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21.5-inch 4K iMac vs. 27-inch 5K iMac: Which Retina desktop should you get?

OS X El Capitan
OS X El Capitan

Apple has not only released an all-new 21.5-inch Retina 4K iMac, but the company has updated the 27-inch Retina 5K iMac as well. One is smaller and more economical, one has more pixels and more power. But which is the right one for you?

Displays

The Retina 4K iMac has a 2.15-inch (diagonal) display boasting 4096 x 2304 pixels at 219 ppi. That's slightly more than standard 3840 x 2160 video 4K, which should allow for full-size editing with some interface room to spare.

The Retina 5k iMac has a 27-inch (diagonal) display boasting 5120 x 2880 pixels at 218 ppi. That's way more than standard video 4K, which allows for full-size editing and much more at the same time.

Both are in-plane switching (IPS) liquid-crystal displays (LCD) with light-emmiting diode (LED) back lighting. Both also include improved red and green phosphors in the LED allowing for full DCI-P3 color space. That's 25% more gamut than standard sRGB displays.

Bottom line: Since both displays are equally fantastic, it really comes down to whether you want less or more pixels. If space is a consideration, 21.5-inches will still look amazing. If money is no object, 27-inches is the biggest, best available.

Processors

The 21-inch Retina 4K iMac comes with a quad-core 3.1 GHz fifth generation Intel Core i5 "Broadwell" processor and can be upgraded to a quad-core 3.3 GHz Intel Core i7 "Broadwell" processor.

The 27-inch Retina 5K iMac starts with a quad-core 3.2 GHz sixth generation Intel Core i5 "Skylake" processor and can be upgraded to a quad-core 4.0 GHz Intel Core i7 "Skylake" processor.

Since Intel hasn't yet shipped the Skylake chips used in the 21.5-inch Retina 4K iMac, Apple went with the just-released Broadwell versions. They're on the same process—14 nanometer—but don't have the new microarchitecture that Skylake brings. Since that architecture once again stresses power efficiency, it's not as critical a deal on desktop as it is on mobile.

Bottom line: Both iMacs are powerhouses but the 27-inch Retina 5K iMac is a super-power house. The 4K will serve almost everyone just fine. The 5K, if you're not price sensitive, will serve even the most demanding power fiends.

Graphics

Intel Iris Pro Graphics 6200 comes standard with the 21.5-inch Retina 4K iMac. It's integrated, it's power-efficient, and... it's fine. It can drive the internal display as well as one external display up to 3840 x 2160 pixels (4K).

AMD Radeon A9 M300 series graphics come standard with the 27.5 inch Retina 5K iMac. M380 with 2 GB of GDDR5 video memory is the baseline, with options for M390, and M395 as options. You can also max it out with M395X and 4 GB of GDDR5 video memory. They're still mobile GPUs, and still not Nvidia, but they're a leap up from integrated.

Bottom line: If graphics aren't a big deal to you, the 4K iMac will be fine. If you want the best performance Apple offers, and are willing to pay for it, the 5K iMac has much more powerful options.

Memory

The 21.5-inch Retina 4K iMac comes with 8 GB of RAM with an option to upgrade at order time to 16 GB. You can't change it later, though, so you need to make that decision right away.

The 27-inch Retina 5K iMac comes with 8 GM of RAM (2x4) with an option to upgrade at order to 16 GB or 32 GB. Unlike the smaller model, though, the dual RAM slots on the big one are user-accessible. So, you can upgrade at any time.

Both iMacs ship with faster 1867MHz DDR3 memory as well.

Bottom line: For the 4K iMac, you need to be happy with a maximum of 16 GB of RAM, and not being able to change it later. If you want more RAM or more flexibility, you'll need to go with the 5K iMac.

Storage

Both iMacs come with 1 TB hard drives standard. The Retina 4K with 5400-rpm and the Retina 5K with 7200-rpm.

You can upgrade the 21.5-inch iMac to 1 TB Fusion Drive (1 TB hard drive + 24 GB of flash) or 2 TB Fusion Drive (2 TB of hard drive + 128 GB of flash) or to 256 GB or 512 GB of pure flash storage.

You can upgrade the 27-inch iMac to 1 TB Fusion Drive (1 TB hard drive + 24 GB of flash), 2 TB Fusion Drive (2 TB of hard drive + 128 GB of flash), 3 TB Fusion Drive (3 TB of hard drive + 128 GB of flash) or to 256 GB, 512 GB, or 1 TB of pure flash storage.

Bottom line: If you don't need much internal storage or external storage is your preference, the Retina 4K iMac has plenty of options for you. If you want maximum internal storage and don't mind paying for it, the Retina 5K iMac offers significantly more, especially for flash.

Media

All the new and updated iMacs include a 720p FaceTime HD camera, stereo speakers, dual microphones, 3.5-mm headphone jack (with digital audio output).

