How you reinstall Office 2021 depends on how your licence was activated the first time. If the key is linked to a Microsoft account, simply sign in at account.microsoft.com and click Install: you will not be asked for the key again. If you have a Professional Plus key that is not linked to an account, you reinstall with the original installer and enter the same product key again; activation goes through online or, if it is refused, via the Activation Wizard's fallback option.
In both cases, formatting does not “use up” your perpetual Office 2021 licence. It can even be moved to a new computer, provided you follow Microsoft's transfer rules. Here is the procedure we apply in support, scenario by scenario.
Before formatting: what should you back up?
Most of the “I can't reactivate Office” tickets we receive come from skipped preparation. Take ten minutes before wiping the drive:
- Note down your product key (25 characters): find it in the order email or on the invoice. Once installed, Office never shows the full key, only the last 5 characters.
- Identify the Microsoft account used during installation: open Word, go to File > Account and look at the address shown under User Information.
-
Export your Outlook data if you use a POP account or PST files: File > Open & Export > Import/Export > Export to a file > Outlook Data File (.pst). PST files are stored by default in
Documents\Outlook Files. -
Copy your templates and customisations: the Word template
Normal.dotm(in%appdata%\Microsoft\Templates) and the custom dictionaryCUSTOM.DIC(in%appdata%\Microsoft\UProof). - Back up your documents to an external drive or OneDrive, not just to another partition on the same disk.
For a full system backup before reinstalling Windows, a disk image (Windows' built-in tool or dedicated software) lets you roll back if something goes wrong.
How do you reinstall Office 2021 with a Microsoft account?
This is the simplest case. If, during the first installation, you entered your key at office.com/setup while signed in to a Microsoft account, the licence is tied to that account for good. On the formatted PC or the new computer:
- Go to account.microsoft.com and sign in with the same account you used at purchase.
- In the Office apps section, find “Office Professional Plus 2021” (or your edition) and click Install.
- If you need the 64-bit version or another language, choose them from the drop-down list before starting the download.
- Run the downloaded
Setup.exefile and accept the User Account Control prompt. - Open Word and sign in with the same account if asked: activation is automatic.
Microsoft describes the same process on its page Download and install or reinstall Office 2021. If the product does not appear in your account, it is almost always because you are signed in with a different address (personal or work account, Outlook.com or Hotmail): sign out and try the other one.
Reinstalling Office 2021 Pro Plus with a key not linked to an account
Many Office 2021 Professional Plus keys are not linked to a Microsoft account: they activate directly in the application. In that case, reinstalling means repeating exactly the same procedure as the first time:
- Download the installer provided with your order (Click-to-Run installer or Office Deployment Tool, depending on the product).
- Install Office, then open Word. The activation window appears: choose the option to enter a product key (or go through File > Account).
- Enter your 25-character key and confirm. Activation completes online in a few seconds.
We will not repeat every screen here: our guide to download, install and activate Office 2021 Pro Plus walks through the installation step by step, with the download links. On Mac, the logic is the same with our Office 2021 Home and Business for Mac installation guide: you sign back in to the associated Microsoft account.
One important point: uninstall any Microsoft 365 version pre-installed on a new PC before installing Office 2021. Two Click-to-Run suites from different channels on the same machine cause conflicts and “Unlicensed Product” messages.
What if activation is refused after reinstalling?
After formatting the same machine, online activation usually goes through without a hitch. On a new PC, or after several reinstallations in quick succession, the server may respond that the key has reached its activation limit. This is not a permanent block:
- Activation Wizard fallback option: the old automated phone activation has been replaced by an online process. Depending on the case, the Wizard's “phone” option now shows a link or QR code to open on a smartphone to obtain the confirmation ID to enter in Office. If that option does not work, contact the seller's support with your order number.
- Check the installed edition: a Professional Plus key will not work on a Home & Business installation, nor on Microsoft 365. The exact name is shown in File > Account.
