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Buying foreign stocks, ETFs, or international mutual funds can be a great way to diversify your portfolio. But first you’ll need to decide how much of your money you want to allocate to foreign investments.

In part, the answer will depend on your appetite for risk and the length of your investment horizon. While there’s no right answer for everyone, there are a few guidelines that can help guide your decision.

Slicing Up the Pie

One way to begin is to look at the size of the U.S. stock market in relation to the rest of the world. Picture a pie chart of all the stock markets in the world, with each slice representing a particular country’s stock market. The bigger each market, the bigger its slice of the pie.

The U.S. accounts for roughly half of the world’s total stock market value. So if you wanted to divvy up your portfolio in the same manner as our imaginary pie, you would simply invest half of your money in U.S. stocks, and the other half in foreign markets.

In practice, however, a 50% allocation to international stocks is probably too aggressive for most investors, especially those who are new to international investing. But don’t be too shy. If you only invest a small amount — say 5% or 10% — overseas, you won’t be able to take full advantage of the many benefits that international markets have to offer.

The 20% Solution

As with a lot of things, the truth lies somewhere in the middle. Most financial advisors recommend putting a range of 15% to 25% of your money in foreign stocks. I think 20% is a good place to start. It’s meaningful enough to make a difference to your portfolio, but not too much to hurt you if foreign markets temporarily fall out of favor. Besides, you can always ratchet up your exposure as you become more comfortable with international markets.

While the precise allocation to foreign stocks will differ from one investor to another, the worst thing anyone can do is flip flop between having too much exposure and not enough. So once you’ve settled on a number that suits your comfort level, stick with it. Don’t make the mistake of trying to outsmart the markets by jumping in and out of foreign stocks.

One last thing: it’s important to make sure your international investments are spread around various regions and countries. Putting 25% of your money in say, China, and the rest in the Dow Jones, isn’t what international diversification is all about. Make sure you have evenly balanced exposure across Europe, Asia and emerging markets.