How to Use a Laptop as a Second Monitor
Turning a spare laptop into an extended display is a genuinely useful trick — but it works very differently depending on your operating system, hardware, and the software you use. Here's what's actually happening under the hood, and what determines whether this setup works smoothly for you.
What "Using a Laptop as a Second Monitor" Actually Means
When you use a laptop as a second monitor, you're not just mirroring a screen — you're treating the laptop's display as an extended desktop, letting you drag windows, apps, or tools onto it as if it were an external monitor plugged in via HDMI.
The catch: laptops are designed to output video, not receive it. Most HDMI ports on laptops are output-only, so you can't simply plug a cable between two laptops and expect it to work. Instead, this setup relies on software-based solutions or specific hardware features that redirect display data over a network or direct connection.
The Two Main Approaches
1. Software Over a Network (Most Common)
Apps like Spacedesk, Luna Display (for Mac), or Apple's Sidecar (for iPads, not laptops) transmit display data over your local Wi-Fi or a wired LAN connection. The host machine (your primary computer) treats the second laptop as a virtual display driver, and the client app on the second laptop renders what it receives.
How it works in practice:
- Install the server app on your main PC or Mac
- Install the client app on the secondary laptop
- Both devices connect over the same network
- The second laptop's screen appears as an additional display in your display settings
Performance depends heavily on network quality. A wired Ethernet connection between both machines will give you noticeably lower latency than Wi-Fi, especially for tasks that involve motion or video.
2. Windows 11 Wireless Display (Built-In)
Windows 11 includes a native feature called Wireless Display (formerly "Project to this PC" in Windows 10). The secondary laptop acts as a Miracast receiver, and the primary machine streams its display wirelessly.
To enable it:
- On the secondary laptop, go to Settings → System → Projecting to this PC
- Set it to allow connections from nearby devices
- On the primary machine, press Windows + K to open the Cast panel and select the secondary laptop
This is built directly into the OS and requires no third-party software, but it does require both machines to support Miracast — a Wi-Fi Direct-based standard. Not all older hardware or network adapters support it reliably.
3. Mac-to-Mac with Sidecar or Universal Control
Apple's ecosystem has its own solutions. Sidecar is designed for iPads, not other Macs. However, Universal Control (macOS Monterey and later) lets you use one keyboard and mouse across multiple Macs or iPads — but it's a KVM-style feature, not a true extended display in the traditional sense.
For true Mac-to-Mac second monitor functionality, third-party software remains the most reliable route.
Key Variables That Affect Your Setup 💻
Not every setup performs equally. These are the factors that actually matter:
| Variable | Why It Matters |
|---|---|
| Network type | Wired LAN is faster and more stable than Wi-Fi for streaming display data |
| OS version | Windows 11 has native wireless display; Windows 10 support is more limited |
| Hardware age | Older Wi-Fi adapters may not support Miracast; older CPUs may struggle with encoding |
| Resolution of secondary display | Higher-res screens require more bandwidth to stream smoothly |
| Use case | Reading documents is forgiving; video editing or gaming is not |
| Software used | Each app has different compression, latency, and feature trade-offs |
What Works Well vs. What Doesn't
Works well:
- Reference windows, documentation, dashboards, chat apps
- Spreadsheets, email, calendars, or any static or low-motion content
- Situations where you need more screen real estate without buying hardware
Works less well:
- High-refresh-rate tasks — the latency of a software-streamed display is almost always higher than a physical monitor
- Video playback or GPU-intensive work on the secondary screen
- Setups where both laptops are on different networks or subnets (corporate VPNs, guest Wi-Fi, etc.)
The Hardware Exception: Capture Cards
There is a hardware path that bypasses the software limitations entirely. Some users connect a USB capture card to the secondary laptop — the primary machine outputs via HDMI to the capture card, which the secondary laptop reads as a video input device. This gives you a near-zero-configuration solution with more consistent latency.
The trade-off is cost and portability. Capture cards add expense and an extra cable, and the secondary laptop needs a USB port capable of handling the data throughput without bottlenecking.
What Determines Whether This Works for Your Situation 🔧
The honest answer is that "use a laptop as a second monitor" covers a wide range of real-world outcomes. Someone on a fast, wired home network running Windows 11 on both machines will have a noticeably different experience than someone trying to do this over shared office Wi-Fi with a five-year-old laptop running Windows 10 Home.
Your use case matters too. If you're extending your workspace to read documentation while coding, the latency tolerance is completely different than if you're trying to stream video or run a live dashboard. The same software that works perfectly for one person's workflow can feel sluggish and frustrating for another's.
Whether the built-in Windows Wireless Display feature covers your needs, or whether you're better served by a third-party app or a capture card, comes down to the specific hardware you have, how your network is set up, and what you're actually planning to put on that second screen.