Your collection file is too large to send to AnkiWeb

If syncing has stopped and Anki has shown you this, the good news is that the error is usually not about the thing people assume it is about.

What the message is measuring

AnkiWeb limits a collection to 100 MB compressed and 250 MB uncompressed. The number Anki shows you in brackets is the uncompressed one, measured against that 250 MB.

It is not measuring your media. Your images and audio live in a separate collection.media folder, not inside collection.anki2, and AnkiWeb does not currently cap how much media you store. Only individual files are limited, to 100 MB each.

That catches most people out, because media is usually by far the biggest thing in an Anki folder. It also means the advice you are most likely to be given is the wrong advice:

What is counted: your notes and the text in them, your cards and their scheduling, your deck and note type definitions — and, very often, the answer nobody looks at: your review history.

First, run Check Database

Tools → Check Database. It is named in the error message for a reason. It rebuilds the file and reclaims space that deleted rows left behind, and Anki's manual is straightforward about the effect: “The procedure optimizes the collection in the process, so it’s normal for the new file to be somewhat smaller than the old one.”

Do this before anything else, especially if you have deleted a lot recently — until the file is compacted, that space is still in it. Then sync again and read the new number. Even if you are still over the limit, you now know how much of the file was slack and how much is real.

Then find out what is actually big

Two things account for almost every oversized collection, and they need completely different fixes.

One heavy deck

A single shared deck can be most of your file on its own. One person tracked 140 MB of a 246 MB collection to one downloaded deck.

Finding it takes an add-on. Anki has no built-in view of how much space each note occupies, which is a large part of why this is so hard to track down — nothing in the Browse window tells you, so people end up guessing. The Note Size add-on (code 1188705668) adds sortable Size, Size (texts) and Size (files) columns to Browse. Sort by Size (texts) and the offenders usually stand out at once.

What you are looking for is notes pasted in from a web page or a word processor. They often carry hidden HTML and inline styling many times larger than the text you can actually see.

Your review history

Every review you have ever done is stored in the collection — including reviews of cards you deleted long ago. On a collection with years of history behind it, this can outweigh all of your note text put together. One user found their notes came to 66 MB in a 246 MB file.

It does not appear anywhere in the Browse screen, which is exactly why it is so rarely the thing people check.

Removing what you found

Export before you delete anything. Right-click the deck → Export, and choose the Anki Deck Package (.apkg) format, with scheduling included if you might want the deck back as it was. That file is yours and counts against nothing.

Then delete the deck and run Check Database again. The space does not come back until you do — which is why deleting a large deck sometimes appears to change nothing.

About deleting review history

It works, and you should know what it costs before you do it.

Your review history is what FSRS uses to decide when to show you each card. Removing it — with an add-on or from the debug console — makes the scheduling less accurate for the cards involved, and Anki's developers have cautioned about exactly this. If your history goes back years, that is a real loss of the thing that makes your reviews well timed.

Clearing review entries for cards that no longer exist is the safer half of this. Deleting your own study history to get under a hosting limit is a trade, not a cleanup — make it deliberately.

If it still does not fit

Sometimes there is nothing left to clean up. Hundreds of thousands of real cards with real history do not compress into 250 MB, and no amount of tidying will change that. Three options, in the order we would try them:

1. Split the collection. Keep separate profiles for separate subjects. You cannot review across them in one session, and each profile needs its own AnkiWeb account — Anki's manual is blunt about why: “Only a single profile can be synced to an AnkiWeb account… If you attempt to link two or more profiles to the same AnkiWeb account, you will overwrite the data from one profile with the data from the other.” Export both profiles before you try this.
2. Run Anki's own sync server. The software ships with Anki and costs nothing to download. Running it is a different question: it needs a machine that stays online, an HTTPS certificate, and more setup time than it looks like from the outside. Anki's manual is candid about who it is for — “targeted at users who are comfortable with networking and the command line.”
   
   Then there are backups, which is the part worth thinking hardest about. A sync server is not a backup. It holds the same collection your devices hold, so whatever corrupts one is copied to the other. If you run your own, the backup routine is yours to design, test and remember — and without it, one failed disk takes your notes and every review you have ever logged, permanently.
3. Use a hosted sync server. That is what we do. You point Anki at one address instead of AnkiWeb's and nothing else about your setup changes. We do not cap the collection separately — it counts toward your plan like everything else — and the backup routine in the paragraph above is ours to run rather than yours: nightly, kept for a month, with weekly copies going back two months beyond that.
   
   Being straight about what that is worth: those backups exist so we can bring the service back after a failure on our side. They are not a personal backup service, and we say so in our terms. Keep exporting your collection to your own computer wherever it is hosted — including here.

We would genuinely rather you worked through the sections above first. Most collections that hit this error can be brought back under the limit, and staying where you are is simpler than moving.