With Apple's (NASDAQ:AAPL) eagerly anticipated December quarter earnings report on tap for Monday afternoon, a few independent analysts are speculating that the company may have sold 60 million iPhones last quarter! This would be well above the average estimate of 54.1 million iPhone shipments among 27 professional analysts surveyed by Fortune's Philip Elmer-DeWitt.

Based on usage data provided by Fiksu and other publicly available information, there is a good chance that Apple will beat the professional analysts' average estimate of 54.1 million iPhone shipments. However, the evidence suggests that Apple probably didn't reach the 60 million mark.

Usage data is the key
Fiksu's iPhone/iPad launch tracker provides a detailed breakdown of iPhone usage by model. As of Thursday, the new iPhone 5s accounted for 13.5% of all iPhone usage and the iPhone 5c represented another 5.3% of total usage. Since Apple aims to keep 4-6 weeks of iPhone supply in channel inventory , it is safe to assume that most of the iPhones in use today had been shipped by the end of the December quarter.


The new iPhone 5s already accounts for 13.5% of all iPhone usage (Photo: Apple)

If we could determine the total number of iPhones still being actively used, it would be possible to translate the 18.8% combined share of the iPhone 5s and iPhone 5c into a unit sales estimate.

Apple sold 150 million iPhones in the fiscal year that ended last September, and another 125 million iPhones in the fiscal year before that . If we include the 54 million iPhones that analysts (on average) believe Apple sold last quarter, Apple has sold roughly 330 million iPhones in the last nine quarters.

The vast majority of those phones are probably still in use, although some have undoubtedly been broken and some may have been "retired" by their users (and not given away, traded in, or sold). Additionally, some of the 72 million iPhones sold in Apple's FY11 may still be in use. Indeed, the iPhone 4 and older models still represent a little over 20% of total iPhone usage, according to Fisku's data.

Estimating sales
Based on all of this information, it seems likely that between 325 million and 350 million iPhones are still active. The combined usage share of 18.8% for the iPhone 5s and iPhone 5c implies that there are currently 61 million-66 million of these new iPhones in use.

Since it is about four weeks since the end of the December quarter and Apple aims for 4-6 weeks of channel inventory, this represents a conservative estimate of how many of the new iPhones had shipped by the end of last quarter.

However, some of those units shipped in the prior quarter. Apple started selling the new iPhones with a little over a week left in the September quarter, and the company sold more than 9 million in the first three days! That early rush more or less depleted Apple's initial inventory, so shipments for the remaining six days of the quarter were probably equal to production: perhaps 3 million units.

If we deduct 12 million units from the combined iPhone 5s-iPhone 5c estimate derived above, it appears that Apple shipped around 49 million-54 million of the new iPhones last quarter.


Apple may have shipped more than 50 million iPhone 5c and 5s units last quarter (Photo: Apple)

It's a little more challenging to estimate Apple's shipments of the older iPhone 4S. However, one helpful signpost is a recent study by Consumer Intelligence Research Partners, which found that the 4S made up 14% of all iPhone sales in the U.S. last quarter.

Older-model iPhones have been somewhat more popular in the U.S. than abroad in recent years. Furthermore, some of the iPhone 4S sales may have been offset by channel inventory drawdowns, as sales of that model are slowing due to the iPhone 5s and iPhone 5c introductions.

It therefore seems reasonable to estimate iPhone 4S shipments at 10% of the total for last quarter. That implies that Apple shipped about 5 million-6 million iPhone 4S units in the December quarter, bringing the total for all three models to 54 million-60 million.

Foolish bottom line
Based on my estimates of the total iPhone user base and Fiksu's iPhone usage data, it appears that Apple shipped somewhere between 54 million and 60 million iPhones last quarter. Thus, while a 60 million iPhone quarter is not impossible, it's a bit of a stretch. On the other hand, the average analyst estimate for 54.1 million shipments appears fairly conservative.

Still, anything in this range will be good news for Apple investors, representing double-digit year-over-year growth. Moreover, with iPad sales also on track for double-digit growth last quarter, Apple could be about to report its first-ever $60 billion quarter! We'll know for sure in a few days, but right now, things are looking very good for Apple.

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Fool contributor Adam Levine-Weinberg owns shares of Apple and is long January 2015 $390 calls on Apple. The Motley Fool recommends Apple. The Motley Fool owns shares of Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.

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