Investment Calculator

What this investment calculator projects

This investment calculator answers one question: if I put money in and keep adding to it, what could the balance look like later? You set four dials: a starting amount, a monthly contribution, a yearly return you are willing to assume, and a number of years. The calculator then shows the future balance, splits it into the money you paid in and the growth on top, and converts the result into today's dollars so inflation does not fool you.

It is a projection, not a promise. Markets do not return the same amount every year, and the calculator uses one smooth rate on purpose, so you can compare plans side by side. The SEC's investor education site, investor.gov, makes the same point: longer horizons and regular contributions do more of the work than any single clever pick.

This extra comparison is useful when checking the calculator by hand. Keep the input units consistent, write down every assumption, and compare the displayed result with an independent calculation. A change in one input can affect every later value, so record the date and the assumptions used. The tool is an educational estimate and does not replace professional advice. Repeating the calculation with a second scenario helps show which inputs matter most and prevents a single rounded number from carrying more certainty than the underlying model deserves.

The math behind the projection

Two formulas are stacked together. The starting balance grows on its own, and every monthly contribution starts its own smaller clock the month it arrives:

FV = P(1 + i)^n + C x (((1 + i)^n - 1) / i)

P is the starting amount, C is the monthly contribution, i is the monthly rate (the yearly rate divided by 12) and n is the number of months. The first term is the lump sum compounding. The second term adds up every contribution together with the growth each one earned from its arrival month onward.

An example of the pieces: $10,000 at 7% a year works out to a monthly rate of 0.5833%, and 240 months pass in 20 years. The calculator applies the formula once per run, which is why changing a single input redraws the whole picture at once.

This extra comparison is useful when checking the calculator by hand. Keep the input units consistent, write down every assumption, and compare the displayed result with an independent calculation. A change in one input can affect every later value, so record the date and the assumptions used. The tool is an educational estimate and does not replace professional advice. Repeating the calculation with a second scenario helps show which inputs matter most and prevents a single rounded number from carrying more certainty than the underlying model deserves.

A 20 year example, dollar by dollar

Take the default plan: $10,000 to start, $500 added every month, a 7% yearly return, 20 years. The projected balance is $300,850.72. You contributed $130,000 of that yourself ($10,000 plus 240 payments of $500), and growth supplied the remaining $170,850.72, which is 56.8% of the total.

The path matters as much as the endpoint. The balance sits at $49,972.70 after 5 years, $106,639.02 after 10 and $186,970.62 after 15. Those five year gaps keep widening: growth of 56,666.32 in years 5 through 10, then 80,331.60 in years 10 through 15, then 113,880.10 in years 15 through 20. Nothing about your behavior changed along the way. The compounding did all of that on its own.

Contributions versus growth over time

The share of the balance that is growth climbs with every year you stay invested:

At year 5 growth is a fifth of the balance. By year 30 it is close to three quarters. That is the whole argument for starting early, compressed into one column.

Holding the opening balance, return, and period constant, $0 monthly produces $40,387.39 after 20 years, $250 produces $170,619.05, $500 produces $300,850.72, and $1,000 produces $561,314.05.

What each dial changes

Rate first. Holding the plan at $10,000 plus $500 a month for 20 years, the balance lands at $205,613.13 at 4%, $232,643.24 at 5%, $264,122.49 at 6%, $300,850.72 at 7%, $343,778.24 at 8% and $394,034.95 at 9%. Each extra point of return is worth more than the one before it: 232,643.24 - 205,613.13 = 27,030.11 for the first step, but 394,034.95 - 343,778.24 = 50,256.71 for the last.

Contribution next, still at 7% for 20 years. $200 a month ends at $144,572.72, $300 a month at $196,665.39, $700 a month at $405,036.05 and $1,000 a month at $561,314.05. Extra contributions scale in a straight line at a fixed horizon: going from $300 to $500 a month adds 300,850.72 - 196,665.39 = 104,185.33, and the same $200 step from $500 to $700 adds another 405,036.05 - 300,850.72 = 104,185.33. Returns compound; contributions stack.

Inflation: what the balance will buy

A dollar in 2046 will not buy what a dollar buys now. To see the real size of the balance, divide by (1 + inflation) once per year. At 2.5% inflation, the 20 year balance of $300,850.72 is worth $183,600.45 in today's dollars. The calculator shows this next to the nominal number so the gap stays visible.

Inflation also quietly shrinks the return itself. A 7% nominal return with 2.5% inflation leaves a real return of about 4.4% a year, which is why two projections can disagree without either one being wrong. The Bureau of Labor Statistics publishes the Consumer Price Index, the series most people use for this kind of adjustment.

Fees and taxes take a slice

Fees come straight off the return. Run the same plan at 6.5% instead of 7%, which is what a 0.5% annual expense drag does, and the 30 year balance ends at $623,007.02 instead of $691,150.47. That fee costs 691,150.47 - 623,007.02 = 68,143.45 even though it never shows up as a line item on any statement.

Taxes wait until you sell in a taxable account, and investments held longer than a year usually get lower rates than wages. The exact rate depends on your income and filing status, and the IRS covers this on its capital gains topic page. Retirement accounts change the timing rather than the arithmetic. This page shows a pre tax projection and is not tax advice.

How to use the projection sensibly

Use a range, not a point. Run the plan at a low rate and a high rate and treat the two answers as a corridor rather than a target. Revisit the inputs once a year: contributions that felt impossible at 25 may be routine at 35, and the assumed return deserves a second look after a few years of real results.

Remember what the smooth rate hides. A plan projected at 7% a year will not deliver 7% in any particular year, and a bad stretch early on leaves less time to recover. That is a feature of investing, not an error in the math, and it is one more reason the early contributions, which get the most years of growth, matter the most.