How to Change BIOS Mode From Legacy to UEFI in Windows 11

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Windows 11 requires UEFI firmware with Secure Boot capability, so if your PC boots in Legacy BIOS (also called CSM or MBR boot) mode, you’ll need to switch before upgrading or to meet the requirements. The catch is that UEFI needs a GPT disk, while Legacy uses MBR. Windows includes a tool, MBR2GPT, that converts the disk without deleting your files. This guide shows how to check your current mode, convert the disk, change the firmware setting, turn on Secure Boot, and fix problems if the PC won’t boot.

Quick Answer

  1. Back up your files and suspend BitLocker if it’s on.
  2. Open Command Prompt as administrator and run mbr2gpt /validate /allowFullOS, then mbr2gpt /convert /allowFullOS.
  3. Restart into your UEFI/BIOS settings, change Boot Mode from Legacy or CSM to UEFI, save, and exit.
  4. Turn on Secure Boot once Windows starts correctly in UEFI mode.
Administrator Command Prompt running mbr2gpt validate and convert with allowFullOS, ending with a message to switch the firmware to UEFI
MBR2GPT validates the disk, converts it to GPT and reminds you to switch the firmware to UEFI.

Step 1: Check Your Current BIOS Mode and Disk Type

  1. Press Windows + R, type msinfo32, and press Enter.
  2. Look at BIOS Mode: Legacy means you need this guide; UEFI means you’re already set.
  3. To see the disk type, right-click Start > Disk Management, right-click Disk 0 > Properties > Volumes, and check Partition style: MBR or GPT.

Step 2: Prepare

  • Back up important files or create a full system image. The conversion is usually safe, but a power loss or disk error could leave the PC unbootable.
  • Suspend BitLocker or device encryption: search Manage BitLocker and click Suspend protection. Have your recovery key saved.
  • Check requirements: the disk needs no more than three primary partitions and no extended partitions, and the PC’s firmware must support UEFI. Most PCs from 2012 onward do.
  • Know your firmware key: often F2, Del, F10 or Esc at startup, or use Windows Recovery (Step 4).

Step 3: Convert the Disk With MBR2GPT

  1. Right-click Start and choose Terminal (Admin) or open Command Prompt as administrator.
  2. Validate the disk (replace 0 with your system disk number if different): 
    mbr2gpt /validate /disk:0 /allowFullOS
  3. If it says Validation completed successfully, convert: 
    mbr2gpt /convert /disk:0 /allowFullOS
  4. Wait for Conversion completed successfully. It adds an EFI system partition and new boot files.

If validation fails: common reasons are too many partitions, an extended partition, or no space to create the EFI partition. The log files in C:\Windows (setupact.log and setuperr.log) show the exact reason. You can also run MBR2GPT from Windows Recovery’s Command Prompt without the /allowFullOS switch.

Step 4: Switch the Firmware to UEFI

  1. Go to Settings > System > Recovery and click Restart now next to Advanced startup. (On Windows 10: Settings > Update & Security > Recovery.)
  2. Choose Troubleshoot > Advanced options > UEFI Firmware Settings > Restart. If that option isn’t there, the PC is still in Legacy mode; restart and press your firmware key instead.
  3. Find Boot Mode, Boot List Option, or CSM, usually on the Boot tab. Set it to UEFI, or disable CSM.
  4. Save and exit (often F10).

Windows should start normally. Check msinfo32 again; BIOS Mode should now say UEFI.

Step 5: Turn On Secure Boot and TPM

  1. Go back into firmware settings and enable Secure Boot. On some PCs you must set Secure Boot mode to Standard or restore default keys first.
  2. Also enable TPM, often called Intel PTT or AMD fTPM, if it isn’t already on.
  3. Save, restart, and confirm in msinfo32 that Secure Boot State is On.
  4. Resume BitLocker protection.

More detail: how to enable Secure Boot for Windows 11.

If the PC Won’t Boot After Switching

  • “No bootable device”: the disk wasn’t converted, or the boot order needs Windows Boot Manager at the top. Check the boot order in firmware.
  • Switch back temporarily: set Boot Mode back to Legacy/CSM to start Windows, then check the MBR2GPT logs.
  • BitLocker recovery screen: enter your recovery key.
  • Startup Repair: boot from Windows installation media and choose Repair your computer > Troubleshoot > Startup Repair.

Frequently Asked Questions

Will converting to GPT delete my files?

MBR2GPT is designed to keep your data, but always back up first.

Can I switch to UEFI without converting the disk?

No. A Windows installation on an MBR disk won’t boot in UEFI mode.

Do I need to reinstall Windows?

Not with MBR2GPT. A clean install on a GPT disk is the alternative if conversion fails.

Summary

  1. Check BIOS Mode in msinfo32.
  2. Back up and suspend BitLocker.
  3. Run mbr2gpt /validate, then /convert, with /allowFullOS.
  4. Set firmware to UEFI, then enable Secure Boot and TPM.

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