How to use the investment calculator
- Choose what you want to find: the end amount, the contribution needed, the return rate needed, the starting amount needed or the time needed.
- If you are solving for anything other than the end amount, enter your target amount.
- Fill in the remaining boxes: starting amount, contribution, how often you contribute (monthly or yearly) and whether it goes in at the start or end of each period, the expected annual return, and the years invested. The box for the value you are solving for is hidden.
- Read the answer, the split between your money and growth, and the balance at the end of every year.
How the growth is calculated
S is the starting amount, c the contribution, r the annual return as a decimal and Y the number of years. The return is treated as an effective annual rate: 7% means the money grows exactly 7% over a year, however you contribute, so monthly growth is a little under 7 ÷ 12. For yearly contributions, A = ((1 + r)^Y − 1) ÷ r.
The other modes turn the formula around. The contribution needed is (target − S × (1 + r)^Y) ÷ A, and the starting amount needed is (target − c × A) ÷ (1 + r)^Y. The return needed has no direct formula, so the calculator narrows it down by repeated halving until the end amount matches the target. The time needed is found by stepping forward a month at a time until the balance reaches the target.
Worked example
The calculator starts with $10,000, plus $500 at the end of every month, at a 7% annual return for 20 years.
- Monthly growth: 1.07^(1/12) − 1 = 0.56541%.
- The starting amount grows to $10,000 × 1.07^20 = $10,000 × 3.869684 = $38,696.84.
- The contributions grow to $500 × (3.869684 − 1) ÷ 0.0056541 = $500 × 507.5364 = $253,768.19.
- End amount: $38,696.84 + $253,768.19 = $292,465.03.
You invest $10,000 + 240 × $500 = $130,000, so $162,465.03 of the end amount is growth.
One plan, five questions
Each row solves for a different unknown, using a $500,000 target where one is needed and the example's other inputs.
| Solve for | Answer |
|---|---|
| End amount | $292,465.03 |
| Monthly contribution to reach $500,000 | $908.91 |
| Annual return to reach $500,000 | 11.24% |
| Starting amount to reach $500,000 | $63,630.98 |
| Time to reach $500,000 | 26 years 6 months |
The table shows how the levers trade off. To hit $500,000 in 20 years you can almost double the monthly contribution, find a return 4 points higher, start with about $64,000, or simply wait six and a half more years.
Reading the result sensibly
Real investments never return the same amount every year. A steady 7% is only a planning assumption, and individual years and even whole decades can come in far above or below it. Check your plan at a lower return too, such as 4% or 5%, and treat the "return needed" mode as a warning sign if it asks for more than about 10% a year. Fees and taxes also come out of real returns; the mutual fund calculator shows what an expense ratio costs over time.
Frequently asked questions
Is the expected return the same as an interest rate?
Not quite. Here it is an effective annual rate: a 7% return grows $1,000 to $1,070 in a year, whether you contribute monthly or yearly. A savings account quoting 7% compounded monthly would actually earn 7.23% a year. Use the compound interest calculator for accounts that quote a rate and a compounding frequency.
Should I use a return before or after inflation?
Either works if you are consistent. Use a nominal return (such as 7%) to see the actual dollar balance, or a real return (such as 4 to 5%) to see the result in today's buying power. The inflation calculator converts a future amount into today's money.
Why does contributing at the start of the period give more?
Each contribution earns one extra period of growth. With monthly contributions the gain is small, but with yearly contributions, investing in January instead of December adds a full year of growth to every deposit.
How do I check how my real investments have done?
Use the average return calculator for your yearly returns or start and end values, or the ROI calculator for a simple gain on one investment.
Related calculators
- Compound Interest Calculator — How a starting balance and monthly contributions grow, with any compounding frequency.
- ROI Calculator — Return on investment as a percentage, the gain or loss, and the annualized ROI from a length or two dates.
- Average Return Calculator — Arithmetic and geometric average returns from yearly results, or the CAGR from a start and end value.
- Retirement Calculator — See what your savings grow to by retirement, how long they last, and how much more to save each month.
Results are estimates for educational purposes and are not financial, tax or legal advice. See our disclaimer.