What this projection shows
Most people build investments by paying in a little at a time — every month from a salary, or once a year. This calculator adds those contributions to a starting amount and grows the whole balance at the return you expect, so you can see three things apart: what you put in, what the growth adds, and how the mix shifts over time.
In the early years the balance is mostly your own money. Later, growth on the growing balance overtakes new contributions, which is why time in the market matters as much as the amount. The chart shows this crossover year by year.
A number decades away is hard to judge, so the calculator can also divide the result by inflation. Today’s money is what the future balance would buy at today’s prices if prices rise at the rate you enter.
Returns are not steady
The calculator uses the same return every year. Real investments rise in some years and fall in others, and the order of good and bad years changes the outcome, especially near the end. Past performance does not predict future returns. Treat the result as one scenario and try a lower and a higher return next to it.
The formulas
The return R is treated as an effective annual rate, so for monthly contributions the monthly return is:
i = (1 + R)^(1 ÷ 12) − 1
This makes a balance grow by exactly R over a year (6% a year is 0.4868% a month, not 0.5%). The end balance is the starting amount grown for the whole period plus every contribution grown from the day it was paid in:
FV = P × (1 + R)^t + C × ((1 + i)ⁿ − 1) ÷ i
- P
- starting amount
- C
- contribution per month (or per year, with i = R)
- t
- years
- n
- number of contributions (t × 12 for monthly)
Contributions made at the beginning of each period earn one extra period of growth, so their part is multiplied by (1 + i). With a yearly raise, each year’s contributions are larger by that percentage and the calculator adds them up one by one. The inflation adjustment divides by the growth of prices:
real value = FV ÷ (1 + inflation)^t
Worked example: $500 a month for 20 years
You start with $10,000 and add $500 at the end of every month for 20 years, assuming a 6% yearly return and 2.5% inflation.
- Monthly return: 1.06^(1/12) − 1 = 0.48676%
- Starting amount: $10,000 × 1.06²⁰ = $32,071.35
- Contributions: $500 × (1.06²⁰ − 1) ÷ 0.0048676 = $226,719.32 (from $120,000 paid in)
- End balance: $258,790.67, of which $128,790.67 — about half — is growth
- In today’s money: $258,790.67 ÷ 1.025²⁰ = $258,790.67 ÷ 1.6386 = $157,932.43
How much the assumed return matters, with everything else the same:
| Return per year | Balance after 20 years |
|---|---|
| 4% | $203,832.10 |
| 6% | $258,790.67 |
| 8% | $331,109.11 |
Paying the $500 at the start of each month instead gives $259,894.24. Paying $6,000 once at the end of each year gives $252,784.90 — less, because monthly contributions are invested earlier on average.
What the projection leaves out
- Volatility. A constant return hides the ups and downs. Two investors with the same average return can end with very different balances depending on when the bad years fall.
- Fees. Fund expenses and advisory fees come out of the return every year. Enter the return you expect after fees.
- Taxes. Growth may be taxed each year or on withdrawal, depending on the account. The result is before tax.
- Inflation is an assumption too. The US consumer price index, published monthly by the Bureau of Labor Statistics, shows how much prices have actually changed; future inflation is unknown.
For a single deposit at a fixed interest rate and how compounding frequency affects it, use the compound interest calculator. To plan for a retirement income, try the retirement calculator.
Frequently asked questions
What return should I assume?
There is no safe number. It depends on what you invest in, fees and the period. Try a cautious, a middle and an optimistic figure and compare them: in the example, $500 a month for 20 years ends at about $204,000 at 4%, $259,000 at 6% and $331,000 at 8%.
Is it better to invest monthly or once a year?
If you have the money either way, investing sooner gives it longer to grow, so monthly contributions from each paycheck usually end a little ahead of one payment at the end of the year. Paying the yearly amount at the start of the year would do better still, if you have it.
What does in today’s money mean?
It is the future balance divided by how much prices are expected to rise. At 2.5% inflation for 20 years, prices rise by about 64%, so $258,790.67 then buys roughly what $157,932.43 buys today. It gives a feel for the real value of a distant number.
Why does growth overtake my contributions?
Growth is earned on the whole balance, including past growth, while contributions add the same amount each time. As the balance gets bigger, the yearly growth gets bigger too, and eventually it exceeds what you pay in each year.
Does the calculator include taxes and fees?
No. Enter a return after fees if you want them reflected. Taxes depend on the type of account and where you live, so the result is a before-tax figure.
Sources
Last reviewed September 15, 2026