What FSRS is, the algorithm that decides when you review
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When a flashcard app tells you “you will see this one in nine days”, an algorithm is behind it. For decades the most used one was SM-2, created in the late 1980s for the SuperMemo program. FSRS, short for Free Spaced Repetition Scheduler, is a modern open-source alternative designed from real study data.
The three things it tracks
FSRS describes the state of each card in your memory with three values:
- Difficulty: how hard that particular card is for you. It goes up when you miss it and down when you get it right.
- Stability: how long the memory holds before it weakens. A card with high stability can go months without a review.
- Retrievability: the probability that you would remember it today. It drops with every day you do not see it.
With those three numbers, the algorithm estimates when your chance of recalling the card will fall to a target level, usually around 90%, and schedules the review for that day.
How it differs from classic systems
Classic algorithms use fixed rules: if you get a card right, they multiply the interval by a factor. They work reasonably well, but they treat all memories in much the same way.
FSRS starts from a model of memory and fits its parameters to review logs from many learners. Its authors presented the basis of the method in 2022 at an academic conference on data mining, and showed that it reaches the same level of recall with fewer reviews.
In practice you notice two things: the cards you know well move away sooner, and the ones you miss do not start again from zero, because the algorithm accounts for the fact that you had partly learned them.
What it means for your daily study
You do not need to understand the maths to use it. The only thing the algorithm asks of you is an honest answer on each card. In Galigo that is two buttons: “Again” if you did not remember it and “Good” if you did.
Two practical consequences:
- If you answer “Good” without really remembering, the algorithm will believe your memory is stronger than it is and show you the card too late.
- If you review several days late and still get it right, the algorithm reads that as a sign the memory is solid and stretches the interval further.
In other words: do not obsess over never missing a day, but do care about answering honestly.
Its limits
No algorithm learns for you. FSRS optimises when to review, not what is worth learning or how much real content you consume. A good choice of sentences and many hours of listening are still the part that depends on you.