What is FSRS?
The Free Spaced Repetition Scheduler (FSRS) is an open-source algorithm that predicts when you are most likely to forget a piece of information — and schedules a review just before that happens. Developed by Jarrett Ye and published in 2022, FSRS is now the default scheduler in many serious flashcard tools, including Neurako.
FSRS is built on the DSR model (Difficulty, Stability, Retrievability), which represents memory using three measurable properties:
- Difficulty (D): How intrinsically hard the card is to remember.
- Stability (S): How long the memory will last before it fades below a target retention rate.
- Retrievability (R): The probability that you can recall the card right now.
Why Does Spaced Repetition Work?
The Spacing Effect — first documented by Hermann Ebbinghaus in 1885 — shows that information reviewed at increasing intervals is retained far more efficiently than information reviewed at fixed or massed intervals ("cramming").
The intuition: each time you successfully recall something, the memory is reconsolidated and becomes slightly more durable. FSRS estimates exactly how much more durable, and uses that estimate to schedule the next review.
How FSRS Calculates Your Next Review
When you rate a card (Again / Hard / Good / Easy), FSRS:
- Updates the card's Stability using a formula that accounts for its current difficulty and how easily you recalled it.
- Sets the next review date so that your expected Retrievability at that date equals your target retention (90% by default in Neurako).
This means harder cards are reviewed more often, and easy cards are reviewed less often — automatically, without any manual intervention.
FSRS vs. SM-2 (the algorithm behind Anki)
| Property | SM-2 | FSRS |
|---|---|---|
| Memory model | Empirical intervals | Explicit DSR model |
| Parameter tuning | Fixed | Personalised per-user |
| Accuracy | Good | Significantly better (peer-reviewed) |
| Open source | Yes | Yes |
A 2022 study comparing spaced repetition algorithms found FSRS reduces the number of reviews needed by up to 25% for the same retention target.
How Neurako Uses FSRS
Every flashcard in Neurako stores its full FSRS state (stability, difficulty, due, reps, lapses). After each review, the algorithm updates the state server-side and schedules the next review. You can see your card stabilities on the Insights page.
The result: you spend less time reviewing cards you already know well, and more time on the ones that need attention.
Want to see your retention rate? Open Learning Insights after a week of consistent study.