Connectivity

An SDXC card slot, four USB 3 ports, two Thunderbolt 2 ports, Gigabit Ethernet, and Kensington locks are built into both iMacs. You can also get Thunderbolt 2 adapters for HDMI, DVI, dual-link DVI, and VGA.

Wireless includes 802.11ac Wi-Fi at up to 1.3 Gbps and Bluetooth 4.0.

Both also have variants available that fit standard VESA mounts.

Accessories

The new Magic Keyboard and Magic Mouse 2 ship standard with both the 21.5-inch Retina 4K iMac and the 27-inch Retina 5K iMac. You can optionally upgrade to the Magic Trackpad 2 for an extra $50.

Pricing

The 21.5-inch Retina 4K iMac starts at $1400, with extra options available via build-to-order.

The 27-inch Retina 5K iMac comes in 3 models, starting at $1799, $1999, and $2299, with extra options also available via build-to-order.

Who should get the 21.5-inch Retina 4K iMac?

If you have minimal desk space or simply prefer smaller screens, if you need 4K but you're on a budget, if you're fine with Broadwell and Irish Pro, up to 16 GB of RAM, and a maximum of 2 TB of Fusion Drive or 512 GB of flash drive, then the 21.5-inch Retina 4K iMac is the space-efficient, economical choice for you!

Buy now from Apple (opens in new tab)

Who should get the 27-inch Retina 5K iMac?

If bigger is better, if 5K is better, if Skylake is better, if up to 32 GB of RAM, 3 TB of Fusion Drive or 1 TB of flash drive are better, and if space and money are secondary concerns, the 27-inch Retina 5K iMac is for you!

Buy now from Apple (opens in new tab)

Still have questions?

If you're having an especially hard time deciding between the 21.5-inch Retina 4K iMac and the 27-inch Retina 5K iMac, check out our iMac forums for extra help and discussion!

Rene Ritchie
Rene Ritchie

Rene Ritchie is one of the most respected Apple analysts in the business, reaching a combined audience of over 40 million readers a month. His YouTube channel, Vector, has over 90 thousand subscribers and 14 million views and his podcasts, including Debug, have been downloaded over 20 million times. He also regularly co-hosts MacBreak Weekly for the TWiT network and co-hosted CES Live! and Talk Mobile. Based in Montreal, Rene is a former director of product marketing, web developer, and graphic designer. He's authored several books and appeared on numerous television and radio segments to discuss Apple and the technology industry. When not working, he likes to cook, grapple, and spend time with his friends and family.

