Investment return calculator

How the formula works

Two things grow side by side: the initial investment \(P\), and the stream of monthly contributions \(C\). The future value is the sum of both — the lump sum compounded on its own, plus the accumulated contributions:

where \(t\) is the number of years and \(i\) the effective monthly rate, derived from the net annual return so that compounding and the monthly deposits stay in step:

The key move is the second equation. Your expected gross return \(r\) is what the market delivers; the fund's fee \(f\) — its expense ratio — is skimmed off every year, so the money actually compounds at the net rate. A fee is not a one-off deduction: it is charged on the whole balance, year after year, so it grows exactly as your returns do. That is why a difference that looks tiny on paper — half a percentage point against one — turns into a large dollar gap over decades. Run the numbers at a 0.5% expense ratio and again at 1%, holding everything else fixed, and the extra half-point quietly removes $30,946 from the default scenario below. The calculator applies the net rate month by month, deposits landing at the end of each month, so any point on the schedule can be checked by hand.

What each input means

Initial investment — \(P\)

What is invested today. It can be zero: the contribution stream does not need a head start, it just benefits from one.

Monthly contribution — \(C\)

What you add every month, assumed constant and deposited at the end of the month. If you invest per paycheck, multiply a biweekly amount by 2.17 to get its monthly equivalent.

Expected annual return — \(r\)

The gross yearly growth you assume before fees, as a nominal figure. A diversified stock-heavy portfolio has returned somewhere near 7–10% nominal over long US history; a lower figure is the more cautious planning choice. Nothing guarantees any of them.

Years — \(t\)

How long the money compounds. Time is the input that does the heavy lifting: doubling the return roughly doubles the growth, but doubling the years far more than doubles it, because growth stacks on growth — and, less happily, so does the fee.

Annual fee / expense ratio — \(f\)

The percentage a fund charges each year, subtracted from the gross return. Index funds commonly run 0.03–0.20%; actively managed funds often 0.5–1% or more; an advisor may add roughly another 1% on top. Enter the total drag on your money. Set it to zero to see the fee-free ceiling.

A worked example

The worksheet's defaults: $10,000 invested today, $500 added at the end of every month, a 7% expected annual return, a 0.5% expense ratio, over 25 years.

The fee comes off the return first: net = 7% − 0.5% = 6.5%. The effective monthly rate is \(i = 1.065^{1/12} - 1 = 0.005262\), and over \(12t = 300\) months the growth factor is \((1+i)^{300} = 4.8277\). The two parts of the formula:

• The initial $10,000 grows by that factor on its own: $10,000 × 4.8277 = $48,277.
• The $500 monthly deposits accumulate to $500 × (4.8277 − 1) / 0.005262 = $363,733.

Together the portfolio reaches $412,010. Of that, $160,000 is money you put in and the other $252,010 is growth. Now run the same numbers with the fee set to zero: the balance would be $445,795. The difference — $33,786 — is what the 0.5% expense ratio costs over the period. You never see that line on a statement, but it is real money — here about 21% of everything you contributed, taken by a fee that looked like a rounding error on the way in.

Assumptions and limits

• The return is a constant 7% every year. Real markets do not move in a straight line: they deliver volatility around the average, and the order of those returns matters. A bad stretch early — sequence-of-returns risk — leaves less invested to recover, so two portfolios with the same average return can end far apart. A smooth line models neither volatility nor sequence risk.
• The fee is modelled as a flat annual drag on the return. Real expense ratios are charged on assets and can change; advisory fees, trading costs and fund turnover add more. Enter your best estimate of the total.
• Taxes are ignored, and the account type is what decides them. The same return compounds tax-free in a Roth, tax-deferred in a traditional 401(k) or IRA, and is taxed along the way in a taxable brokerage account — three materially different after-tax results this model treats alike.
• Contributions are constant and monthly, deposited at the end of the month. Most people's contributions rise with income, which helps, and pause in bad years, which hurts.
• This is arithmetic, not advice. It shows what the inputs imply; it cannot know what return you will actually earn.

Questions people ask

What return should I assume?

Lower than the headline. US stocks have returned roughly 10% a year in nominal terms over the last century, or closer to 7% after subtracting inflation — a real return. A diversified portfolio that holds bonds too returns less, and fees subtract further. These are descriptions of the past, not promises about the future; the market has no obligation to repeat them. Many people plan with 6–7% nominal and check the result at 5% as well, to see how sensitive the plan is to being wrong.

How much do fees really cost?

Far more than the percentage suggests, because the fee compounds against you every year. Take the default scenario at a 0.5% expense ratio: it grows to $412,010. Change nothing but the fee, raising it to 1%, and the balance falls to $381,063 — the extra half a percentage point costs $30,946 over 25 years. That is the case for low-cost index funds in a sentence: the fee you pay is small and certain, the return you give up for it is large and compounding.

Does this account for inflation?

No — the result is in future dollars. A portfolio worth $412,010 in 25 years will not buy $412,010 of today's goods. To think in today's purchasing power, subtract expected inflation from the return before you enter it: a 7% return during 3% inflation is about a 4% real return. Run the calculator at that lower figure and the balance is stated in today's money instead.

Is it better to invest a lump sum or spread it out?

On average, investing a lump sum sooner beats spreading it out, because markets rise more often than they fall, so time in the market usually wins. Spreading a fixed sum in over time — dollar-cost averaging — lowers your average return in exchange for lowering the risk of buying everything right before a drop. This calculator models an initial amount plus a steady monthly stream, which is how most people actually invest: a paycheck at a time, whatever the market is doing.

What is a realistic long-run assumption?

A conservative planning number sits below the historical average, leaving room to be wrong. For a stock-heavy portfolio, 6–7% nominal (or 4–5% real, after inflation) is a common, defensible choice; a portfolio with more bonds warrants less. Whatever you pick, hold fees down — they are the one input on this page you fully control — and treat the output as a projection of an assumption, not a forecast of your balance.