Space needed for update 26.5.2.

It is generally recommended that you maintain at least 4-6GB of free space at all times - as your iPad needs some temporary space to complete many tasks. When performing system software updates, you may temporarily require more...

A system software update requires sufficient space to be downloaded, unpacked and verified prior to installation - major version updates (such as updating from iPadOS 18.x to iPadOS 26) requiring considerably more space than an incremental point-update. The required space, to perform an OTA update, is typically double the stated space for the installed update; after the update is successfully completed, temporary files are automatically deleted - returning free space.

If your internal storage is full, or is insufficient for the task that you need to perform, your only option is to better manage the storage that you have - and delete unused Apps or unneeded data. 

These support pages should prove to be helpful:

About storage on your device and in iCloud – Apple Support

What's the difference between device storage and iCloud storage? – Apple Support

Manage your iCloud storage – Apple Support

How to check the storage on your iPhone, iPad and iPod touch – Apple Support

Clear Other storage on your iPhone or iPad - Apple Support

“Offloading” Apps can be a helpful method of creating temporary space for a software update:

https://9to5mac.com/2020/01/21/how-to-offload-apps-in-ios-to-save-space-without-deleting-their-data/

Additional information about creating sufficient space for installation of a software update can be found here:

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

External storage options can often help with managing limited internal storage. Depending upon your iPad model, the iPad will either have a Lightning or USB-C port - and may therefore require either an Adapter or USB hub to connect the USB Storage device. If your iPad has a Lightning port, you’ll need an Apple Lightning to USB3 Camera Adapter:

• Lightning to USB 3 Camera Adapter

https://store.apple.com/xc/product/MK0W2ZM/A

Using a Mac computer or Windows PC can help with system software updates - as the update package is initially downloaded to the computer, unpacked and verified by the computer, prior to the computer installing the updated system software on the target device.

As is, perhaps a workable remedy is to ensure that you have either an iCloud or iTunes backup of your iPad - and then completely erase the iPad. With no user-data present, you will be able to update the iPad; once updated, you can restore your backup (that includes all your data and device settings) to the iPad.

How to back up your iPhone, iPad, and iPod touch - Apple Support

Erase all content and settings:

Settings > General > Transfer or Reset iPad > Erase All Content & Settings

After the reset, set-up the iPad with your AppleID - but do not restore the backup. Perform the system software update using whichever method works for you.

Finally, restore the backup to your iPad:

Restore all content to iPad from a backup - Apple Support

bobseufert wrote:

The Upgrade always needs more space to download than it will eventually need. Why that is so is above my pay grade in the ASC Forum. If you want the Update you need to delete everything possible to make room and reinstall the most important stuff when it’s done.

When updating on the device, the update file first downloads to the device, then it has to be unpacked. The update proceeds from the file that was unpacked and after it is done, the update file is deleted.

When performing an update using a computer attached to the device, the update file is downloaded and unpacked on the computer, so that space is not needed on the device for the install. The update proceeds using the unpacked file on the computer. The update will still likely need more storage than the previous OS, but you will not have to free up as much space on your device in order to install the update using a computer.

Rudegar offers one potential solution.

See this article.

If you need more space for an update on your iPhone, iPad or iPod touch - Apple Support

As iPadOS 26.5.2 updater was over 9G in size compressed, 20GB sounds normal when you consider the decompression and file swap-outs involved in an update.

If you cannot free up space, try connecting to a computer to do the update. You are NOT requred to have a Mac to do so:

How to back up your iPhone or iPad with your Mac - Apple Support

or

How to back up your iPhone, iPad, or iPod touch with Windows - Apple Support

That way, decompressing is doe ont eh compuer, not your iPad.

you could restore it to factory settings first

then upgrade

and then restore your prior back up ?

The Upgrade always needs more space to download than it will eventually need. Why that is so is above my pay grade in the ASC Forum. If you want the Update you need to delete everything possible to make room and reinstall the most important stuff when it’s done.

Keep in mind the operating system along with its storage is replaced each time with the newer version. it does not add on, meaning it is not 20 GB bigger than the previous update.

Also if you haven't been catching up with the updates, to update to the latest version will require even more storage as it includes all the previous updates you missed.

Consider updating with any computer where it will take less storage and is often more reliable.

Thank you Mac Jim ID for the information, it’s very helpful.