What are all the differences between the "Late 2015" Aluminum iMac models? Which one should I buy?

Please note that the "Late 2015" iMac models have been discontinued. However, this Q&A has been updated subsequently and can be quite helpful to anyone buying or selling one of these systems on the used market.

With a quick glance at the "Late 2015" Aluminum iMac models, whether the smaller ones with 21.5-Inch displays -- the iMac "Core i5" 1.6 21.5-Inch, "Core i5" 2.8 21.5-Inch, "Core i5" (4K) 3.1 21.5-Inch, and "Core i7" 3.3 (4K) 21.5-Inch -- and the larger ones with 27-Inch displays -- the iMac "Core i5" 3.2 (5K) 27-Inch, "Core i5" 3.3 (5K) 27-Inch, and "Core i7" 4.0 (5K) 27-Inch -- one might assume that the displays are the primary difference.

Although it certainly is true that the displays are an important difference, there are other internal differences that are almost as important, or even more so, if upgrades are important to you.

Consequently, a detailed evaluation can be quite helpful to determine which model is best for your needs.

Photo Credit: Apple, Inc. (21.5" & 27" Late 2015 iMac Models)

External Differences

Each of the "Late 2015" iMac models use a completely aluminum case design that tapers sharply to a 5 mm edge at the sides of the display. As shipped by default, none of the 21.5-Inch or 27-Inch models are VESA compliant, but both sizes can be configured with a VESA mount option instead of the aluminum stand at the time of purchase for an extra US$40.

The entry-level 21.5-Inch models -- the iMac "Core i5" 1.6 21.5-Inch and "Core i5" 2.8 21.5-Inch -- have a "traditional" 21.5" 1920x1080 LED-backlit 16:9 widescreen IPS display. The mid-range 21.5-Inch models -- the iMac "Core i5" (4K) 3.1 21.5-Inch and "Core i7" 3.3 (4K) 21.5-Inch -- have a much higher-quality 21.5" 4096x2304 LED-backlit 16:9 widescreen IPS DCI-P3 "Retina 4K" display that runs "pixel doubled" to look like 2048x1152 at twice the density. Finally, the high-end 27-Inch models -- the iMac "Core i5" 3.2 (5K) 27-Inch, "Core i5" 3.3 (5K) 27-Inch, and "Core i7" 4.0 (5K) 27-Inch -- have a 27" 5120x2880 LED-backlit 16:9 widescreen IPS DCI-P3 "Retina 5K" display that looks like 2560x1440 at twice the density. All of these displays have a fully laminated glass cover and an anti-reflective coating.

All of these models also have an integrated 720p "FaceTime HD" webcam embedded in the upper display bezel, speakers integrated at the bottom edge of the display, and dual integrated microphones. Neither has an integrated optical drive, although an external one may be added at additional cost (site sponsor Adorama has the official external Apple SuperDrive for US$79 and Other World Computing has a variety of third-party external optical drive options).

By default, each "Late 2015" iMac model included a compact aluminum Apple Magic Keyboard and the choice of either a multi-touch "Magic Mouse 2" or a traditional wired Apple mouse. For an additional US$50, one also could upgrade from the Magic Mouse 2 to the "Magic Trackpad 2," which provides multi-touch and pressure-sensitive "Force Touch" capabilities.

Connectivity Differences

Connectivity is identical on all "Late 2015" iMac models with one notable exception.

Each "Late 2015" iMac model -- regardless of display size or quality -- has an SDXC-capable SD card slot, four USB 3.0 ports, two Thunderbolt 2 ports, one Gigabit Ethernet port (10/100/1000Base-T), and one "headphone/optical digital audio output (minijack)" that also supports the Apple iPhone headset with microphone.

The "Late 2015" iMac systems also support 802.11ac Wi-Fi, which is backwards compatible with the slower 802.11a/b/g/n standards, as well as Bluetooth 4.0.

The notable difference is that because of variations in internal graphics, the entry-level 21.5-Inch models -- the iMac "Core i5" 1.6 21.5-Inch and "Core i5" 2.8 21.5-Inch -- can support up to 3840x2160 on an external display, the mid-range 21.5-Inch "4K" models up to 4096x2304 on an external display, and the high-end 27-Inch models can support either a single external 5120x2880 dual-cable external display or two external 4096x2160 displays.

Identification Differences

The "Late 2015" iMac models share model numbers with previous "Tapered Edge" Aluminum iMac models. Specifically, these 21.5-Inch models share model number A1418 and these 27-Inch models share model number A1419.

As a result, the model numbers are not sufficiently unique for most tasks, like upgrading the RAM and storage. Consequently, other identifiers are necessary for more precision.

