Investment Calculator

How to use this calculator

1. Enter your starting amount — the lump sum you begin with, or 0 if you are starting from nothing.
2. Set how long you will invest for and the return rate you expect.
3. Choose how often the return compounds, and add any regular contribution.
4. Say when that contribution lands — the beginning or end of each month or year. It matters more than it looks.
5. Read the end balance, the split between what you put in and what it earned, and the accumulation schedule year by year or month by month.

How compounding builds an investment

Every dollar you invest earns a return, and those returns then earn returns of their own — the engine behind long-term growth. Regular contributions amplify it, because each new deposit starts its own compounding clock. Early on the balance is mostly the money you have put in; over a long horizon the earnings quietly overtake the contributions and become the larger share.

Investing $500 a month at 7% from nothing, the crossover lands in year 19 — that is when what the money has earned first exceeds everything you have paid in. Carry on to forty years and $240,000 of contributions has become about $1.31 million.

The formula

Each period the balance grows by the return and the contribution is added, month after month:

B_next = B × (1 + i) + C

• B — the balance at the start of the month
• C — the contribution, added before the growth if you contribute at the beginning of the period and after it if you contribute at the end
• i — the monthly growth the chosen compounding frequency implies: (1 + r/n)^n/12 − 1, where r is the annual return and n the compounding periods per year

Running month by month rather than using a closed formula is what lets the two settings be independent: you can contribute monthly to an account that credits interest once a year, which is exactly what most people do.

A worked example

Start with $20,000, add $1,000 at the end of every month, and assume 6% compounded annually for 10 years. The end balance is $198,290.40. Of that, $20,000 is what you started with and $120,000 is what you paid in — leaving $58,290.40 of return, about 29% of the total.

The schedule shows the shift. Year one earns $1,526.53 on deposits of $32,000; year ten earns $10,852.79 on the same $12,000 of deposits. The contributions never change — the balance doing the earning does.

Timing changes the answer

Three settings people skip past are each worth real money on that same plan:

• Contribute at the beginning rather than the end of each month and the end balance rises to $199,081.24 — $791 for paying on the 1st instead of the 30th.
• Contribute $12,000 once a year instead of $1,000 monthly and it falls to $193,986.49 — the same money in, $4,304 less out, because it spent most of each year uninvested.
• Compound monthly instead of annually and it rises to $200,267.28.

None of these change a dollar of what you contribute. They change how long each dollar spends invested, which is the only thing compounding responds to.

Contributions versus earnings

The ring splits your end balance into three parts: the amount you started with, the contributions you added over time, and the return compounding produced. The accumulation chart shows the same three stacked year by year, so you can watch the earnings band widen. Over a 20- or 30-year horizon it typically becomes the biggest of the three, which is why consistency and time usually beat trying to time the market.

What return should you assume?

Historical stock-market returns have averaged roughly 10% before inflation and about 7% after over the long run, but any given year can be sharply up or down. Because returns compound, small differences in the rate produce large gaps over time. Using the same $10,000 plus $500 a month for 20 years:

Annual return	Projected value	Earnings
5%	$232,643	$102,643
7%	$300,851	$170,851
9%	$394,035	$264,035

A four-point spread in the assumed return swings the result by more than $160,000 here. Because the future is uncertain, it is worth running a low, medium and high rate to see the range of plausible outcomes rather than trusting a single figure.

Measuring return: ROI and the rule of 72

To judge an investment after the fact, use ROI = (net gain ÷ cost) × 100 — turn $1,000 into $1,300 and the ROI is 30%. To judge one before, use the rule of 72: 72 ÷ your return is roughly how many years the money takes to double, so about 10 years at 7% and 8 years at 9%. Both are quick sanity checks on how powerful a given return really is.

Limitations

• Results are nominal — not adjusted for inflation, so future dollars buy less than today's.
• The projection assumes a constant return, which real markets never deliver; actual results vary year to year and can be negative.
• It does not deduct taxes or investment fees, both of which reduce real-world returns.

To translate a nominal projection into today's money, use the inflation calculator; to explore the underlying mechanics of compounding frequency, see the compound interest calculator. This is educational information, not investment advice.