Use an external display with your iMac
The Thunderbolt 3 (USB-C) ports 
on your iMac support video output. You can use an external display, a projector, or an HDTV with your iMac.
To connect this device to a Thunderbolt 3 (USB-C) port | Use this cable or adapter |
|---|---|
Thunderbolt 3 (USB-C) display | Thunderbolt 3 (USB-C) cable |
USB-C display | USB-C (USB 3.1) cable |
Thunderbolt 2 display | Thunderbolt 3 (USB-C) to Thunderbolt 2 adapter |
HDMI display or HDTV | USB-C to HDMI adapter or a USB-C Digital AV Multiport adapter |
VGA display or projector | USB-C VGA Multiport adapter |
DisplayPort or Mini DisplayPort display | USB-C to DisplayPort or Mini DisplayPort adapter |
DVI display or projector | USB-C to DVI adapter |
If you have an HDTV connected to an Apple TV, you can use AirPlay to mirror your iMac screen in up to 1080p HD. See Use AirPlay on your Mac for details.
Adapters and other accessories are sold separately. Visit apple.com, your local Apple Store, or other resellers for more information and availability. Review the documentation or check with the manufacturer to make sure you choose the right product.
Learn more. See the Apple Support article Use external monitors with your Mac for details about display preferences and more. To troubleshoot an issue, see Get help with video issues on external displays connected to your Mac.