Spaced repetition for learning Chinese: how the algorithm works

Chinese asks for a lot of memory: 11,000 words and 3,088 characters across the nine HSK 3.0 levels. Nobody keeps that by reading lists twice. This guide explains how spaced repetition works, what the algorithm actually computes, and what it costs you in minutes a day. We build Gushi, so every number about Gushi below comes from its own scheduler, the code that runs in the app.

The forgetting curve

In 1885 Hermann Ebbinghaus measured how fast he forgot lists he had learned. Most of it went in the first day, then the loss slowed down. That curve is the reason a word you learned on Monday feels new again on Friday.

There is a second finding next to it. Every time you recall something just before it slips away, the memory lasts longer than before. So the best moment to review is not tomorrow for everything. It is a different day for every word, and the days get further apart.

From boxes to algorithms

• Leitner boxes (1970s). Paper cards in a row of boxes. A card you know moves one box on and waits longer; a card you miss goes back to the first box.
• SM-2 (1987). Piotr Woźniak's algorithm for SuperMemo. Each card has an "ease", and every interval is the last one times that ease. Anki was built on a version of it.
• FSRS (2022 onward). The Free Spaced Repetition Scheduler, open source, by Jarrett Ye and the open-spaced-repetition project. Anki has offered it since version 23.10.

SM-2 works, but every miss lowers a card's ease, and little raises it again. Learners call that "ease hell": a card missed a few times comes back every few days for months. FSRS models memory itself instead.

What FSRS computes

FSRS keeps three numbers for every card:

• Stability: how many days until your chance of recalling it drops to 90%.
• Difficulty: how hard this card is for you, from 1 to 10.
• Retrievability: your chance of recalling it right now. It falls every day you do not see the card.

When you answer, you press one of four buttons: Again, Hard, Good or Easy. FSRS updates difficulty and stability from that answer and from how long it has been. Then it schedules the next review for the day your chance of recall would drop to the target, 90%. A word you still remember after a long gap gets a bigger jump than one you saw yesterday, because the long gap proved more.

Real intervals, from Gushi's scheduler

One word, first met on day 0, answered Good every time it comes back:

Five sittings in the first year, and the sixth comes after day 440. On the first meeting the four buttons read: Again in 10 minutes, Hard in 1 day, Good in 3 days, Easy in 15 days.

Forgetting is part of it. Say you press Again on day 14. The word comes back 10 minutes later in the same sitting. Then Good brings it back after 3 days, 10, 30 and 81. The memory restarts lower, but it climbs again faster than the first time.

What it costs a day

Every new word brings its reviews with it. With the intervals above, a word you always know comes back 4 times in its first year. So:

That is the sum for words you know every time. Words you miss come back more often, so plan for a little more. A review takes a few seconds, a new word longer. How many new words a day each HSK level takes by the end of 2027 is in our level by level guide.

How Gushi schedules your cards

Everything in this list is what the app does today, on the iPhone and in the browser alike:

• FSRS with its published default weights. The same 19 weights for every learner; each card still gets its own stability and difficulty from your answers. Anki can fit the weights to your own history. Gushi does not do that yet.
• A 90% target. A card comes back when your chance of recall falls to 90%. You cannot change it.
• Again means 10 minutes. A card you just missed comes back in the same sitting, like Anki's default. If it is the last card of the day, it comes back at once so you can finish.
• No smoothing. A review is due exactly when FSRS says, never moved to even out a busy day.
• Separate cards for separate skills. Reading a word, hearing it, writing a character and saying a word each have their own card and their own schedule. Reading texts, listening practice and sentences are free practice without a schedule.
• You set the pace. You choose how many new words a day, and there is no cap on reviews unless you set one. New cards are mixed into the reviews, not saved for the end.
• Undo is exact. A grade you take back is gone, as if you had never pressed it.

The other half of remembering a character is a good story for it. That part is in how to remember Chinese characters.

Five habits that make it work

• Answer honestly. Press Again when you did not know it. A kind Good today is a forgotten word next month.
• Reviews first. Do the cards that are due before you take new ones.
• A steady number of new words. 10 a day every day beats 50 on Sunday.
• Missed a day? Just continue. The cards wait for you, and FSRS counts the real gap.
• Learn it before you review it. Spaced repetition keeps a memory. It does not make one. A story or a sentence makes one.

How we wrote this

The intervals and the daily sums come from Gushi's scheduler, the FSRS code in the app, run for one card answered as described. The history is the published record: Ebbinghaus's study of 1885, SM-2 from SuperMemo, and the FSRS project with its benchmark. We build Gushi, and we say where it stops: its weights are the defaults, not fitted to you.

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