Apple reports earnings today. Five numbers to watch by Philip Elmer-DeWitt @FortuneMagazine July 19, 2011, 11:46 AM EST E-mail Tweet Facebook Google Plus Linkedin Share icons A big Q3 is a given. Just look at what the stock has done lately. But the devil is in the details The Street has fallen back in love with Apple AAPL . “Get ready for the return of the wow factor,” J.P. Morgan’s Mark Moskowitz told clients last week just before the stock put the finishing touches on a 28-day, $58.48 (18.5%) run that took it to $373.80 Monday afternoon, an all-time record close. The impetus for this recent infatuation, of course, is the earnings report due out this afternoon. Revised estimates from analysts have been piling in so quickly that the aggregators can hardly keep up. As of Tuesday morning Thomson Reuters still had the consensus calling for earnings of $5.80 on sales of $24.92 billion. Recent estimates, however, have been much higher. Moskowitz expects Apple to report earnings of $6.58 on sales of $27.42 billion, up 87% and 75%, respectively. Some of the independent analysts we’ve been tracking are even more bullish. See here. Although Apple almost always beats the consensus, nothing is certain. It could still surprise Wall Street — positively or negatively — in the details of its report. Here are the numbers we’ll be watching. iPhone unit sales. This is where Apple gets nearly 50% of its income, and there are two confounding factors this quarter that make iPhone sales particularly hard to predict: 1) how many potential customers have held off buying an iPhone in anticipation of the new model reportedly pushed back to this fall, and 2) the rate at which subscribers are leaving their old cell phone plans to switch to the iPhone for Verizon that was introduced in February. In the face of these uncertainties, the highest iPhone unit sales estimate we’ve seen is 20.25 million units. The lowest is 15 million. Anything above 17 million would be a plus. iPad unit sales. The issue is not how fast Apple could sell iPads last quarter, but how fast it could build them. A fire in a Foxconn plant slowed production in May and it was only last week that Apple was finally able to meet demand. Estimates for units sold in the quarter range from 7.21 million to 9 million. The number to beat is 8.33 million. Earnings per share. This is the number that produced the largest discrepancy between the amateur analysts we polled and the pros. The consensus among the amateurs is that Apple will report earnings of $6.72 per share, nearly 17% higher than the pro’s $5.76. The amateurs, for what it’s worth, are usually right. Gross margin: Some believe that the gross margin ratio — the difference between revenue and cost of goods divided by revenue — is the most important number of all, because it tells us how efficiently Apple is turning sales into profits. Three months ago, Apple warned investors that third-quarter margins might be as low as 38% — far below the 41.4% the company reported for Q2. Our amateurs are looking for 40.4%. The pros for 39.2%. June guidance: This is the number that savvy investors have learned to discount but which can still send the stock spiraling down in after-hours trading. Apple traditionally guides “conservatively” — which is to say it low-balls its forward-looking estimates so it can handily beat them three months later. Piper Jaffray’s Gene Munster says that if Apple guides the way it has over the past several years, it is likely to tell investors to expect earnings in the September quarter of $5.73 on sales of $27.08 billion. Ticonderoga’s Brian White issued a note Tuesday morning telling clients that “Apple holds the hottest hand in the tech world” and he expects the rally to continue. J.P. Morgan’s options trading desk issued a report Monday advising clients to buy the $365 August calls, presumably hoping to collect some extra revenue on the shares it snapped up in June, when Apple was going for as little as $310.50 a share. Apple opened at $377.92 Tuesday, up $4.12. But be forewarned: speculating on this stock is a mug’s game. As often as not traders buy on the rumor and sell on the news. Apple usually reports its earnings about a half-hour after the markets close. A conference call with analysts is scheduled to begin at 5 p.m. EDT (2 p.m. PDT) today and should last about an hour. Tune in here for our analysis.