Investment Calculator

This calculator projects what an investment account could be worth after a number of years if you start with a lump sum, add a fixed amount every month and earn a steady annual return that you choose. It splits the final balance into the money you put in and the growth earned on it, converts the result to today's dollars with an inflation rate, and shows how much a delay in starting would cost by the same end date.

Use it to compare plans: a larger starting amount against bigger monthly deposits, contributions that rise with your pay, or a shorter or longer horizon. Because real returns are not steady, the table under the calculator repeats the projection at two percentage points below and above your rate.

FV = P × (1 + r)ⁿ + C × ((1 + r)ⁿ − 1) ÷ r

P is the starting amount, C the monthly contribution, r the monthly rate (annual return ÷ 12) and n the number of months. This is the spreadsheet function FV(rate/12, n, −C, −P). If you contribute at the start of each month, the contribution term is multiplied by (1 + r) because every deposit earns one extra month of growth.

value in today's dollars = FV ÷ (1 + inflation)^years

Monthly compounding at a nominal 6% gives an effective annual return of 6.17%. When contributions rise each year, the calculator applies the increase every 12 months and compounds month by month rather than using a single formula.

$10,000 to start and $500 a month for 30 years

At an assumed 6% annual return, the account reaches $562,483. You contribute $190,000 and growth supplies $372,483, so about 66% of the balance is earnings. With 3% inflation, that is worth $231,736 in today's dollars.

Waiting ten years to start

Starting the same plan ten years later and stopping on the same date leaves $298,361 less. Most of that gap is not the missing deposits ($60,000 of them) but the growth they and the earlier balance would have earned.

Raising contributions 3% a year

If the $500 rises 3% each year, in line with pay, the final balance is $760,246 from $295,452 contributed, $197,762 more than with flat deposits.

A single $50,000 investment

With no further deposits, $50,000 at 6% for 20 years grows to $165,510, or $91,639 after 3% inflation.

No starting balance, deposits at the end of each month, monthly compounding. Scale linearly: $400 a month gives four times these figures.

The return you enter is an assumption, not a forecast. Investment returns vary from year to year and can be negative, and the order of good and bad years matters once you start withdrawing. A steady-rate projection shows what compounding does at a given average; it cannot tell you what average you will get. Fees and taxes reduce the return you keep, so subtract fund expense ratios and advisory fees from the rate, and use an after-tax rate for taxable accounts.

The 4% figure is the first-year withdrawal from William Bengen's 1994 study of historical US stock and bond returns, which found that withdrawing 4% of the starting balance and raising it with inflation lasted at least 30 years in every period he tested. It is a rule of thumb from past data, not a guarantee, and many planners adjust it for longer retirements and current conditions.