Free up Microsoft To Do storage

Microsoft To Do tasks live in your Microsoft account (Exchange Online). This phone stores a local copy, file attachments you opened, and cache. Microsoft does not document an in-app cache button. Clearing cache does not delete synced lists.

See how much space Microsoft To Do is using

iPhone

Open the iPhone Settings app.
Tap General > iPhone Storage.
Tap To Do or Microsoft To Do.

Android

Open your phone Settings > Apps.
Tap To Do or Microsoft To Do.
Tap Storage or Storage and cache.

Attachments

Microsoft Support: open a task, tap + Add a file, and upload from the phone. Each task can hold up to 25 MB. Files added in Outlook Tasks also show in To Do.

Open a task with a large photo or PDF.
Remove the file only if you mean to delete it from the task in your mailbox.

Copies you saved to Files are separate. Clearing cache does not remove those.

Mailbox full is not phone cache

Microsoft Support: a “ran out of storage space” message in To Do means the Outlook.com mailbox or Microsoft cloud quota is full, not that this phone is full. Tasks share mailbox space with mail. Manage that in Outlook.com storage settings.

Local storage errors

Microsoft Support for “Sync failed due to an issue with local storage”: on iPhone, reinstall To Do from the App Store. Wait until lists finish syncing first. Tasks already in the mailbox stay.

Clear Microsoft To Do cache on Android

Open your phone Settings > Apps > To Do.
Tap Storage or Storage and cache.
Tap Clear cache.

Do not tap Clear storage / Clear data unless you mean to sign out. Unsynced edits on this phone can be lost. Synced tasks stay in the Microsoft account.

iPhone does not offer a separate To Do cache button. Offload the app in iPhone Storage if the system total stays high.

Reinstall the app if storage is still high

If you already cleaned media and Microsoft To Do still looks huge, a fresh install can clear leftover cache and temp files that are hard to reach by hand.

Wait until offline tasks and file uploads finish syncing. Lists in your Microsoft account stay. Attachments that never uploaded live only on this phone.
Delete the app from your phone.
Restart your phone.
Install Microsoft To Do again from the App Store or Google Play.
Sign in and restore from backup if the app offers it.

This often frees a lot of space. You may need to download some media again after restore.

What to do next

After Microsoft To Do is lighter, check your photo library for duplicates and similar shots. That is what Dedupo is for.

More app guides: Free up app storage.