Laptop as second monitor: 4 ways that work, and why HDMI won’t

The right method depends on what operating system each computer runs and how much delay you can live with. Here is the quick version:

• Both computers run Windows 10 or 11: use the built-in Wireless Display feature, which works over Wi-Fi through a standard called Miracast.
• Both are Macs: turn on AirPlay Receiver on the spare MacBook. The spare one needs to be a 2018 model or newer.
• One Windows PC and one Mac, or anything with Linux: install Deskreen, a free and open-source app that shows your screen in a web browser.
• You want the lowest lag: buy an HDMI-to-USB video capture card and go wired.
• Your spare screen is an iPad, not a laptop: Apple’s Sidecar feature handles it.

Two terms come up a lot. Mirroring shows the same picture on both screens, which is handy for presenting. Extending turns the two screens into one wide desktop, so you can drag windows from one to the other. Most people setting up a work desk want extend.

On Windows, the spare laptop needs a free add-on from Microsoft before it can receive a picture.

On the spare laptop (the one that will act as the screen):

1. Open Settings, choose System, then go to the page called Projecting to this PC.
2. Pick Optional features, then View features, and search for the add-on named Wireless Display.
3. Tick its box, press Next and then Install.
4. When it finishes, head back to the projection page and choose Available everywhere so other PCs can find the laptop.
5. Launch the Wireless Display app. The quickest way is to type its name into the taskbar search box.

Microsoft says that app exists on Windows 11 version 22H2 and later. On older versions, search for the Connect app instead and open that.

Now go to your main computer:

1. Press the Windows key + K. A list of nearby displays appears.
2. Pick the spare laptop from that list.
3. Type in the PIN it shows, and allow keyboard and mouse input if you want to control things from either machine.

If you’d rather click than use shortcuts, type Cast into Windows search and choose the option to connect to a wireless display. You’ll find the same option in Display settings, under the section for multiple displays.

Switching between mirror and extend

Once connected, press the Windows key + P on the main PC to pick a display mode. Choose duplicate to mirror, or extend for one wide desktop. You can switch at any time without reconnecting.

If the spare laptop doesn’t show up

Microsoft’s advice is simple: check that Wi-Fi is turned on for the spare laptop, and that the Wireless Display app is installed and actually open.

The catch with this method is lag. Everything travels over Wi-Fi on both machines, so expect a slight delay. It’s fine for spreadsheets, email and a wall of browser tabs, but it isn’t the best choice for fast games.

Two Macs can link up over Wi-Fi with AirPlay, Apple’s wireless streaming system, as long as the MacBook acting as the screen is a 2018 model or later.

On the spare MacBook:

1. Open System Settings and go to General.
2. Open AirDrop and Continuity, and switch on AirPlay Receiver.
3. Under the setting that controls who can use AirPlay, choose Current User.

Sign both Macs into the same Apple ID, which keeps the connection smooth. Then, on your main Mac, open System Settings, go to Displays, click the plus button and pick the spare MacBook from the list. Choose whether you want it to mirror your screen or extend it.

Like the Windows method, AirPlay runs over Wi-Fi, so a small delay is normal. If that bothers you, the wired capture card method further down works on Macs too.

Got an iPad instead? Use Sidecar

If the spare screen you own is an iPad, Apple’s Sidecar feature turns it into a second display for your Mac. Apple’s requirements are short:

• Both devices signed in to the same Apple Account, with two-factor authentication turned on.
• Wi-Fi, Bluetooth and Handoff on for both.
• The two devices within 10 meters (30 feet) of each other.
• The iPad not sharing its cellular connection, and the Mac not sharing its internet connection.

To start it on macOS Ventura 13 or later, open System Settings, click Displays, press the plus button and choose your iPad. Use the menu labeled Use as to pick mirroring or extending, and click Arrange to drag the screens into the right positions. Apple suggests plugging the iPad into the Mac with a USB cable if you want it to stay charged; a wired link also works if your iPad trusts the Mac.

When the two computers run different systems, built-in tools won’t talk to each other, so you need an app. Deskreen is free and open source, runs on Windows, macOS and Linux, and only asks the spare laptop to have a web browser.

1. Connect both computers to the same Wi-Fi network.
2. Install Deskreen on your main computer, the one whose screen you want to share.
3. Deskreen shows a QR code and a web address. Open that address in the spare laptop’s browser.
4. Once the spare laptop connects, your screen shows up in its browser window.

Here’s the catch. Deskreen mirrors your screen by default. To make it act as a true extra screen, you need a virtual display adapter, often called a dummy plug, in the main computer’s HDMI port. It is a cheap add-on.

A few other Deskreen details are useful. You can share a single app window instead of the whole screen, which is handy for presentations. The developer says the app uses end-to-end encryption, and it can run on a local Wi-Fi network with no internet connection at all. There’s also a mode that flips the picture, meant for people using a tablet or phone as a teleprompter.

As with every Wi-Fi method, speed depends on your network, so expect some lag.

Most laptop HDMI ports are output only. They’re built to send a picture to a TV or monitor, not receive one, so cabling two laptops together does nothing.

The workaround is an HDMI-to-USB video capture card, a small adapter that converts an HDMI signal into something a USB port understands.

1. Plug the HDMI cable from your main computer into the capture card.
2. Plug the capture card’s USB end into the spare laptop.
3. On the spare laptop, open the camera app or a free program such as OBS Studio, which runs on Windows, Mac and Linux.

The spare laptop treats the capture card like a webcam, so the main computer’s screen shows up as a video feed. There is still a small delay, but usually less than with Wi-Fi, which makes this the best pick for gaming. The same trick lets you play a PlayStation or Nintendo Switch on a laptop screen when there’s no TV free. Only competitive players are likely to notice the lag.