Here's the number everybody will be waiting for on Apple's earnings call tomorrow

Tomorrow, Apple reports earnings for its fiscal fourth quarter, which ended Sept. 30.

As usual, everybody’s eyes will be focused on the number that matters most to Apple’s business: iPhone sales. The iPhone has become Apple’s most important business, making up between 60% and 70% of its revenue in each of the last three quarters, and sales are still growing.

This chart from Statista shows quarterly sales since the iPhone first went on sale.

The new iPhone 6S went on sale in the U.S., U.K., and a dozen other countries right at the end of the quarter. This quarter, average analyst estimates are around 49 million, which would be a 25% jump over a year ago. The high-end estimates are around 50 million, while 47 million or less would be a bad miss. But the most important quarter will be the end-of-year quarter — that’s when we’ll see whether the 6S can help break the year-ago’s quarterly sales record of 74.47 million.

Get THE MID-YEAR SMARTPHONE MARKET REPORT now! A comprehensive look at the global smartphone market from BI Intelligence by platform, vendor, country and more. Insights into the power struggles between the biggest platforms and the underdogs. Get the Report Here »

NOW WATCH: Tech Insider videos

Want to read a more in-depth view on the trends influencing Australian business and the global economy? BI / Research is designed to help executives and industry leaders understand the major challenges and opportunities for industry, technology, strategy and the economy in the future. Sign up for free at research.businessinsider.com.au.