Under Armour (NYSE:UA) reported promising second-quarter earnings earlier this week, and the stock has soared 18% in the past two days. Shareholders are doubtlessly happy, but at almost $65 per share, Fools should be wondering whether the stock is overpriced.

Based on earnings per share over the past 12 months, Under Armour stock is trading at a P/E ratio of 73. That's sky-high compared to other sportswear and apparel makers such as Timberland (NYSE:TBL), Hidden Gems rec Columbia (NASDAQ:COLM) at 18, Nike (NYSE:NKE) at 19, and even Oakley (NYSE:OO) at 37. Of course, Under Armour deserves a higher earnings multiple because of its growth potential, but is that too high a price? I did a back-of-the-envelope reality check to find out.

In the most recent earnings announcement, management reiterated long-term earnings growth estimates of 20%-25%. If earnings grow at the higher end of that estimate, the company would earn $2.12 during the fifth year. Let's assume that the current multiple contracts to 45; it's a reasonable P/E for a growth company, and it implies a PEG ratio of 1.8. The stock would then trade around $95 per share. Based on today's prices, that would be about an 8% compounded annual return over five years. Not a particularly attractive return, especially for a small-cap stock.

In a more optimistic scenario, assuming that Under Armour delivers 25% earnings growth and the earnings multiple stays above 65, investors would net a 16% annual return over five years. In that case, investors would double their money, and the company's market cap would be $6 billion -- still significantly less than Nike's current $29 billion.

Of course, that's a fairly simplistic analysis, and P/E ratios are notoriously difficult to predict. I'd urge Fools to be cautious with this stock. The company has serious long-term potential, but the market has priced in most of it already.

For more on the companies mentioned, check out:

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Fool contributor Brendan Mathews owns shares in Nike. The Fool's disclosure policy is built for speed.