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What is max external monitor resolution supported by iOS?

wfcrr used Ask the Experts™
If I buy an iPhone X, what is the maximum resolution for an external monitor?  Will it run a 4K monitor at 3840x2160?
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Tom CieslikIT Engineer
Distinguished Expert 2017
It will not.

Maximum resolution for iPhone X is

5.8in Super Retina Display (2436x1125, 458ppi) OLED

So maximu you can transfer to VT is 2436x1125
IT Manager
Top Expert 2010
Only 1080P using the adapter below.

"Use the Lightning Digital AV Adapter with your iPhone, iPad, or iPod with Lightning connector. The Lightning Digital AV Adapter supports mirroring of what is displayed on your device screen — including apps, presentations, websites, slideshows, and more — to your HDMI-equipped TV, display, projector, or other compatible display in up to 1080p HD.
It also outputs video content — movies, TV shows, captured video — to your big screen in up to 1080p HD. Simply attach the Lightning Digital AV Adapter to the Lightning connector on your device and then to your TV or projector via an HDMI cable (sold separately)."

Lightning Digital AV Adapter

Even if you mirror the screen of your iPhone X using Apple TV 4 which supports 4K resolution, the resolution of the mirroring is only 1080P.
Jackie Man IT Manager
Top Expert 2010

The first comment is not correct.

If external resolution is equal to internal resolution, it means iPhone X is using screen mirroring via HDMI which has a maximum resolution of 1080P only.

You will never get  2436x1125 resolution using screen mirroring via HDMI unless Apple has launched a new lightning to HDMI adapter.