Monthly investment

Monthly investment: the basics

Estimate how a fixed monthly investment could grow, separating the money you contribute from the return assumed in your scenario.

How to use the inputs

• Monthly investment: Enter the same amount you plan to invest every month.
• Expected annual return: Enter an assumed annual percentage return, not a guaranteed result.
• Investment period: Choose a whole number of years from 1 to 60.

Change an input to update the results. Add up to four scenarios to compare separate assumptions; reset affects only the selected scenario.

Understanding the results

• Estimated future value includes all contributions and assumed growth.
• Total invested is your monthly amount multiplied by the number of months. Estimated returns are future value minus total invested.
• The chart samples the estimated value at each year-end. Each scenario uses its own inputs.

The formula, in words

Future value = M × ((1 + r) raised to n − 1) ÷ r.

M is the monthly contribution. R is the annual percentage entered (for example, 8 means 8%). The monthly decimal rate r = R ÷ 1200. Y is years and n = 12 × Y months.

At zero interest, future value = M × n. Total invested = M × n; estimated returns = future value − total invested.

Assumptions and limits

• Contributions arrive at the end of each month, so the final contribution earns no return before the end date. The starting balance is zero.
• The assumed annual percentage is divided by 12, then by 100; growth compounds monthly at a constant rate. This is not a conversion from an effective annual yield.
• No skipped or increasing deposits, withdrawals, taxes, fees or inflation are modeled. Input returns are limited to 0–50%, so this calculator does not model negative investment returns.

Currency results display whole units; other numbers show at most one decimal place. Calculations keep full precision unless whole units must be rounded up for break-even. Rounded figures may not add up exactly.

Examples below use USD and local number formatting. Country selection changes currency labels, formatting and terminology; it does not convert money or automatically apply local tax or lender rules.

Worked example

This fixed example uses the calculator’s default inputs; it does not change when you edit a scenario. Any example interest or return rate is an assumption, not a guaranteed outcome.

Monthly investment

USD 500

Expected annual return

8%

Investment period

20 years

There are 240 month-end deposits. Contributions are USD 500 × 240 = USD 120,000. Using r = 8 ÷ 1200 in the formula gives USD 294,510; subtracting contributions leaves USD 174,510 of assumed growth.

Estimated future value

USD 294,510

Total invested

USD 120,000

Estimated returns

USD 174,510

Common questions