Did you know that every year 2.5 percent of the average stock mutual fund’s total return is lost to taxes?

And did you know that if you invest in a mutual fund outside of a tax-deferred account, you could get hit with a tax bill even if you haven’t sold a single share?

By the way, how tax efficient is your mutual fund? Don’t know? You can easily find out now, because as of Feb. 15 the Securities and Exchange Commission requires mutual fund companies to list, in a standard table in their prospectus, their after-tax returns for periods of one, five and 10 years.

This new and important disclosure rule now gives investors the quintessential bottom line look at a fund’s return. It is designed to help investors understand the magnitude of tax costs and compare the impact of taxes on the performance of different funds.

Only 33 percent of investors in one poll felt that they were very knowledgeable about the tax implications of investing, according to the SEC. Only 18 percent were able to identify the maximum rate for long-term capital gains (it is currently 20 percent for most tax brackets).

Again, not so surprising because, according to the Eaton Vance survey, 42 percent of investors who use a broker or other financial adviser say that the adviser rarely or never discusses the tax implications of their investments with them. That was actually higher than last year’s 36 percent.

In real numbers, what does that mean?

Here’s an example, set up by Dickson: Suppose you had $10,000 to invest and you bought shares in a mutual fund that had an annual return of 10 percent over a 10-year period. At the end of the 10 years you would have $25,937.

However, if during those 10 years you lose 2.5 percent each year to taxes, your investment would be reduced to $20,610.

Unlike investors who own individual stocks and pay taxes only when they sell their shares, mutual-fund investors pay taxes on annual distributions regardless of whether the fund’s return is up or down.

The amount of taxes a mutual-fund shareholder pays can vary widely from fund to fund. One recent study cited by the SEC found that the annual impact of taxes on the performance of stock funds varied from zero for the most tax-efficient funds to 5.6 percent for the least tax-efficient. The size of the tax bite depends on several factors, including the amount of trading a fund manager does, as well as the degree to which the manager uses portfolio losses to offset realized gains.

According to the SEC, the after-tax returns have to be presented in the following two ways:

After taxes on fund distributions. This number represents investors who hold onto their shares and are taxed based on any capital gains and dividend distributions each year. This return will give shareholders an idea of how tax-efficient a portfolio manager has been.

After taxes on fund distributions and sale of fund shares. This number represents the after-tax return for shareholders who get taxed while holding onto their fund shares and any taxable gain or loss once they sell their shares. This figure is the big picture. It reflects the after-tax effects on shareholders based on both a fund manager’s purchases and sales of stock, and an individual’s decision to sell his or her shares at a given time.

The SEC rule requires companies to show the worst-case scenario in their calculations. Therefore, fund companies have to use the maximum individual federal income tax rate in effect when the distributions were made.