How many iPads did Apple sell last quarter?

That iPad sales declined year over year in Apple’s December quarter should come as no surprise to anyone who follows the business.

At the core of the “massive deceleration” in worldwide tablet sales that IDC predicted in November — and illustrated with the attached chart — was the expectation that Apple in 2014 would rack up its first full year — and fourth quarter in a row — of falling iPad shipments.

Why the slump? Commentators have offered plenty of reasons; Time supplied five.

The only question left is how much.

Of the Apple analysts we’ve heard from so far — 10 amateurs and 16 pros — iPad unit sales estimates ranged from a low of 16.3 million (Stifel Nicholas’ Aaron Rakers) to high of 25 million (Jefferies Peter Misek and Piper Jaffray’s Gene Munster).

The average estimate, 21.5 million, represents a drop of 17.4% from last year’s 26 million — the worst year-over-year falloff in four quarters.

For the record, the amateur analysts this time around were ever-so-slightly more bearish about the iPad than the pros. We’ll find out who was closest to the mark when Apple reports its fiscal Q1 2015 earnings after the markets close on Tuesday Jan. 27.

Below: The individual analyst’s Q4 estimates — pros in blue, indies in green. Thanks as always to Posts at Eventide‘s Robert Paul Leitao for pulling together the Braeburn Group numbers.

Follow Philip Elmer-DeWitt on Twitter at @philiped. Read his Apple AAPL coverage at coins2day.com/ped or subscribe via his RSS feed.