FSRS EXPLAINED
FSRS is a spaced repetition scheduler that predicts when you are about to forget each card and shows it just before then. Here is how it works, how it differs from SM-2, and how Klappa sets it up.
THE SHORT ANSWER
FSRS is a spaced repetition algorithm that models your memory of each card and schedules the next review for the moment your chance of recalling it drops to a target retention. Klappa schedules with FSRS-6, the 21-weight version of the model.
Older schedulers such as SM-2 multiply the last interval by a per-card ease factor that fixed rules nudge up or down. FSRS instead estimates how strong each memory is, then works out how long that memory will last. The result is a review date set by your own answer history.
In Klappa, Study and Learn run on FSRS-6 in every deck you make, and so does the Anki Portal by default. You never have to choose an algorithm or set a parameter to get it.
THE FORGETTING CURVE
Klappa uses the FSRS-6 forgetting curve to estimate each card's current chance of recall in your decks. That curve describes how memory fades: recall is near certain right after you study, then falls quickly at first and more slowly later.
Each successful review flattens the curve. A card you have recalled several times across separate days decays more slowly than one you saw once, so it can safely wait longer before the next review.
Spaced repetition puts that to work. Review too early and you spend time on something you still know. Review too late and you have to relearn it. The best moment is just before the curve crosses the recall level you are aiming for, and finding that moment for every card is the scheduler's whole job.
HOW FSRS WORKS
Klappa's FSRS-6 scheduler tracks three values for every card. Difficulty is how hard the card is for you, on a 1 to 10 scale. Stability is how many days the memory lasts before your chance of recall decays to 90%. Retrievability is your chance of recalling it right now.
Each time you answer, FSRS updates difficulty and stability from your rating, then sets the next interval so retrievability will have fallen to your target by the due date. In Klappa Study you rate with Again, Hard, Good or Easy, and each button shows the interval it will actually schedule. In Learn, a correct multiple choice answer counts as Good and a wrong one as Again.
New and missed cards do not jump straight to days. In Klappa decks a new card steps through short learning steps of 1 and 10 minutes, and a review card you miss comes back after 10 minutes before it returns to day-long intervals.
FSRS VS SM-2
Klappa replaced SM-2 with FSRS-6 in Study: SM-2 is the classic formula behind early spaced repetition apps, and FSRS swaps its rule-based multiplier for a fitted memory model. The table compares them as Klappa implements and converts them.
| SM-2 | FSRS-6 | |
|---|---|---|
| What it stores per card | An ease factor (2.5 by default), the interval and a repetition count | Difficulty, stability and the date of the last review |
| How the next interval is set | The previous interval scaled by the card's ease factor | The day your predicted recall falls to the retention target |
| Retention target | None you can set directly | A recall probability; Klappa decks use 95% |
| Model | Fixed rules and constants | 21 weights |
| In Klappa | Older Study progress was converted to FSRS-6, with each card's interval carried over as its starting stability | Study and Learn in every Klappa deck, and the Anki Portal by default |
RETENTION VS WORKLOAD
Klappa decks schedule at 95% retention with a hard 60-day maximum interval, a setting in place since 26 October 2026. The retention target is the trade between how much you remember and how much you review: a higher target schedules each card sooner, so you remember more of the deck at any moment and do more reviews to keep it there.
A card you keep answering Good in a Klappa deck comes back after 1, 3, 8, 19 and 43 days, then every 60 days. At a 90% target with no cap, the same answers would have spaced it 2, 11, 46 and then 163 days apart.
The cap matters most before an exam: no card in a Klappa deck is ever scheduled more than 60 days out, so material you learned early in a course still comes back before you are tested on it.
Decks in the Anki Portal use their own settings instead. They default to a 90% target, and Deck Options lets you set desired retention anywhere from 70% to 97%. More on the setup is on scheduling in Klappa.
WHERE FMSR-1 SITS
FMSR-1 is Klappa's own scheduling model. In Study and Learn it can change the interval of a card in review once that card has at least three reviews of history, and it is on for every account.
It changes only the interval and the due date. Difficulty, stability and your progress bar stay exactly as FSRS-6 computed them, and its intervals obey the same 60-day cap. For a new card, a card still in learning steps, or a card with too little history, FSRS-6 schedules it unchanged.
So FSRS-6 is the foundation and FMSR-1 is a per-card refinement on top of it, computed from that card's own review history. The full picture is on the scheduling page.
FAQ
FSRS stands for Free Spaced Repetition Scheduler. Klappa uses its sixth version, FSRS-6, for Study and Learn in every Klappa deck and as the default in the Anki Portal.
FSRS sets each interval from a model of your memory and a retention target, where SM-2 scales the last interval by an ease factor that fixed rules adjust. That is why Klappa schedules with FSRS-6 rather than SM-2.
Higher retention means more reviews and fewer forgotten cards. Klappa decks are fixed at 95% with a 60-day cap; Anki Portal decks start at 90% and can be set from 70% to 97% in Deck Options.
Yes, through the Klappa Anki Portal. It imports each card's interval, due date, ease, lapses and any FSRS memory values from an .apkg exported with scheduling information; the per-answer review log is not imported. See how to import Anki decks.
Yes. Anki has offered FSRS alongside SM-2 since version 23.10 and FSRS-6 since 25.07. Klappa schedules with FSRS-6 as well and adds FMSR-1 in Study and Learn; the differences are on the Anki alternative page.
No. Klappa decks schedule with FSRS-6 at 95% retention from the first card you grade. Klappa is free, with no paid tier, and more answers are on the FAQ.