Mutual Fund Returns: What the Projection Leaves Out

The projection a fund calculator produces is not a forecast. It is a piece of arithmetic that answers one question: if a return held steady, what would this contribution schedule be worth? Markets never behave that way, but the exercise is still the most useful thing you can do before signing up to twenty years of direct debits.

What the calculator is really modelling

It compounds a fixed monthly contribution at a fixed rate. Real fund returns arrive in a jagged sequence, with several down years scattered through any long run, and the order they arrive in changes the outcome even when the average is identical.

That matters most when you are drawing money out. While you are still contributing, a bad early stretch is close to a gift, because the same $500 buys more units. Which is why the honest way to read the output is as a central estimate with a wide band around it, not a number to plan a retirement date on to the month.

The expense ratio does more damage than it looks like it should

Fees are quoted as a fraction of a percent, which makes them feel negligible. Work the numbers. $500 a month for 20 years at a 7% gross return: with a 0.05% index fund you finish near $258,900. With a 0.75% actively managed fund, $238,000. The gap is about $21,000, and the fund charging more has to beat the cheap one by 0.7 percentage points every single year just to draw level.

UK funds quote the same thing as the ongoing charges figure, or OCF, and it appears on the key information document. Add any platform fee on top, because that is charged separately and does the same damage.

Lump sum against monthly contributions

If you already hold the money, investing it in one go usually finishes ahead, simply because it spends more time in the market. Spreading it over twelve months lowers the chance of putting everything in the week before a fall, at the cost of a slightly lower expected result. Both are defensible and the difference is smaller than the argument about it suggests.

Monthly contributions from salary are a different case entirely. There is no choice being made there, because the money only exists monthly. Run those in the calculator as a regular contribution and leave the lump sum field empty.

How to use the mutual fund calculator

Initial investment. What you are putting in on day one, or zero. Monthly contribution. The standing order amount. Expected annual return. Your assumption, discussed below. Years. Your actual horizon. Expense ratio. From the fund factsheet, listed as the expense ratio in the US or the OCF in the UK.

Choosing a return assumption you can defend

Never type in the fund's best recent year. Use a conservative long-run figure for the asset mix you hold, and if you want a number that survives inflation, subtract your inflation assumption from the return and read the result in today's money. Run the projection twice, once at your central figure and once two percentage points lower, and plan against the lower one. The mechanics behind the growth are set out in how compound interest builds over time, and our investment calculator guide walks through the same inputs for a general portfolio.

Common questions

What return should I assume? Something you would still be comfortable with after a bad decade, then run it again two points lower to see how much rides on the guess. At 7%, $500 a month for 20 years reaches about $260,000; at 5% the same contributions reach about $206,000, a $54,000 gap produced by nothing but the assumption. Treat whichever figure you pick as an assumption rather than a promise, and re-check it every couple of years against your real balance.

Does the projection account for tax? No. Hold funds inside a tax-sheltered account where you can, an ISA in the UK or a 401(k) or IRA in the US, and the projection is close to what you keep. In a taxable account, dividends and gains are taxed along the way and the real figure lands lower.

Why does my actual balance differ from the projection? Three usual reasons: the return has not matched your assumption, contributions were missed, or fees beyond the expense ratio are being charged, such as a platform or adviser fee. Check the fee schedule before blaming the market.

Are ETFs cheaper than mutual funds? Often, though the gap has narrowed a great deal. Index mutual funds and index ETFs tracking the same benchmark now charge similar amounts. Compare the specific expense ratios rather than the categories.