What is FSRS? The spaced-repetition algorithm, explained

FSRS stands for Free Spaced Repetition Scheduler. It is a modern, open algorithm that decides when you should review each flashcard so that you see it right before you would forget it — no earlier, no later. That single decision, made well, is what separates efficient studying from hours wasted on cards you already know.

This article explains what spaced repetition is, how FSRS models your memory, and why it schedules reviews more efficiently than the fixed intervals older apps used.

What is spaced repetition?

Spaced repetition is a study method built on two findings from memory research:

• The spacing effect: you remember more when reviews are spread out over time than when you cram them together.
• The testing effect: actively recalling an answer strengthens the memory far more than re-reading it.

The catch is timing. Review a card too soon and you waste effort on something you already know. Review it too late and you have already forgotten it, so you are relearning from scratch. The ideal moment is just before the memory fades — and that moment is different for every card and every person. An algorithm is what turns this idea into a concrete schedule.

How FSRS models your memory

FSRS describes each card with three numbers, often called the DSR model:

• Difficulty (D) — how inherently hard the card is for you. Harder cards grow their intervals more slowly.
• Stability (S) — how durable the memory is, defined as the number of days it takes for your chance of recall to fall from 100% to 90%. Every successful review increases stability, so intervals get longer over time.
• Retrievability (R) — the probability that you could recall the card right now. Retrievability starts high after a review and decays as time passes.

Together these let FSRS estimate, for any card on any day, how likely you are to remember it.

How FSRS schedules your next review

FSRS aims for a target retention — by default, a 90% chance of recall. After each review it does three things:

1. You rate how the recall went (for example: again, hard, good, or easy).
2. FSRS updates the card’s difficulty, stability, and retrievability from that rating.
3. It schedules the next review for the day your predicted retrievability drops to your target — so you meet the card again right as it is about to slip.

Because stability grows with each success, a card you keep getting right returns after minutes, then days, then weeks, then months. A card you struggle with comes back soon and often. You can also raise or lower your target retention: aiming for higher retention means more frequent reviews, lower retention means fewer.

Why FSRS instead of fixed intervals

Older schedulers — the classic example is the SM-2 algorithm from the 1980s — multiply your interval by a fixed factor after each review. That works, but it treats memory as a rigid formula and cannot adapt well to how a specific card or a specific learner behaves.

FSRS is different in three ways that matter:

• It is a memory model, not a fixed rule. It predicts a probability of recall and schedules against it, rather than applying the same multiplier to every card.
• It can learn from your history. FSRS parameters can be optimized to your own review data, so the schedule fits how you actually remember.
• It is open and free. The algorithm is public and community-developed, which is why a growing number of study apps have adopted it.

In practice this means fewer reviews for the same retention — or better retention for the same number of reviews.

FSRS in Repeto

Repeto uses the proven FSRS algorithm to schedule every card. It shows each card right when you are about to forget it and tells you honestly when the card will come back — anywhere from one minute to several months. No mindless cramming; only the reviews that truly matter.

And when you have finished the reviews due today, Repeto never locks you out — you can study ahead instead of being shut out. It is the modern, science-based way to learn for a real goal, whether that is med school, a language, or a professional certification.

Repeto is free to download for iPhone and iPad; learning, the FSRS scheduler, manual cards and sync are unlimited and free.