FSRS is the newer spaced repetition algorithm in Anki. Here is what changes in practice, when it matters, and when it does not matter much yet.
FSRS is an algorithm that tries to show you a card shortly before you would forget it. Compared with Anki's older SM-2 system, it tends to schedule hard cards more intelligently and cut some wasted reviews. In practice that usually means fewer cards per session while keeping about the same retention.
| Feature | FSRS | SM-2 (older Anki default) |
|---|---|---|
| Forgetting prediction | Memory state model (stability + difficulty) | Interval multiplier formula |
| Hard cards | Shorter intervals, closer tracking | Can over-schedule, leading to failures |
| Calibration | Learns from your review history | Fixed formula, no personal calibration |
Page design based on study habits around PDFs, papers, and class materials.
SM-2, the older default, mainly stretches or shrinks intervals based on how you rated the card. FSRS tries to model the card more directly: how stable the memory seems and how difficult that card is for you. The result is usually fewer unnecessary reviews on cards you already know and closer tracking on the ones that keep slipping.
If your deck is easy and you almost never miss cards, the difference can feel pretty small. The gains show up more when the material is difficult or uneven, where some cards are obvious and others keep coming back wrong.
FSRS is built into Anki from version 23.10 onward. Open deck options, find the spaced repetition settings, and switch the algorithm to FSRS. If you already have review history, you can also run the optimizer so the schedule reflects your own data instead of the default values.
For most users, yes. It predicts forgetting more accurately and reduces total review time while maintaining the same retention rate.
Go to deck options, find the spaced repetition algorithm setting, and select FSRS. Available from Anki version 23.10 onward.
No. FSRS works from your existing review history and gets more accurate as it collects more data.
Some alternatives, like SocriFlow, use FSRS-based scheduling without requiring the full Anki setup workflow.
Not at the start. The algorithm matters less than building the card-writing and review habits first. Switch once you have a deck you're actively using.
