Learn vocabulary with spaced repetition: how it works (FSRS explained)

The forgetting curve: why new words disappear

In the 1880s the German psychologist Hermann Ebbinghaus memorized lists of nonsense syllables and measured how much he still knew after different delays. The result is known as the forgetting curve: memory drops fast in the first hours and days after learning, then the decline slows down.

The more useful part of his work is what happens when you review. Each time you successfully recall something, the curve gets flatter, so the next review can wait longer. A word you have recalled several times at growing intervals can stay with you for months between reviews. That is the whole principle behind spaced repetition.

Spaced repetition and active recall

Spaced repetition answers the question “when should I review?”. Active recall answers “how should I review?”. Rereading a list feels productive because the words look familiar, but recognition is much easier than producing the word yourself. Research on the testing effect consistently finds that retrieving an answer from memory strengthens it more than looking at it again.

Flashcards combine both ideas: you see one side, try to remember the other, and only then check. A scheduler then decides when that card comes back.

• Look at the front and say or type the answer before flipping.
• Be honest with yourself about whether you really knew it.
• Let the schedule decide which cards are due instead of reviewing everything every day.
• Learn in both directions for words you need to produce, not just recognize.

Again, Hard, Good, Easy: what the buttons mean

Many flashcard apps ask you to rate each answer with four buttons. The rating is not a grade for you. It is information the scheduler uses to plan the next review.

What FSRS is

FSRS stands for Free Spaced Repetition Scheduler. It is an open-source scheduling algorithm developed in the spaced repetition community, and Anki added it as an option in 2023. Older schedulers, such as the SM-2 family that many apps have used for decades, mostly multiply an interval by a fixed factor. FSRS instead keeps a small model of your memory for each card.

That model tracks how difficult a card is for you, how stable the memory is (roughly, how long it lasts before you are likely to forget), and the current probability that you can still recall it. After every answer the values are updated, and the next review is scheduled for the point where your recall probability is expected to drop to a target level, commonly around 90 percent. The model’s parameters can be fitted to real review histories, which is why it adapts to how you actually remember rather than to a one-size-fits-all formula.

In practice that means fewer unnecessary reviews of words you already know, and quicker returns for the ones that keep slipping. Cardfox schedules every card with FSRS, so the Again/Hard/Good/Easy answers you give (or a swipe) feed directly into that model.

A daily routine that works

Spaced repetition only pays off if you show up regularly. Short daily sessions beat long weekly ones because the schedule is built around specific days.

• Review your due cards first, every day. That is the part that keeps old words alive.
• Add new words in small batches, for example one textbook unit at a time, rather than hundreds at once.
• Don’t skip Again out of pride. Marking a forgotten word honestly is what makes the schedule accurate.
• If a word keeps failing, change the card: add an example sentence, an image or a memory aid.
• Expect more reviews in the first weeks after adding many new cards. The load evens out as intervals grow.

Exams and deadlines

Pure spaced repetition doesn’t know about your test on Friday. If you have a fixed date, start early enough that each word gets several reviews before it, and keep adding new units at a steady pace. In Cardfox you can set an exam date for a deck, and the app takes it into account so that everything sticks by then.

Getting started with Cardfox

Cardfox is a vocabulary flashcard app for iPhone and iPad that uses FSRS for scheduling. You can scan a textbook page to create cards, add your own, or import them, then study by flipping cards or typing the answer, in both directions. A reminder only shows up on days when cards are due. If you want to try the method described here without setting anything up by hand, it’s a straightforward place to start.