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Dual Boot with Windows

Dual boot installation is one of the most common setups — keep your existing Windows system while also installing Ubuntu, and choose which system to boot into via the boot menu at startup.

Pre-Installation Preparation

Dual boot installation requires more preparation. Please carefully complete the following steps.

1. Back Up Important Data

Dual boot installation involves partition operations. While the risk is minimal, it’s still recommended to back up important files in Windows.

Warning

Before proceeding, be sure to check the BitLocker and Windows Fast Startup status. Partitioning without disabling BitLocker may prevent Windows from booting; leaving Fast Startup enabled may prevent Ubuntu from properly reading/writing Windows partitions.

2. Check BitLocker Status

Windows 10/11 may have BitLocker disk encryption enabled by default. BitLocker must be addressed before adjusting partitions, or Windows may fail to boot. See the BitLocker Guide for details.

3. Check Intel RST Mode

Some computers default to Intel RST mode for the disk controller, which prevents Ubuntu from detecting the hard drive. See the Intel RST / AHCI Guide for details.

4. Disable Windows Fast Startup

Windows’ Fast Startup feature locks hard drive partitions, which may prevent Ubuntu from properly accessing Windows partitions.

Control Panel -> Power Options -> Choose what the power buttons do -> Change settings that are currently unavailable -> Uncheck "Turn on fast startup"

5. Shrink the Windows Partition

You need to free up space from the Windows partition for Ubuntu.

Shrinking the Partition in Windows

  1. Right-click the Start menu and select Disk Management
  2. Right-click the Windows partition (usually C:) and select Shrink Volume
  3. Enter the amount of space to shrink (at least 50 GB recommended, 100 GB+ preferred)
  4. Click Shrink

After shrinking, you’ll see a block of Unallocated space — this is reserved for Ubuntu.

Tip: If you can’t shrink enough space, it may be because unmovable files occupy the end of the partition. Try running Windows disk defragmentation first, or disable virtual memory and hibernation before trying again.

Installing Ubuntu

1. Boot from USB

Refer to Boot from USB to boot into the Ubuntu installation environment from your prepared bootable USB drive.

2. Choose Installation Type

At the partitioning step, the installer will typically detect the existing Windows system and offer these options:

  • Install Ubuntu alongside Windows Boot Manager: The installer handles partitioning automatically — the simplest method
  • Manual partitioning: Control the partition scheme yourself; for advanced users

After selecting “Install alongside Windows”:

  • The installer will show a slider letting you adjust how much space Windows and Ubuntu each get
  • Drag the slider to allocate appropriate space (at least 50 GB recommended for Ubuntu)
  • After confirming, the installer will automatically complete all partition operations

Manual Method

If you’ve already freed up unallocated space in Windows, you can choose manual partitioning to create Ubuntu partitions in the unallocated space. See Manual Partitioning for details.

3. Complete the Installation

Follow the remaining steps in the Installation Walkthrough for timezone, user setup, etc., then wait for installation to complete.

GRUB Boot Menu

After installation, when you restart you’ll see the GRUB boot menu:

  • Ubuntu: Boot Ubuntu
  • Windows Boot Manager: Boot Windows

Use the arrow keys to select and press Enter to confirm. If you don’t make a selection, it will automatically boot into Ubuntu after a few seconds.

Changing the Default Boot Entry

If you want Windows to boot by default, modify the GRUB configuration in Ubuntu:

# Edit GRUB configuration
sudo nano /etc/default/grub
 
# Change the GRUB_DEFAULT value
# 0 = first entry (Ubuntu); Windows is typically the last entry in the list
# You can use the menu title as the value, but it must match the actual title exactly
GRUB_DEFAULT="Windows Boot Manager"
 
# After saving, update GRUB
sudo update-grub
Tip

When using a menu title for GRUB_DEFAULT, it must match the title shown in the GRUB menu exactly (the Windows entry title can differ across motherboards/versions). First check the actual title with:

grep menuentry /boot/grub/grub.cfg

If the title is hard to match, you can use a numeric index instead (e.g. GRUB_DEFAULT=2, counting from 0).

Changing the Timeout

# In /etc/default/grub, modify
GRUB_TIMEOUT=5  # Wait 5 seconds
 
# Update GRUB
sudo update-grub

Time Desync Issue

Dual boot users often encounter incorrect time when switching between Windows and Ubuntu. This is because the two systems interpret the hardware clock differently.

Run the following command in Ubuntu to fix it:

# Make Ubuntu treat the hardware clock as local time (consistent with Windows)
sudo timedatectl set-local-rtc 1

FAQ

After installation, goes straight to Windows with no GRUB menu

  • Enter UEFI settings and confirm that ubuntu is listed before Windows Boot Manager in the boot order
  • Some computers require manually adding the Ubuntu boot entry in UEFI

Windows won’t boot after installation

  • Try selecting Windows Boot Manager from the GRUB menu
  • If that option is missing, boot into Ubuntu and run sudo update-grub to re-detect Windows
  • Confirm you didn’t accidentally delete the Windows EFI partition during installation

Want to uninstall Ubuntu and restore Windows-only

  1. In Windows, use Disk Management to delete the Ubuntu partitions
  2. Extend the Windows partition to reclaim the space
  3. Clean up the GRUB boot entry so the machine boots straight into Windows. The exact method depends on the firmware mode:
    • UEFI systems (the vast majority of newer PCs): Do not use bootrec /fixboot (it usually returns “Access is denied” and has no effect under UEFI). Instead, remove the leftover ubuntu EFI boot entry — in Windows you can run bcdedit /enum firmware as administrator to inspect it and use bcdedit to delete the corresponding entry; or delete the ubuntu entry in your motherboard’s UEFI boot-order settings; or use a disk tool to delete the \EFI\ubuntu directory on the EFI partition. Afterwards UEFI falls back to Windows Boot Manager.
    • Legacy BIOS systems (older MBR installs): Only here do you boot from a Windows installation disk into the recovery environment and run bootrec /fixboot and bootrec /fixmbr to rewrite the MBR boot loader.
  • BitLocker Guide — Handle Windows BitLocker disk encryption before dual boot installation
  • Intel RST / AHCI Guide — Resolve Intel RST mode preventing Ubuntu from detecting the hard drive
  • Dual Boot Repair — Fix GRUB loss, missing boot entries, and other dual boot startup issues
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