Spaced repetition calculator. See exactly when each card comes back.

In the 1880s Hermann Ebbinghaus memorized lists of nonsense syllables and tested himself on how much effort it took to relearn them after different delays. His results gave us the forgetting curve: memory drops off steeply in the first hours and days, then levels out. They also showed that relearning is cheaper each time, and that repetitions spread over several days hold up better than the same number crammed into one sitting.

That second finding, the spacing effect, is one of the most replicated results in learning research. When Cepeda and colleagues pooled hundreds of verbal-learning experiments in 2006, spaced study beat massed study across a wide range of tasks and test delays. A 2008 follow-up by the same group asked how big the gaps should be, and found that the best gap between sessions grows as the time you need to remember something grows - though more slowly than the delay itself. For an exam a few weeks away, gaps of a few days to a week or so work well; for something you need next year, gaps of weeks.

The difficulty is bookkeeping. If every card needs its own schedule, nobody can track hundreds of them by hand. That is all a spaced repetition scheduler does: it keeps the gap for each card, and widens or shrinks it depending on how well you answered.

Most flashcard schedulers descend from SM-2, the algorithm Piotr Woźniak wrote for the SuperMemo program around 1987. SM-2 gives each card an ease factor that starts at 2.5. When you remember a card, its next interval is the last one multiplied by the ease, so the gaps grow geometrically. When you forget it, the card starts over with short intervals. Your grades nudge the ease up or down, so cards you find hard come back more often for good, and the ease never falls below 1.3.

This calculator uses an SM-2-style scheduler with four grades. A new card answered Good comes back in 3 days, Easy in 5. After that, Good multiplies the last interval by the card's ease, and Easy multiplies it by the ease and a bonus of 1.3 while raising the ease. Hard grows the interval by only 20% and lowers the ease, and Again brings the card back the next day and lowers the ease more. Ease stays between 1.3 and 3.0, and every interval is a whole number of days.

So a card you answer Good four times in a row comes back after 3, 8, 20 and then 50 days. Forget it on the third review and it returns the next day; a Good after that starts the growth again at 3 days, with a slightly lower ease than before. The calculator calls a card mastered once its gap reaches 21 days, because a single good answer isn't proof you'll still know it in a month.

SM-2 is simple and predictable, and it ignores a lot of information: how long you actually waited before a review, how you did on similar cards, how hard a card is compared with how much time has passed. Newer schedulers try to use that. FSRS, an open-source scheduler that Anki added as an option in 2023, models each card's memory stability and difficulty from your own review history, then schedules each review for the point where you're likely to still recall it at a retention level you choose.

For a long-running deck with years of reviews, that can mean fewer reviews for the same retention. For a semester course, the difference matters less than how you use whichever scheduler you have. The rules below make more difference than the algorithm does.

Review what's due, every day. The schedule only works if the reviews happen near their dates. Ten minutes daily beats an hour on Sunday, because a backlog of overdue cards turns every review into relearning.

Grade honestly. Pressing Good on a card you half-remembered pushes it out too far, and you'll meet it again when it has already faded. Again is not a failure; it's the scheduler doing its job.

Don't review the whole deck before an exam. Cramming every card at once undoes the spacing. Keep doing the due reviews, add one final pass the day before, and spend the rest of your time on practice questions. The calculator's exam planner suggests that final pass when your last scheduled review would otherwise fall too early.

Write cards that can be graded. One fact per card, with a cue that only has one right answer, so Good and Again mean something. There's a full guide to writing better flashcards, and a longer explanation of the method in the spaced repetition guide.