What Is FSRS? The Modern Spaced Repetition Algorithm Explained

Short answer. FSRS is the Free Spaced Repetition Scheduler — an open-source algorithm released in 2022 that decides when to review each flashcard or quiz question for optimal long-term retention. It replaced SuperMemo's SM-2 as the default in modern spaced-repetition tools, including modern Anki versions and [SimpleQuizMaker's review queue](/review).

How FSRS works (in one paragraph)

FSRS models each card with three values: difficulty (how hard it is for you), stability (how long the memory will last before forgetting), and retrievability (the probability you still remember it now). After each review, those values update based on whether you got it right and how confident you were. The algorithm then schedules the next review just before retrievability drops too low.

Why FSRS beats SM-2

SM-2 (the 1980s algorithm Anki used to use by default) treats every card with a single "ease factor." FSRS' three-variable model captures more of what actually happens in memory. Empirical studies show roughly 20-30% better scheduling efficiency — same retention with fewer reviews, or higher retention for the same review effort.

Where you see FSRS in 2026

If a tool markets "spaced repetition" but doesn't specify FSRS or SM-2, ask. The algorithm matters more than the UI.

Should you tune FSRS parameters?

For most users: no. Defaults are good. Power users with 10,000+ reviews can tune their personal parameters using FSRS Helper add-ons in Anki — but only after you have enough data for the tuning to be meaningful.

Related reading

Try a quiz with FSRS-scheduled review built in.