- Contact our support team with a screenshot of the error: we check the key's status and guide you through reactivation.
Tip: before getting in touch, open a Command Prompt as administrator and run cscript "C:\Program Files\Microsoft Office\Office16\OSPP.VBS" /dstatus. The output shows the installed edition, the licence status and the last 5 characters of the key, which speeds up the diagnosis.
Transferring an Office 2021 licence to a new PC: the rules
A one-off purchase Office 2021 licence is valid for one computer at a time, but it is not tied for life to the first PC. Microsoft states on its page Transfer your Office licence to another device that you can move Office 2021 to another computer you own, no more than once every 90 days (except in the case of hardware failure), and that the software must be removed from the old computer.
The licence type does, however, make a difference:
| Licence type | After formatting the same PC | On a new PC |
|---|---|---|
| Key linked to a Microsoft account | Install from the account, automatic activation | Yes, after uninstalling from the old PC |
| Pro Plus key not linked (retail) | Reinstall and re-enter the key | Yes, online activation (or fallback via the Wizard) |
| 3 PC or 5 PC licence | Reuses one of the included seats | Yes, within the number of seats |
| MAK volume licence | Uses one activation from the MAK counter | Yes, as long as activations remain |
| Pre-installed Office (OEM) | Reinstall on the same PC | No, tied to the original machine |
On the old computer, uninstall Office via Settings > Apps > Installed apps, then click the three dots next to “Microsoft Office Professional Plus 2021” and choose Uninstall. If the old PC is out of action, there is nothing to do on that side: that is precisely the hardware-failure case Microsoft allows for.
Reinstalling on several PCs or in a business
If you manage a small fleet, two options save you juggling individual keys. The Office 2021 Pro Plus 3 PC licence covers three computers in a single order: when a PC is replaced, you uninstall from the old one and reactivate on the new one. The Office 2021 Pro Plus MAK (volume) licence is installed with the Office Deployment Tool and activated with ospp.vbs; each activation counts down one unit on the counter, so you can track reinstallations precisely.
Field tip: keep a simple spreadsheet (PC name, installation date, last 5 characters of the key). The day a drive fails, you will know immediately which key to reactivate.
Should you reinstall Office 2021 or move to Office 2024?
Office 2021 support ended on 13 October 2026. Your Office 2021 keeps working normally, and you can reinstall it as many times as needed within the licence terms: only the security updates have stopped. However, if you are changing PC now and plan to keep the suite for several years, this is the right time to look at Office 2024, supported until October 2029. Our comparison Office 2024 vs Office 2021: which to buy sets out the practical differences.
Frequently asked questions
Do I lose my Office 2021 licence when I format my PC?
No. The licence is perpetual and remains valid after formatting. Just reinstall Office, then sign back in to the associated Microsoft account or re-enter the key.
How many times can Office 2021 be reinstalled?
There is no fixed number of reinstallations on the same PC. When changing computer, Microsoft allows one transfer every 90 days, except in the case of hardware failure. If online activation is refused, the Activation Wizard offers a fallback option (a link or QR code to open on a smartphone); failing that, contact the seller's support with your order number.
My Office 2021 key does not appear in my Microsoft account. Is that normal?
Yes, if your Professional Plus key activates directly in the application without being linked to the account. In that case, keep the order email safe: it is your only record of the full key.
Can I install Office 2021 on a PC and a Mac with the same key?
No. An Office key for Windows does not work on Mac, and vice versa. You need a licence that matches the operating system.
Do I need to uninstall Office from the old PC before activating the new one?
Yes, it is a condition of the licence transfer. If the old PC no longer starts, nothing needs to be done on that side.
Need a licence for your new PC?
If your old licence is an OEM version tied to the machine, or if you are setting up an additional PC, you will find the Office 2021 Professional Plus licence for 1 PC with the key delivered by email, as well as the 3 and 5 PC editions. Our support team answers your reinstallation and activation questions by email and chat, 7 days a week.