FSRS-6 spaced repetition for language learners

Lexica makes flashcards for language learners and schedules every review with FSRS-6, an open-source spaced repetition algorithm. Each word comes back just before you'd forget it, so your study time goes to the words that need it.

What spaced repetition does for vocabulary

You forget a new word quickly at first, and more slowly each time you remember it. Spaced repetition uses that. Instead of reviewing every word every day, you see each one again around the moment you'd lose it. A word you know well waits weeks or months; a word you keep missing comes back the same day.

For vocabulary, that's the difference between cramming a list the night before a test and still knowing it at the end of the year, without spending more time on it.

FSRS-6 in plain words

FSRS, the Free Spaced Repetition Scheduler, is an open-source algorithm that keeps track of two things for each card: how stable your memory of it is, and how difficult the word is for you. From those it predicts how likely you are to remember the word on any given day, and schedules the next review for when that chance drops to your retention target, 90% unless you change it.

Lexica runs version 6 of FSRS on every card from the start, on the web and on iPhone, so a review on one counts on the other.

One card, five reviews

Rated Good each time at a 90% retention target, la sobremesa comes back after 10 minutes, then 2 days, 11 days and about a month and a half, and the next review is five months away. Rate it Again and the gaps shrink; rate it Easy and they stretch.

Again, Hard, Good and Easy

• Again: you didn't remember. The card comes back later in the same session: a new card starts its learning steps again, and a card you'd already learned goes through your relearning steps before FSRS schedules it again.
• Hard: you remembered, with effort. The next gap is shorter than Good's.
• Good: you remembered. The usual next gap.
• Easy: you knew it at once. The gap is longer, and the word counts as easier for you from then on.

On the web, Space shows the answer and the keys 1 to 4 rate it.

Your settings

In Profile › Study you choose how many new cards to learn each day and your retention target. Raising the target means more reviews and fewer forgotten words; lowering it means the opposite.

Learning steps are the short waits a new card goes through on its first day, 1 and 10 minutes by default: rate it Good and it comes back after 10 minutes, rate it Again and it starts over at 1 minute. After the last step, FSRS starts counting in days. Relearning steps do the same for a card you forgot. You can also cap the longest gap between reviews, and keep a little randomness so cards learned together don't all fall due on the same day.

Scheduling is half the work. The other half is the cards themselves: Lexica builds them from a topic or a word list, each with a translation, IPA and an example sentence.