Apple's iPhone 6 unit sales will continue to impress, according to UBS. Image Source: Flickr user Kārlis Dambrans

For Apple (NASDAQ:AAPL) investors, the newest iterations of the iPhone have turbocharged the company's stock. After being released late in the fourth fiscal quarter, the real effects of the phone were felt during the seasonally heavy first fiscal quarter. The company reported selling 74.5 million iPhones during its fourth quarter, registering 46% year-over-year unit growth, and blowing past analyst expectations of 65 million units sold.

In the company's fiscal second quarter, growth in Apple's signature product continued to impress. Led by strong performance in China, aided by the observance of the Chinese New Year, the company sold 61.2 million units -- registering year-over-year unit sales growth of 40%. As far as actual revenue is concerned, the higher prices of the newer units actually led to revenue growth figures higher than the units-sold figures: For a visual representation of the iPhone's growth, see the chart below:

Source: Apple's 10Qs. Revenue figures in millions

Apple's year-over-year units sold growth figures of 46% and 40%, respectively, led to year-over-year iPhone revenue increases of 57.5% and 54.6%. Not only that, the iPhone has been performing so well the product is now responsible for nearly 70% of Apple's total revenue, up from a little over 50% in the third fiscal quarter of 2013. And if the newest report from UBS is correct, investors can continue to expect strong performance from Apple's iPhone.

UBS expects overperformance
In a research note, UBS notes Apple will smash consensus Wall Street estimates. While Wall Street is expecting Apple to sell 45 million units, UBS predicts Apple will move 51.1 million units -- a figure nearly 13.6% greater than the existing consensus. And while that's certainly an aggressive estimate, it's not unheard of for Apple to blow past consensus estimates. Furthermore, Business Insider notes that UBS' Evidence Lab research wing has been more accurate than Wall Street estimates in the past in regards to iPhone sales.

If past is prologue, it's entirely possible that UBS is correct. Multiplying last year's third fiscal quarter units sold (35.2 million) by the average of the last two quarters' growth (43%), gives a units sold total of 50.3 million units sold in the upcoming quarter. Not exactly 51.1 million units, but closer to 51.1 million than 45 million non-UBS consensus.

And the difference is rather important when it comes to Apple's top line. Using last quarter's per-unit average selling price (ASP) as a proxy for next quarter's, the difference amounts to $4 billion of revenue.

Quarter/ScenarioQ3 '14Q3 '15, Consensus Q3 '15, UBS
Units Sold 35,203 45,000 51,100
Average Selling Price $561.08 $658.52* $658.52*
iPhone Revenue $19,751 $29,633 $33,650
iPhone Revenue Growth, YoY   50% 70%

*Projected. Units sold figures in thousands; Revenue figures in millions. Q3 '14 figures extracted from Apple's 10Q. Consensus is ex-UBS.

As you can see, the differences between the two are $4 billion in revenue and 20 percentage points of year-over-year growth. Of course, this is dependent on the ASP keeping pace with last quarter's figure. And while it's fair to note that last year's third-quarter ASP slipped from the second quarter as more shoppers chose off-year models and the lower-priced iPhone 5c model, demand for the new, larger form factors should keep ASPs high.

That said, even if there's a little slippage in ASP, it's good to see analysts -- especially ones that have been rather accurate in the past -- provide iPhone units sold numbers higher than consensus. For investors, it appears Apple's growth engine isn't stopping anytime soon. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.