Spaced repetition

Spaced repetition, timed by FSRS

Every word you keep in Vocaby is scheduled by FSRS — the Free Spaced Repetition Scheduler. Instead of moving cards through fixed boxes, it models how strongly you remember each individual word and books the next review for the moment that memory is about to slip. Here is exactly what that looks like.

How FSRS times every review A retention chart. Memory of a word decays from 100% and FSRS schedules the next review at the moment it reaches 90% — after 1, 3, 7, 16, 35 days. Each successful review makes the curve flatter, so the gaps stretch out. A second, dashed line shows the same word never reviewed: it falls to about 25% over 62 days. 100% 50% 0% 90% 1 day3 days7 days16 days35 days interval until the next review Reviewed with FSRS Never reviewed FSRS-4.5 · 90% desired retention
Each review makes the curve flatter, so the next gap is longer. Review five times and one word has gone from a one-day interval to a five-week one.

What the chart is actually showing

FSRS tracks two things about every word: how likely you are to recall it right now, and how durable that memory has become. The dip in each tooth is recall probability falling. The 90% line is your desired retention — the point where Vocaby decides a review is worth your time. Because a successful review makes the memory more durable, the next curve falls more slowly, and the interval that reaches 90% gets longer. That is the entire mechanism.

What Again, Hard, Good and Easy actually do

After each card you tell Vocaby how that went. FSRS uses your answer to update both how hard it thinks the word is for you and how durable the memory now is, then recomputes the next interval from scratch. You are not nudging a card up or down a ladder — you are giving the model a data point.

The four answers

Again

You blanked. The word becomes a short interval again and is marked harder for you than average.

Hard

It came back, but slowly. The next interval still grows — just less than it otherwise would.

Good

Normal recall. The interval grows the way the model expects for a word this durable.

Easy

Instant. The word is treated as easier than average and jumps a noticeably longer interval.

Why FSRS and not the classic schedule

Older apps mostly use SM-2, a rule from the 1980s that multiplies your interval by a fixed ease factor. It works, but it treats every learner and every word the same and it is famous for burying you in reviews. FSRS fits a memory model instead, which means fewer reviews for the same retention.

The science behind it: forgetting curves, SM-2 vs FSRS

You set the pace

Desired retention is a dial, not a law. Push it up and Vocaby reviews you more often and forgets less; ease it down and you review less for a slightly leaner memory. You also choose how many new words a day you take on and when the reminder arrives. If you would rather just drill something now, on-demand review ignores the schedule entirely.

Not sure how many words your level needs? CEFR explained

Frequently asked questions

What is FSRS?
FSRS stands for Free Spaced Repetition Scheduler. It is an open, modern scheduling algorithm that models memory with three quantities — how hard a card is for you, how durable the memory is, and how likely you are to recall it right now — and picks the review date from that model rather than from a fixed rule.
How is FSRS different from SM-2?
SM-2, the classic algorithm from the 1980s, multiplies your interval by an ease factor that goes up or down as you answer. FSRS fits an actual memory model instead, so it can hit the same retention with noticeably fewer reviews. Both are spaced repetition; FSRS is the more accurate scheduler.
What does desired retention mean?
It is the recall probability you want to be sitting at when a word comes back. Vocaby aims for 90% by default, which means you should recognise roughly nine out of ten words you are shown. Raise it to forget less at the cost of more reviews, or lower it to review less often.
Do I need to build my own decks?
No. Vocaby ships more than 29,000 English words that already have IPA, audio, definitions, and example sentences, grouped into decks by exam and topic. You swipe to keep the ones worth learning and FSRS schedules them from there. Words you add yourself join the same reviews.
How long does a daily session take?
That depends on how many new words you let in each day, which you control. Because FSRS only surfaces words you are actually at risk of forgetting, a typical session is a few minutes rather than a grind through everything you have ever saved.

Let the algorithm decide when to review.

Vocaby schedules every word with FSRS so you study what you're about to forget and skip the rest. Free to start.