You bought the iMac because of that screen. It’s gorgeous. Whether it’s the 4.5K Retina on the M3 models or the older 5K panels, it’s basically the gold standard for desktop clarity. But then you start editing video, or you’re juggling fourteen Chrome tabs and a Slack thread that won't quit, and suddenly that beautiful 24-inch or 27-inch canvas feels... tiny. You need more space.
Adding a second monitor for iMac setups isn't just about plugging in a cable and calling it a day. If you treat it like a standard PC build, you’re going to end up with blurry text, weird color shifts, or a cursor that feels like it’s moving through molasses.
I’ve seen people spend $600 on a high-end gaming monitor only to realize it looks "fuzzy" next to their iMac. It’s frustrating.
Why the "Retina Gap" Ruins Most Dual-Monitor Setups
Apple handles resolution differently than Windows. This is the biggest hurdle. macOS is designed around "Retina" scaling, which essentially means it wants to see a high pixel density (PPI). Your iMac screen sits at roughly 218 PPI. If you buy a standard 27-inch 1440p monitor—which is a great resolution for Windows—it only has about 108 PPI.
When you put those two side-by-side, your eyes have to physically readjust every time you glance over. The text on the second screen looks jagged. The icons are the wrong size. It’s a recipe for a headache.
To get a seamless experience, you really have two choices. You either go "one-to-one" with something like the Apple Studio Display or the LG UltraFine 5K, which match that 218 PPI density. Or, you go the 4K route. A 27-inch 4K monitor hits about 163 PPI. It’s not perfect, but it’s the "sweet spot" for most people's budgets. Avoid 1080p entirely. Just don't do it. It’ll look like you’re looking through a screen door.
The Refresh Rate Trap
Here is something honestly annoying: if your iMac has a ProMotion display (120Hz) and you buy a cheap 60Hz second monitor, you might notice "stuttering" on the main screen. MacOS sometimes struggles to sync different refresh rates perfectly across displays. While it’s gotten better with recent Sequoia and Sonoma updates, it's still a thing. If you’re sensitive to motion, try to match the refresh rates, or at least be prepared for a slight visual "hiccup" when dragging windows between screens.
Connectivity: Dongles, Docks, and Disappointments
The modern iMac (M1, M2, M3) uses USB-C/Thunderbolt ports. That’s it. No HDMI. No DisplayPort.
If you buy a monitor that only comes with an HDMI cable, you’re buying a dongle. But not all dongles are equal. I’ve seen cheap $15 adapters from Amazon overheat and cut the signal every twenty minutes. If you’re going HDMI, you need an adapter that specifically supports HDMI 2.0 or 2.1 to get 4K at 60Hz. If you use an old HDMI 1.4 adapter, you’ll be stuck at 30Hz.
30Hz is painful. Your mouse will lag. Your windows will ghost. It’s unusable for real work.
The Magic of Thunderbolt
If your budget allows, get a monitor with Thunderbolt 3 or 4 passthrough. One cable. That’s the dream. The cable goes from the iMac to the monitor, and the monitor acts as a hub for your hard drives, your webcam, and your keyboard. It keeps the desk clean. Brands like BenQ (the PD series) and ASUS ProArt are doing a great job with this lately. They aren't cheap, but they save you from the "dongle hell" that plagues most Mac users.
The M1/M2/M3 Limitation You Need to Know
This is the part that catches everyone off guard. If you have a base-model 24-inch iMac with the standard M1, M2, or M3 chip, Apple officially supports only one external display.
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One. That’s it.
You can’t just plug in two monitors and expect them to work. The second one simply won't turn on. Now, there is a workaround called DisplayLink (not to be confused with DisplayPort). You have to buy a specific DisplayLink-certified dock or adapter and install a driver. It works by using your CPU to "compress" the video signal and send it over USB. It’s okay for spreadsheets and email, but it’s not great for gaming or high-end video editing because there’s a tiny bit of latency.
