it. For advice specific to your circumstances, speak with a qualified financial professional.
If you find an error in the math or the fund data, please report it through the contact page. Corrections are made promptly and the underlying figures are reviewed periodically.