The two ways to resize your VirtualBox window

You can change how large your virtual machine appears on your screen in two ways: resize the VirtualBox window itself, or adjust the display settings inside the guest operating system. The window resize works immediately but may leave black bars around your screen. Changing the guest display settings fills your screen properly but requires a few extra steps inside the virtual machine.

Most people start by resizing the window because it is the fastest option. If that leaves your screen looking wrong, you will then adjust the guest settings to match. Both methods work together — you are not choosing one or the other, but using them in sequence.

Key Takeaways

  • Dragging the VirtualBox window edge resizes the display immediately, but the guest operating system may not fill the space properly until you adjust its display settings.
  • The Guest Additions software must be installed on your virtual machine before the display will resize automatically to match your window.
  • If your virtual machine screen stays small with black bars around it, you need to change the display resolution inside the guest operating system.
  • Right-clicking the desktop inside the virtual machine and selecting display or screen settings is usually the fastest way to adjust resolution.
  • Restarting the virtual machine after installing Guest Additions often fixes display scaling problems without any manual adjustment.

Resizing the VirtualBox window directly

The simplest way to change your virtual machine screen size is to drag the edge of the VirtualBox window larger or smaller, just like resizing any other window on your computer. Click and hold the corner or edge of the VirtualBox window and drag it to the size you want. The virtual machine display will resize as you drag.

This method works immediately, but your guest operating system may not automatically adjust to fill the new space. You might see black bars on the sides or bottom of the screen. This is normal and does not mean anything is broken — it just means the guest operating system is still using its old display resolution. The next section explains how to fix that.

Installing Guest Additions for automatic resizing

Guest Additions is software that runs inside your virtual machine and allows it to communicate with VirtualBox about screen size, mouse movement, and clipboard sharing. Without Guest Additions installed, your virtual machine cannot automatically resize to match your window — you have to manually change the resolution every time.

To install Guest Additions, start your virtual machine and then click the Devices menu at the top of the VirtualBox window. Select Insert Guest Additions CD Image. A disc icon will appear inside your virtual machine, usually on the desktop or in a file manager window.

Double-click that disc icon to open it. You will see a folder with several files. Look for a file named VBoxWindowsAdditions.exe (on Windows), VBoxLinuxAdditions.run (on Linux), or VBoxDarwinAdditions.pkg (on macOS). Double-click the correct file for your guest operating system. A window will open asking for permission to install software — click yes or enter your password if prompted.

After installation finishes, restart your virtual machine. When it boots back up, resizing the VirtualBox window will automatically resize the guest display to match. If it does not, move to the next section.

Manually changing the display resolution inside the guest

If your virtual machine screen still has black bars after resizing the window, you need to change the display resolution inside the guest operating system itself. The steps differ depending on whether you are running Windows, Linux, or macOS inside the virtual machine.

On Windows: Right-click an empty area of the desktop inside the virtual machine. Select Display settings or Screen resolution from the menu. A settings window will open. Look for a dropdown or slider labeled Resolution or Display resolution. Click it and select a larger resolution from the list — usually the largest option available is the one you want. Click Apply or OK. Windows may ask if you want to keep the new resolution; click Yes to confirm.

On Linux: Right-click the desktop and look for Display Settings, Screen Resolution, or Settings. If that does not work, open the Activities menu (top left) and search for "display" or "settings". A settings window will open showing available resolutions. Select the largest resolution available and close the window. The change usually takes effect immediately.

On macOS: Click the Apple menu and select System Preferences or System Settings. Click Displays. Select the Resolution tab if one appears. Choose a larger resolution from the list. The change takes effect immediately.

What to do if Guest Additions will not install

Guest Additions requires that your virtual machine has enough disk space and that the guest operating system is fully updated. If the installation fails, try restarting your virtual machine and attempting the installation again. Sometimes the disc image does not mount properly on the first try.

If the installation still fails, check that you have at least 500 MB of free space inside the virtual machine. Open a file manager or terminal and look at your available disk space. If you are low on space, delete files you do not need or expand the virtual machine's hard drive — that is a separate process covered in VirtualBox documentation.

If you are running an older version of VirtualBox, Guest Additions for that version may not work with newer guest operating systems. Check the VirtualBox download page to see if a newer version is available. You can also manually change the display resolution without Guest Additions — it just requires doing it by hand each time you resize the window.

Scaling and full-screen mode

VirtualBox also offers a Scaled Mode that stretches the guest display to fill your window without changing the actual resolution inside the guest. This is useful if you want a larger view without adjusting settings. Click View at the top of the VirtualBox window and select Scaled Mode. The display will stretch to fill your window, though it may look slightly blurry.

For a cleaner look, use Full-screen Mode instead. Click View and select Full-screen Mode. Your virtual machine will take up your entire monitor. Press the right Ctrl key (or Cmd on macOS) plus F to exit full-screen mode and return to windowed view. The right Ctrl key is the Host Key — it tells VirtualBox you are giving it a command rather than typing inside the virtual machine.

Frequently Asked Questions

Why does my virtual machine screen have black bars even after I resized the window?

The guest operating system is still using its old display resolution. You need to change the resolution inside the guest to match the new window size. Right-click the desktop inside the virtual machine, select display or screen settings, and choose a larger resolution from the list.

Do I have to install Guest Additions?

No, but without it you will have to manually change the display resolution every time you resize the window. Guest Additions makes resizing automatic and also enables better mouse movement and clipboard sharing between your computer and the virtual machine.

What is the Host Key and why does it matter?

The Host Key (usually right Ctrl or Cmd) tells VirtualBox that your next keystroke is a command for VirtualBox, not for the virtual machine. For example, right Ctrl + F exits full-screen mode. Without the Host Key, every keystroke would go to the virtual machine and you could not control VirtualBox itself.

Can I change the screen size while the virtual machine is running?

Yes. You can resize the VirtualBox window at any time, and if Guest Additions is installed, the guest display will resize automatically. If Guest Additions is not installed, you will need to manually change the resolution inside the guest after resizing.

My virtual machine is very small and hard to read. What is the fastest fix?

Drag the corner of the VirtualBox window to make it larger, then right-click the desktop inside the virtual machine and change the display resolution to the largest option available. If that does not work, install Guest Additions and restart the virtual machine.