27 Comments
  • I'm interested to see the performance impact of the 24GB fusion drive. Whilst I'm used to Apple soldering RAM and making things impossible to upgrade, reducing the total SSD space on a fusion drive is a big change. As someone with a Mac Mini, the iMac lineup is something I look at for my next desktop. The original fusion was ok, the new one is unbalanced either at 1TB or 2TB.
  • 2 TB with 128 GB of flash seems okay, what's unbalanced to you?
  • Just that the 1TB fusion drive used to have 128GB, I'd prefer the 2TB to offer 256GB of SSD space. If I double the drive size I'd like both bits to double. I think the price differences justify this. From 1TB to 1TB fusion is $100, up to 256SSD is a further $100 and then finally a 2TB fusion an additional $100.
  • Good article as usual, Rene. I can understand why you would choose to compare the two Retina models but in reality, I suspect the decision for knowledgeable buyers will be the 21.5-inch non-Retina vs. the 27-inch Retina. In other words, the cost-conscious light user vs. the productivity-oriented user. If the standard display weren't world class to begin with, I wouldn't say this but let's face it – for the average user, Retina is overkill with an unjustifiable price tag. You don't need ultra-high resolution to check email, post on Facebook, surf the Web, write the occasional letter, or even manage photos. The $1,299 stock 21.5-inch model with 8GB of RAM and spinning hard drive is an incredible machine and a bargain to boot. I would say if you're considering the smaller Retina iMac, go non-Retina instead.. Spend the $200 savings on AppleCare (from B&H) and a TimeMachine backup drive. If you really want Retina, see the next paragraph. As someone who has made his living on the Mac since System 7, I'm the productivity-oriented type. My efficiency (and pleasure, I might add) would go way down if I were to use Photoshop, Illustrator, Excel, or even Quicken on a 21.5-inch display regardless of resolution. And yet as fussy as I am about each pixel, Retina is still borderline overkill for me. I know this because my last machine was a 2012 27-inch iMac, non-Retina of course. Will my new 27-inch Retina (Fusion and extra RAM) iMac serve me well for years to come? Is it a dream machine? YES. Do my emailing, Facebook-posting friends need a Retina display of any size? NO.
  • I'd say that no matter what I'm doing from checking email to editing video to simply opening an closing an app, the thing I will never go back to is a spinning hard drive. It has to be pure SSD or fusion. It makes such a huge difference in the user experience--even more than a retina screen or maxed out Ram. Sent from the iMore App
  • You can't argue with fusion being the first choice for an upgrade. Wicked fast and pretty good bang for the buck.
  • But fusion has a spinning hard drive.
  • "You don't need ultra-high resolution to check email, post on Facebook, surf the Web, write the occasional letter, or even manage photos" Try telling that to every iPhone user and every iPad 3 and post-iPad 3 user.
  • But Iphone users don't have high resolutions- 750p on the 6 and 6s and 1080p on the plus versions. 2k is the standard for most flagships (2560x1440), and the xperia z5 premium has a 4k screen, tell that to users of the z5, not Iphone users. Posted via the iMore App for Android
  • Oh. I thought we were talking about computers.
  • I'm talking about computer not phones or iPads furthermore you want to talk resolution go android Posted via the iMore App for Android from my iPhone 6s with a cracked screen by 3D Touching too much
  • I'm thinking the lower-priced P3 4K with the integrated graphics. The low price and integrated graphics are the clinchers for me.
  • Seriously though, 5400rpm hard drive in a $1,500 machine in 2015? Just when you thought Apple couldn't go any lower than $650 smartphones with 16GB storage. Disgusting.
  • the 16GB iPhone doesn't bother me that much but the I definitely agree regarding the inclusion of the 5400rpm HD as standard. All Retina iMacs should include Fusion Drives as standard.
  • They also put as much ram as my computer which I bought in 2013 for just over $500 in a computer that costs $1700, then they make you pay extra to get more ram. Posted via the iMore App for Android
  • While its nice to see Apple continue to update the iMac line up when the hell are they going to update their Cinema Thunderbolt Display, that is what 7-8 years old and still costs $999. I guess the answer is never and I should look to LG http://www.lg.com/us/monitors/lg-34UC97-S-ultrawide-monitor who makes their displays anyways. BTW this $1300 monitor can be found on Amazon.com for UNDER a grand.
  • I'm willing to bet that the minute the Mac Pro, 27" Retina iMac, and rMBP get TB3, you will see an updated Cinema Display. A very high chance, given that Apple's updated the vast majority of their lineup, you don't see that till next year.
  • @Rene, the 5K iMac has 4 slots for RAM, not just 2.
    It comes with 2x4GB, options for 4x4GB or 4x8GB.
    I've read that it (and previous generations) will accept 4x16GB aftermarket RAM. Sent from the iMore App
  • Wouldn't it be cool (and super expensive) if one could option for 1TB SSD + 3TB HDD = 4TB Fusion?
    I suppose to could be done- order the 1TB SSD, open the case (voiding your warranty) and installing a 3(or more)TB HDD, then change them to fusion (I've read it's possible). If only money were no concern. Sent from the iMore App
  • Or, you could use an external hard drive via thunderbolt and still make the combo a fusion drive.
  • "Who should get the 21.5-inch Retina 4K iMac? ... Broadwell and Irish Pro..." *Iris Pro Sent from the iMore App
  • Irish would be kind of apt in the case of the bas model. With that 5400rpm HDD, you'd surely be cussing a lot.
  • "Graphics AMD Radeon A9 M300 series graphics come standard with the 27.5 inch Retina 5K iMac." *M380
    *27 inch Actually this whole sentence should be deleted, the next sentence is correct. M380/2GB, M390/2GB, M395/2GB, M395/4GB Sent from the iMore App
  • Has anyone tested the performance of 1 TB fusion drive?
    Is the boot / restart as fast as SSD?
    24GB seems sufficient to store the entire OS and some of the frequently used apps. (8GB OS, 8GB swap file, 8 GB for daily use apps)
  • My first gen 5K iMac (base model, came with 1TB Fusion) is noticeably faster than my sister in-law's 21.5" (1TB 5400rpm 2.5" HDD) But it's not as fast as my PCs with SATA SSD's. I haven't ran any benchmarks, and you really can't accurately benchmark a Fusion drive, since the system dictates if the real data you are using is on the SSD part of the volume, or the HDD. I've noticed that iMovie can be fairly slow when I start out, but it speeds up later on, or if I fire it up the next day. I assume that it has moved the files I'm working on into the SSD.
    Either way, I STRONGLY recommend getting the Fusion drive vs. the HDD-only option. It will serve you well if you are a light-duty user. If it is going to be your main machine, 4+hours/day usage, splurge for the 1TB SSD, if it's in your budget.
    Don't forget that you can always add an external HDD via USB-3, or a RAID box with Thunderbolt-2, if 1TB isn't enough.
    If you're not afraid of voiding your warranty, you could try this- order the 1TB SSD, then open it up to install a 6TB HDD, then rebuilt them as a Fusion volume. I'm assuming the SSD-only model will still have the connection for a HDD.
  • If I'm a Lightroom user (who edits simple videos on occasion) will I see a benefit from the AMD Radeon R9 M395X with 4GB over the M395 with 2GB video memory or even the M390 with 2GB video memory?
  • Sorry to be 'that guy', but by the question 'who should buy the 21.5 inch 4k', you accidentally wrote "Irish pro". Excellent article, as usual though. Posted via the iMore App for Android