In software, the Model Identifier is sufficiently unique for just about all purposes. Externally, the EMC number is best as it is less likely to be shared by future models using the same case type, although it is inconveniently located on the underside of the stand.

These identifiers for the "Late 2015" iMac models are as follows:

"Late 2015" iMac	Model ID	EMC No.
"Core i5" 1.6 21.5"	iMac16,1	2889
"Core i5" 2.8 21.5"	iMac16,2	2889
"Core i5" 3.1 (4K) 21.5"	iMac16,2	2833
"Core i7" 3.3 (4K) 21.5"	iMac16,2	2833
"Core i5" 3.2 (5K) 27"	iMac17,1	2834
"Core i5" 3.3 (5K) 27"	iMac17,1	2834
"Core i7" 4.0 (5K) 27"	iMac17,1	2834

Just as the site has for many years, EveryMac.com has meticulously hand documented all of these details for your convenience.

Additionally, EveryMac.com's Ultimate Mac Lookup feature also can uniquely identify each one of the "Late 2015" iMac models by their serial numbers.

Internal Differences

In addition to displays and connectivity, the most important differences between the "Late 2015" iMac models are internal.

The 21.5-Inch models use "Fifth Generation" Intel "Broadwell" processors that are soldered in place, comparatively slow integrated graphics of varying capability, and 1867 MHz "Low Power" LPDDR3 SDRAM soldered onboard that cannot be upgraded at all after initial system purchase. If these models only were configured with a 2.5" hard drive at the time of initial system purchase, rather than being configured with a "blade" SSD or "Fusion Drive", the PCIe connector (PCIe 2.0 x4 NVMexpress interface) is not onboard to make it possible to install one after the initial system purchase, either.

The 27-Inch models, by contrast, use more advanced, and faster "Sixth Generation" Intel Mobile Core i5 "Skylake" processors mounted on a LGA 1151 socket that can be upgraded after purchase, have much faster graphics with dedicated GDDR5 memory, four easy-to-upgrade 1867 MHz PC3-14900 DDR3 SDRAM SO-DIMM slots (courtesy of a small "pop off" door on the rear), and have both a Serial ATA (6 Gb/s) connector for a 2.5" hard drive and a PCIe connector (PCIe 2.0 x4 NVMexpress interface) for a compatible "blade" SSD whether or not one is pre-installed.

"Late 2015" iMac Comparison Chart

The major differences between the entry-level "non-Retina" 21.5-Inch, Retina 4K 21.5-Inch, and Retina 5K 27-Inch "Late 2015" iMac models -- size, displays, architectures, processors, graphics, upgrades and more -- are summarized below:

	21.5"	4K 21.5"	5K 27"
Std. Processor Speeds:	1.6, 2.8 GHz	3.1 GHz*	3.2, 3.3 GHz**
Std. Processor Type:	Core i5	Core i5*	Core i5**
Std. Processor Cores:	2, 4	4	4
Turbo Boost:	2.7, 3.3 GHz	3.6 GHz	3.6, 3.9 GHz
Intel Generation:	Broadwell (5th)	Broadwell (5th)	Skylake (6th)
Processor Mounting:	Soldered	Soldered	LGA 1151
Std. Storage:	Hard Drive	Hard Drive	Hard Drive Fusion Drive
NVMe Std:	No	No	Yes
Std. RAM:	8 GB	8 GB	8 GB
Max. RAM:	16 GB†	16 GB†	64 GB
RAM Upgrade:	Soldered	Soldered	Easy (Door)
Video System:	HD Graphics 6000 Iris Pro 6200	Iris Pro 6200	Radeon R9‡
Video Type:	Integrated	Integrated	GDDR5
Video Memory:	1.5, 1.7 GB (Shared)††	1.7 GB (Shared)††	2 GB‡
Display Size:	21.5-Inch	21.5-Inch	27-Inch
Native Resolution:	1920x1080	4096x2304	5120x2880
Ext. Display:	3840x2160	4096x2304	5120x2880 or 4096x2160 x2
Apple Order No (US):	MK142LL/A MK442LL/A	MK452LL/A	MK462LL/A MK472LL/A MK482LL/A
Apple Model ID:	iMac16,1 iMac16,2	iMac16,2	iMac17,1
EMC Number:	2889	2833	2834
Original Price (US):	US$1099 US$1299	US$1499	US$1799 US$1999 US$2299
Original Price (CA):	C$1399 C$1649	C$1899	C$2299 C$2599 C$2999
Original Price (UK):	£899 £1049	£1199	£1449 £1599 £1849
Original Price (AU):	A$1699 A$1999	A$2299	A$2799 A$3099 A$3599

* By custom configuration, the iMac "Core i5" (4K) 3.1 21.5-Inch could be configured with a 3.3 GHz Quad Core "Core i7" processor (I7-5775R) for an extra US$200. EveryMac.com also has documented this custom configuration as its own model.