If you have an iMac with a "Pro" or "Max" chip (though those are mostly reserved for the Mac Studio and MacBook Pro), you get more native displays. But for the vast majority of iMac owners, you’re looking at a dual-screen setup (iMac + one external), not a triple-screen setup.
Color Accuracy: Don't Trust the Box
Most monitors claim "100% sRGB" or "99% Adobe RGB." Don't believe them blindly. iMacs are factory-calibrated to the P3 color gamut. If you do photo work, your second monitor will likely look "yellow" or "pink" compared to the iMac right out of the box.
You’ll spend hours in the macOS "Color" settings trying to eyeball it. Honestly? You’ll never get it perfect by eye. If color matters to you, you need a hardware calibrator like a Datacolor Spyder or an X-Rite i1Display. Or, just accept that the second monitor is for your palettes and email, while the iMac is for the actual creative work.
Ergonomics and the "Chin"
The iMac has that famous "chin" at the bottom. It also isn't height-adjustable unless you paid the extra $400 for the tilt-and-height adjustable stand (which most people don't). This makes lining up a second monitor a nightmare.
Most external monitors come with stands that are either too short or too tall. Your eyes will hate jumping up and down.
- Get a VESA mount.
- Get a dual-monitor arm if you can.
- If you can't mount them, buy a sturdy monitor riser for the second screen.
The goal is to have the top edges of both screens perfectly level. It sounds like a small detail. It isn't. Your neck will thank you after a ten-hour shift.
Real-World Recommendations
I’m not going to give you a list of twenty monitors. I’m going to give you three that actually make sense for specific people.
The "I Want It to Look Like Apple" Choice: LG UltraFine 5K
This is the closest you’ll get without spending $1,600 on a Studio Display. It’s 27 inches, it’s 5K, and it integrates with macOS so you can control the brightness using your Apple keyboard. It’s been around for a few years, but it’s still the gold standard for PPI matching.
The "Prosumer" Choice: BenQ PD2725U
This is a 4K 27-inch monitor that has a "M-Book mode." Basically, BenQ tried to color-match the iMac’s profile. It’s not perfect, but it’s much closer than your average Dell or HP office monitor. It also has Thunderbolt 3, which simplifies your cabling.
The "Budget but Decent" Choice: ASUS ProArt PA278CV
Look, it’s 1440p. I know I said 1440p has lower PPI, but the ProArt series is factory-calibrated and actually looks decent. If you just need more room for Slack, Spotify, and some code, and you don't want to spend $700, this is the one. Just know that text won't be as "crisp" as your iMac.
Actionable Steps for a Better Setup
To get your second monitor for iMac running correctly, stop looking at "top 10" lists and follow this workflow instead:
- Check your chip first. If you have a base M1/M2/M3 and want two external screens, stop. You need a DisplayLink adapter (like the ones from StarTech or Sonnet). If you only want one extra screen, carry on.
- Prioritize the connection. Buy a USB-C to DisplayPort 1.4 cable. Avoid HDMI if possible. DisplayPort is generally more stable with macOS and handles sleep/wake cycles much better.
- Resolution is king. If you can afford 4K, get 4K. If you can afford 5K, get 5K. Anything less will look blurry next to the Retina display.
- Match your heights. Measure from your desk to the top of your iMac screen. Ensure your new monitor’s stand can reach that height, or buy a $20 VESA adapter if the monitor doesn't have one built-in.
- Adjust the "Scaling" in System Settings. When you plug it in, macOS might try to run the monitor at "Default" resolution. Go to System Settings > Displays and hold the 'Option' key while clicking 'Scaled' to see all resolution options. Find the one that makes the UI size match your iMac.
Setting up a second screen shouldn't be a chore, but Apple’s high standards for display quality mean you can't just cut corners. Focus on pixel density and connection types, and you’ll actually enjoy the extra screen real estate rather than squinting at it.