In late October, Apple (AAPL -0.57%) CEO Tim Cook predicted that it would be an "iPad Christmas." From the beginning, Cook's optimism about the holiday season seemed justified. The introduction of the new iPad Air and iPad Mini Retina tablets, along with a price drop for the original iPad Mini, gave Apple its best iPad lineup ever.

The new iPad Air has been very successful for Apple (Photo: Apple)

Based on real-time usage data collected over the last two months, it seems clear that it was indeed an iPad Christmas. Apple has experienced strong uptake of the iPad Air, and to a lesser extent the iPad Mini Retina (which has had tighter supply constraints ). Additionally, the iPad Mini has continued to sell well, driven by additional discounts that made it very affordable this holiday season.

Usage soars
Fiksu provides real-time usage data for iOS devices, which it gathers from a variety of apps that use its marketing platform. Fiksu's usage data shows that the iPad Air has seen stronger adoption than last year's iPad 4 (which itself was a good seller). By Thursday, the new iPad Air represented 6.4% of all iPads in use. That's up more than 2 percentage points from the beginning of this week, indicating that the iPad Air was a popular Christmas present.

The iPad Mini Retina has not sold as well, largely due to limited availability. By Thursday, it accounted for about 2% of all iPad usage. Nearly half of those devices have been activated this week, according to the Fiksu data.

iPad Mini Retina usage jumped on Christmas Day (Photo: Apple)

The original iPad Mini also saw a spike in usage around Christmas, suggesting that it too was a popular gift item. For most of November and December, the iPad Mini represented about 20% of all iPad usage. However, by Thursday, the iPad Mini accounted for 23.3% of the total.

How many iPads did Apple sell?
Last year, Apple sold 22.9 million iPads during the holiday quarter. Based on this year's Fiksu usage data, it seems likely that iPad sales grew at least 20% year over year.

In October, Apple revealed that it had sold 170 million iPads in total. Given that the sales pace probably picked up after the new iPads went on sale in November, cumulative iPad sales should have reached at least 190 million units by now (estimating conservatively).

While some older iPads have been "retired" -- as evidenced by low usage of the original iPad and falling usage for the iPad 2 -- there are probably at least 150 million still being actively used. Based on this figure, it's possible to estimate sales of the new iPads. The iPad Air and iPad Mini Retina combined for 8.4% of total iPad usage by Thursday, implying that roughly 12 million-13 million had been sold through to end users.

Based on the recent activation pace, up to 1 million more of these new iPads could be in use by the end of the month. Additionally, based on Apple's target of maintaining at least four weeks of supply in the channel, resellers should have purchased at least 3 million of the new iPads this quarter for their own inventory. Adding this all up, Apple probably sold at least 16 million units of its two new iPad models last quarter.

The original iPad Mini was also a popular holiday gift (Photo: Apple)

The iPad Mini was also a big seller this quarter. Its usage has grown from around 20% to about 23% of total iPad usage since the beginning of November. Apple would have needed to sell at least 5 million iPad Minis during the quarter just for it to maintain its 20% usage share. Since the iPad Mini actually gained 3 points of share, sales must have been higher than that: probably around 10 million for the quarter.

Older full-size iPads, such as the iPad 2 and the fourth-generation iPad (which was on sale until the new iPad Air became available), contributed a smaller amount to sales. It's harder to estimate sales of these older models, but it seems reasonable to guess that Apple could have sold 2 million-3 million in the quarter.

Adding it up
Combined, these sales estimates point to total iPad sales of 28 million-29 million units this quarter -- and it's even possible that Apple will break the 30 million mark! Even the bottom of this range would represent better than 20% growth in unit sales.

Furthermore, strong iPad Air sales probably caused the iPad ASP to rise somewhat year over year. This would cause revenue to rise even faster than unit sales, unlike last year when the popularity of the cheaper iPad Mini led to rapid unit sales growth but much slower revenue growth.

Foolish bottom line
While not a perfect indicator, Fiksu's iPad usage data can provide investors with a good sense of where iPad sales are headed this quarter. The new iPad Air looks to have been the strongest seller, with around 10 million sold through to end users, plus up to 3 million shipped to resellers. The iPad Mini was close behind, with perhaps 10 million units sold.

In total, Apple may have sold 28 million-29 million iPads this quarter. That's a strong enough performance to call it "an iPad Christmas." If iPhone sales also turn out to be as strong as they seem, Apple investors will be very happy when the company reports its earnings next month.