Apple has taken away the yardstick everybody measured its business with — but the replacement might be better (AAPL)

Tim Cook Auburn

Apple won't provide unit sales for products such as the iPhone and the iPad to investors anymore.
Analysts and investors had relied on those numbers as key metrics to judge Apple's growth.
Instead, Apple is now disclosing how many iPhones are in use — 900 million, according to Tuesday's earnings call.

Tuesday's earnings report was the beginning of a new era for Apple.

Instead of providing investors and observers a simple number for the total amount of iPhones sold during a quarter — for example, 47 million at its previous earnings call — Apple now provides information for only the amount of revenue that it generated through sales of devices.

Apple has a new number that it would like investors to look at instead: 900 million.

That's the number of iPhones out there in use.

"We are disclosing that number now for the first time as it has surpassed 900 million devices, up year-over-year in each of our five geographic segments and growing almost 75 million in the last 12 months," Apple chief financial officer Luca Maestri said in a call with analysts on Tuesday.

Apple has previously provided the number of total active Apple devices in usage, which totaled 1.4 billion on Tuesday. But active iPhones are a more important statistic, given that while some people own multiple Apple devices, it's much less likely that people have two iPhones. So it gives investors a rough size of Apple's user base.

The disclosure also seemed to mollify investors, who were bracing for the worst after Apple's stock was halted earlier this month when it said it would miss its revenue target by at least $5 billion. Apple stock was up more than 5% in after-hours trading on Tuesday.

The number is also the key statistic that investors need to watch if they buy into Apple's services narrative. Apple is telling investors that it is undervalued when you consider how much money Apple can make from its existing customers by selling them products like Apple Music or iCloud subscriptions.

"Not only is our large and growing installed base a powerful testament to the satisfaction and loyalty of our customers but it's also fueling our fast growing services business," Apple CEO Tim Cook said on Tuesday.

It wasn't the only new data point that Apple revealed on Tuesday. The company had new statistics on the number of paid Apple Music subscribers (50 million) and the number of Apple News monthly readers (85 million.)

But those appeared to be one-off data points on Tuesday's call. For analysts and investors going forward, the number of active iPhones will be a key metric to watch, and it will be disappointing if Apple does not disclose them next quarter.