About this calculator

A free tool for exploring how your starting amount, monthly contributions, rate, and time could affect your balance.

Clear numbers, fewer distractions

Change four inputs and compare the results. No accounts, subscriptions, ads, or affiliate links. Copy a link to save a scenario or share the exact calculation with someone else.

Calculation methodology

The annual percentage is a nominal rate, which does not include the effect of compounding within the year. We divide the percentage you enter by 1,200 to get the monthly decimal rate, apply it to the existing balance, then add your contribution. That repeats for 12 months per year, with no withdrawals and the same contribution every month.

The final balance, chart, and annual breakdown all use the same monthly calculation. We keep numerical precision during the calculation and round only for display. Automated tests compare the monthly calculation with the independent closed-form formula. You can find the formula and a worked example on the homepage.

Supported inputs

You can enter an initial investment from $0 to $100,000,000, monthly contributions from $0 to $1,000,000, a nominal annual rate from −100% to 100%, and a duration from 1 to 100 whole years. Amounts and rates can include decimals. These are software limits, not recommended financial assumptions.

A 0% rate adds up only your initial investment and contributions. Negative rates model losses before each monthly contribution is added. Even at the lowest accepted annual rate, the monthly rate stays above −100%, so a single month cannot reduce the existing balance below zero.

Reading the results

Total invested is all the money you put in. Interest / growth is the difference between your final balance and total invested. The growth percentage divides that difference by total invested; it is a cumulative comparison, not an annualized return. If nothing is invested, that percentage is undefined and displays as “Not applicable.”

Currency amounts display to the nearest cent. Adding rounded table values can produce a one-cent difference. Amounts of $1 quadrillion or more use scientific notation to stay readable: for example, $1E15 means $1 followed by 15 zeros.

What the model leaves out

Real investment returns can fluctuate or be negative. This calculation uses a fixed rate and excludes taxes, fees, inflation, changing contributions, and withdrawals. It helps compare assumptions; it cannot predict what an investment will earn or tell you which investment to choose.

Sharing and privacy

Calculations run in your browser. The shareable URL contains your inputs as readable numbers, so anyone with the link can see that scenario. Invalid link values fall back to the defaults for those fields. Read our privacy policy for details about shared links and hosting logs.