** The higher-end configuration of the iMac "Core i5" 3.2 (5K) 27-Inch (MK472LL/A) as well as the iMac "Core i5" 3.3 (5K) 27-Inch additionally could be configured with a 4.0 GHz Quad Core "Core i7" processor (I7-6700K) for an extra US$300 or US$250, respectively. EveryMac.com also has documented this custom configuration as its own model.

† In these models, 8 GB of RAM is onboard. They could be upgraded to 16 GB at the time of purchase as an optional upgrade for US$200. The RAM cannot be upgraded later at all.

†† The Intel HD Graphics 6000 processor reserves up to 1.5 GB of system memory for video function whereas the Intel Iris Pro 6200 graphics processor reserves up to 1.7 GB.

‡ The iMac "Core i5" 3.2 (5K) 27-Inch has either an AMD Radeon R9 M380 (MK462LL/A) or AMD Radeon R9 M390 (MK472LL/A) graphics processor with 2 GB of dedicated GDDR5 memory. The high-end iMac "Core i5" 3.3 (5K) 27-Inch (MK482LL/A) has an AMD Radeon R9 M395 graphics processor also with 2 GB of dedicated GDDR5 memory. At the time of purchase, this high-end model also could be upgraded to an AMD Radeon R9 M395X graphics processor with 4 GB of dedicated GDDR5 memory for an additional US$250.

So, which one should I buy?

Even though they only now are available on the used market, which "Late 2015" iMac is best for you still largely depends on your budget and your intended usage.

The 21.5-Inch models are cheaper upfront, but unfortunately, essentially have been designed to be disposable, which means that they may not last as long as the 27-Inch models if you're the type of user who likes to upgrade a system later to eke out a bit more performance and keep it going for another year or two, essentially lowering your overall cost of ownership on an annual basis.

On the other hand, if you like to buy a new computer every couple of years, or don't perform upgrades, a 21.5-Inch model could be sufficient.

If price is your primary concern, the entry-level iMac "Core i5" 1.6 21.5-Inch was cheapest when new and still will be cheapest on the used market, no doubt, but it's not a particularly good value compared to the other models in the series, and many similar Aluminum iMac models from previous years are faster and cheaper still.

The next step up the ladder -- the iMac "Core i5" 2.8 21.5-Inch -- is worth considering if you don't see the value of a higher-resolution "4K" display, and you want a better buy than the entry-level option. When new, it provided roughly 50% more multicore performance than the entry-level option for about 18% more money.

The middle range option -- the iMac "Core i5" (4K) 3.1 21.5-Inch -- was a bit muddled, as is often the case for options in the middle of a price chart. It has a beautiful, high-resolution display, but is hobbled somewhat by relatively slow integrated graphics, and is effectively sealed shut and disposable. It's a power user's system without the power and really only would make sense for someone who wants a beautiful display, but who either needs a physically smaller computer and/or doesn't have the extra money to spend on a 27" model that provides a much better value.

For most users, just comparing these models on the used market, EveryMac.com would suggest either configuration of the iMac "Core i5" 3.2 (5K) 27-Inch. It has a big, beautiful display, solid performance and graphics, it is easy to upgrade the RAM later, and it even is possible to upgrade the storage and processor if you're a tinkerer or don't mind paying a professional to extend its life later. The high-end stock option -- the iMac "Core i5" 3.3 (5K) 27-Inch -- also is a solid value. When it was new, it provided roughly 5% more overall performance, better graphics, and twice the storage for roughly 15% more money than the MK472LL/A configuration of the iMac "Core i5" 3.2 (5K) 27-Inch.

Please also see EveryMac.com's Ultimate Mac Comparison feature to dynamically compare any iMac model to any other Mac.

Used iMac Purchase Options

There are many places to purchase a used "Late 2015" iMac. However, buying from a quality merchant with an extensive track record in the Mac market will provide the best experience and save you money and time, too.

In the US, site sponsor Adorama sells new iMac models with free shipping. Other World Computing and JemJem sell used and refurb iMac models at bargain prices with free shipping, as well. Finally, if you need to sell an iMac, A+ BBB-rated Cash for Your Mac will buy your older iMac with an instant quote and prompt payment.

In the UK, site sponsor Hoxton Macs sells used iMac models with a one-year warranty and free next day delivery throughout the UK. Delivery across Europe also is available starting at just £9.99 for two-day delivery to France and Germany.

In Australia, site sponsor Mac City likewise has a variety of used iMac models sold at low prices and available with a free warranty and fast shipping across Australia.