DecksWhat is FSRS in Anki, and should you turn it on?
FSRS stands for Free Spaced Repetition Scheduler. It's an open-source alternative to SM-2, the scheduler Anki has used since the start, and it's been built into Anki since version 23.10. Instead of fixed rules, it estimates how likely you are to remember each card and schedules the card for the day that chance drops to a level you choose, 90% by default. If you review most days and have a few hundred reviews behind you, turning it on is usually worth it.
What the name means
FSRS was developed by the Open Spaced Repetition community, with Jarrett Ye as a core contributor. "Free" refers to it being open source. It is a scheduler, not a new kind of card: your notes, decks and card types stay exactly as they are.
How it decides when a card comes back
FSRS tracks three numbers for every card:
| Number | What it means |
|---|---|
| Retrievability | The chance you can recall the card right now. It falls the longer you go without seeing it. |
| Stability | How many days it takes for that chance to fall from 100% to 90%. Each successful review raises it. |
| Difficulty | How hard the card is for you. Harder cards gain stability more slowly. |
When you review, FSRS updates those numbers from your rating and works out the day your recall will reach your desired retention. That date is the next due date. The formulas have parameters (the latest version, FSRS-6, uses 21), and Anki can fit them to your own review history with the Optimize button.
What you gain
- Fewer reviews for the same retention. The FSRS project reports that users do 20–30% fewer reviews than with SM-2 for the same retention level. That figure comes from simulations, so treat it as an estimate, not a promise.
- One setting that means something. Desired retention says how much you want to remember. With SM-2 you nudge ease and interval multipliers instead. See Anki desired retention.
- No ease hell. FSRS has no ease factor that sinks every time you press Hard. See what is ease hell.
- Late reviews handled better. It accounts for how long it actually was since you last saw a card.
Should you turn it on?
Turn it on if:
- You review most days and already have a few hundred reviews. FSRS learns from your history, so more history means a better fit.
- Your reviews feel too frequent for cards you know well, or too rare for cards you keep failing.
- You've been fighting low ease on SM-2.
You can leave it off if:
- You're brand new to Anki and happy with how it schedules.
- You want to predict exactly when a card comes back. SM-2's rules are simpler to follow by hand.
Do you need to rate cards differently?
Mostly no. Anki's FSRS FAQ says using only Again and Good is fine, and that in some cases FSRS is even more accurate that way. The one habit to drop: pressing Hard when you actually forgot. FSRS treats Hard as a pass. If you didn't remember, press Again. See Hard vs Again.
Next steps
- How to turn on FSRS in Anki, on desktop, AnkiDroid and AnkiMobile.
- Anki FSRS settings explained, for every option in the FSRS section.
- SM-2 vs FSRS, for a closer comparison of the two schedulers.
In Decks
Decks uses SM-2 with Anki's standard settings, so it has no FSRS switch. How spaced repetition works in Decks explains what it does instead.
Sources
Published 2026-10-24.