Investment Calculator
Free investment calculator. Project how a lump sum plus monthly contributions grow over time, with CAGR and inflation-adjusted values. No signup.
An investment calculator projects what a lump sum plus a regular monthly contribution grows to over a chosen number of years, then splits the total into your money and investment growth. It also reports the inflation-adjusted value and the portfolio CAGR. At the defaults — $10,000 to start, $500 a month, 20 years at 7% with 2.5% inflation — the future value is $300,851, of which $170,851 is growth, and the real value is $183,600.
The formulas
Compounding is monthly, with r = annual return ÷ 12 and n = years × 12.
- Starting balance grows to: lump × (1 + r)n
- Contributions grow to: monthly × ((1 + r)n − 1) ÷ r
- Future value: the two added together
- Inflation-adjusted: future value ÷ (1 + inflation)years
- CAGR: the single annual rate that would reproduce the same future value from the same contributions, found by numeric search on the same formula
Worked example with the defaults
7% ÷ 12 = 0.5833% a month, over 20 × 12 = 240 months.
| Step | Calculation | Result |
|---|---|---|
| Starting balance grows | $10,000 × (1.005833)240 | $40,387 |
| Contributions grow | $500 × ((1.005833)240 − 1) ÷ 0.005833 | $260,463 |
| Future value | $40,387 + $260,463 | $300,851 |
| Total contributed | $10,000 + $500 × 240 | $130,000 |
| Investment growth | $300,851 − $130,000 | $170,851 |
| Inflation-adjusted | $300,851 ÷ (1.025)20 | $183,600 |
Contributions are $130,000; the other $170,851 is compounding. At 2.5% inflation, that final balance buys what $183,600 buys today.
How return and time change the outcome
$10,000 starting, $500 a month:
| Return | 10 years | 20 years | 30 years |
|---|---|---|---|
| 5% | $94,111 | $232,643 | $460,807 |
| 7% | $106,639 | $300,851 | $691,150 |
| 10% | $129,493 | $452,965 | $1,328,618 |
Doubling the horizon at 7% more than doubles the result, and the same contributions at 10% instead of 7% produce nearly twice the 30-year value. Return and time are the two levers that matter most.
Common mistakes
- Quoting nominal returns as real ones. At 2.5% inflation, a 7% nominal return is roughly 4.4% real. The inflation-adjusted row is the spending-power number.
- Ignoring fees and taxes. A 1% expense ratio quietly reduces the return you should enter.
- Reading the CAGR as a prediction. It is the constant rate that reproduces this result, not a forecast; real markets are uneven.
- Forgetting that contributions rise over time. The tool holds the monthly amount fixed, which understates what most people invest as income grows.
Why does the CAGR equal my return input?
The CAGR is solved from the same annuity formula the projection uses, so for a level contribution schedule it matches the annual return you entered. It becomes a separate, useful number only when the cash flows are irregular.
Does the timing of contributions matter?
Yes, slightly. The formula assumes contributions arrive at the end of each month, so a contribution made at the start of the month would earn marginally more.
How do I account for a one-off lump sum added later?
Run the projection in two parts, or discount the future lump sum back to the target date and add it to the reported future value. The tool itself handles one starting lump plus level monthly contributions.
How to use the result
- Work from the inflation-adjusted number, since it is the spending power you can actually plan with.
- Check the growth share. If it is small relative to contributions, your horizon is short and compounding has not had time to work.
- Raise the contribution rather than the return assumption; the first is under your control, the second is not.
Should I reduce the return for fees?
Yes. If your funds charge 0.5% a year in total, entering 6.5% instead of 7% gives a more honest projection. Over 20 years that difference compounds to tens of thousands on balances of this size. Fees are also the most predictable drag on a long-horizon account, which is why low-cost index funds are common in retirement portfolios.
The same compounding drives retirement saving, with ages and an income estimate added by the retirement calculator. If you are deciding whether to invest or repay debt, compare the return here with the loan rate from the simple loan calculator.