FSRS Terms Explained: Retrievability, Stability, Difficulty
A plain-English map of the three ideas behind modern review scheduling
TL;DR
FSRS describes a card with three practical ideas: how likely you are to recall it now, how long the memory tends to last, and how hard the card is for you. You do not need to calculate these values by hand. You do need to write cleaner cards and review consistently so the scheduler gets useful signals.
FSRS can sound more complicated than it feels in daily study. Learners see words like retrievability, stability, and difficulty, then assume they need a statistics lesson before reviewing a deck.
This guide translates the terms into study decisions. Use it with our FSRS algorithm guide and the FSRS vs SM-2 comparison when you want the deeper scheduler context.
The short version
Retrievability asks, can you recall this card right now? Stability asks, how long does this memory usually survive before fading? Difficulty asks, how stubborn is this card for you compared with other cards? Together, those ideas help FSRS choose a review date that is not too soon and not too late.
Retrievability: the now question
Retrievability is the scheduler's estimate of your chance of recalling a card at the current moment. If a card is due today, the app is trying to show it near the point where review is useful. For learners, the practical lesson is simple: answer honestly. Marking a shaky answer as easy tells the scheduler the memory is stronger than it really is.
Stability: the memory-life question
Stability is about how long the memory tends to hold after a successful review. A card with high stability can wait longer. A card with low stability needs a shorter interval. Stability grows when recall is successful and clean. It grows less when the answer is vague, overloaded, or only recognized because the card wording gives away the answer.
Difficulty: the stubborn-card question
Difficulty is not a moral judgment about you. It is a signal that some cards are naturally harder because the concept is abstract, the wording is muddy, or several facts are packed into one prompt. When the same card keeps failing, rewrite it before blaming your memory. Flica is useful here because AI drafting can split long notes into smaller prompts before review starts.
How the three terms affect review choices
Use this table as a learner's translation, not a technical specification.
| FSRS term | Learner translation | What to do |
|---|---|---|
| Retrievability | Can I recall it now? | Grade recall honestly |
| Stability | How long will it last? | Review consistently so intervals can grow |
| Difficulty | Why does this card keep fighting me? | Rewrite overloaded cards |
Checklist before changing settings
Most review problems are card-design problems first. Before tuning FSRS parameters, check the deck itself.
- One idea per card.
- No paragraph-length answers.
- No trick wording.
- Review at a similar time most days.
- Suspend and rewrite repeated failures.
FAQ
Do I need to know FSRS math to use it?
No. The terms help you understand why cards appear when they do, but daily use only requires honest grading and consistent review.
Is retrievability the same as retention?
They are related. Retrievability is an estimate for a specific card at a specific moment, while retention is the outcome you want across many reviews.
What makes a card difficult?
The concept may be hard, but wording and card design often create avoidable difficulty. Split big prompts before changing scheduler settings.
How does Flica help with FSRS?
Flica helps create smaller reviewable cards and keeps spaced repetition built into the workflow, which gives any scheduler better material to work with.
Learn the terms, then fix the cards
FSRS is easier to trust when its terms map to everyday study choices. Retrievability is now, stability is how long, and difficulty is how stubborn.
If your deck is messy, start by making cleaner cards. The scheduler can only be as useful as the prompts you give it.
Turn rough notes into cleaner review cards
Use Flica to draft focused cards from notes or links, then review them with spaced repetition built in.
References
- Anki manual: FSRS documentation.
- FSRS project documentation and scheduler overview.
- SuperMemo: spaced repetition background.