Spaced repetition is a study method where you review information at increasing intervals, timing each review for just before you would forget it. It works because memory fades on a predictable curve, and each well-timed review slows that fade, so you can remember more while spending less time on each item.
If you have ever crammed for a test and lost most of it a week later, you have met the problem it solves.
The forgetting curve in plain language
In 1885, the German psychologist Hermann Ebbinghaus published experiments in which he memorised lists of nonsense syllables and tested himself at different delays. He found that forgetting is steepest right after learning and then slows down. That shape is what we call the forgetting curve.
Two practical points follow from it:
- Most forgetting happens early. The first review matters most, and it should come soon.
- Every successful recall changes the curve. When you retrieve something from memory, it tends to decay more slowly afterwards, so the next gap can be longer.
The exact speed varies with the person, the material and how meaningful it is to you. Treat the curve as a shape, not a stopwatch.
What spaced repetition actually does
A spaced repetition system combines two well-studied ideas:
- Retrieval practice. Trying to recall an answer, rather than rereading it, strengthens memory. In their 2006 work, Roediger and Karpicke showed that being tested on material often beat restudying it for long-term retention. This is usually called the testing effect.
- Distributed practice. Spreading reviews over time beats packing the same amount of study into one sitting. A 2006 meta-analysis by Cepeda and colleagues pulled together many experiments with this conclusion.
In a later review, Dunlosky and colleagues (2013) rated both practice testing and distributed practice as high-utility techniques, while highlighting and rereading rated low. Spaced repetition is essentially those two high-utility techniques automated: flashcards supply the testing, and a scheduler supplies the spacing.
Why expanding intervals work
Reviewing a fact you already know perfectly is wasted effort. Reviewing it after you have forgotten it means relearning from scratch. The sweet spot sits in between: the moment recall is possible but takes some effort.
Expanding intervals aim for that spot. The first gap is short, because new memories are fragile. Each successful recall earns a longer gap, because the memory is now sturdier. If you fail, the gap shrinks back, because you have shown the memory is not yet stable.
The “some effort” part matters. A review that feels slightly difficult does more for memory than one that feels effortless, which is why trying to recall before flipping a card beats glancing at the answer.
How a scheduler decides: Again, Hard, Good, Easy
Most modern flashcard schedulers, including the classic SM-2 family and the approach used in Anki and Lernoa, ask you to grade each card after you try to recall it. There are usually four buttons:
| Button | What it means | What the scheduler does |
|---|---|---|
| Again | You forgot it or got it wrong | Sends the card back into short learning steps and shortens its future interval |
| Hard | You recalled it, but with real struggle | Grows the interval only a little |
| Good | You recalled it with reasonable effort | Grows the interval by its normal step |
| Easy | You recalled it instantly | Grows the interval the most |
Behind the buttons sit a few moving parts:
- Learning steps. A new card is first shown a few times over short gaps, such as minutes, before it graduates to day-long intervals.
- An ease factor per card. Cards you find easy earn faster-growing intervals; cards you keep struggling with grow more slowly. The scheduler adapts to each card, not just to you.
- Lapses. A failed review on a mature card moves it back into relearning.
- Retirement. In Lernoa, a card that reaches a long interval and has been recalled enough times is treated as mastered and leaves your daily queue. Resetting the card brings it back.
The grading is honest self-assessment, so be straight with yourself. Pressing Good on a card you guessed at just pushes the review past the point where you would have forgotten it.
A sample schedule (illustrative)
The numbers below are illustrative only. Real schedulers vary, and your own intervals will depend on how each review goes.
| Review | Gap since last | You press | Card due again in |
|---|---|---|---|
| Day 1 (new card) | first time | Good | about 1 day |
| Day 2 | 1 day | Good | about 3 days |
| Day 5 | 3 days | Good | about 1 week |
| Day 12 | about 1 week | Good | about 2 to 3 weeks |
| Day 33 | about 3 weeks | Hard | about 4 weeks |
| Day 61 | about 4 weeks | Again | back to short steps, then about 1 day |
Notice the pattern: the gaps grow roughly geometrically while recall holds, and a miss pulls the card back. Across a whole deck, that means a card you know well costs you a couple of seconds a few times a year, while a stubborn card gets the attention it needs.
Common spaced repetition mistakes
- Making bad cards. Cards that are vague, overloaded or copied word for word from a textbook are hard to review well. Our guide on how to make good flashcards covers the basics: one idea per card, a clear question, an answer you can check.
- Flipping without trying. If you read the answer before attempting recall, you have skipped the part that builds memory.
- Grading too kindly. Pressing Easy on everything makes intervals balloon and recall crumble.
- Skipping days, then bingeing. Reviews pile up when you vanish for a week. Short daily sessions are far easier to keep; see a 20-minute daily study habit.
- Adding too many new cards at once. Every new card becomes a future review. A modest daily number keeps the load manageable.
- Memorising without understanding. Spaced repetition preserves what you put in. Learn the idea first, then use cards to keep it.
- Expecting it to replace everything. It is excellent for facts, terms and comparisons. For skills, you still need practice problems and hands-on work. If you want to see where it sits among other methods, see study techniques ranked by evidence.
Doing it in practice
You can do spaced repetition with paper boxes (the Leitner system), a general-purpose tool such as Anki, or a dedicated spaced repetition app. Whichever you pick, look for a scheduler with the four grades, learning steps and sensible handling of missed days, and for an easy way to get your content in and out. If you are currently in Anki, see how Lernoa compares on our Anki alternative page; it imports Anki decks and CSV files and lets you export whenever you like.
Try it
Pick a topic you need to remember for the next few months, write ten clear question-and-answer cards, and review them tomorrow, in three days and then in a week. If you would rather not build the cards by hand, you can start free and create them from your own notes or documents with the AI flashcard generator. Reviewing and studying are always free; only AI generation uses credits.
Frequently asked questions
What is spaced repetition in simple terms?
Spaced repetition means reviewing something just before you would forget it, then waiting longer each time you remember it. You spend your effort on the things you are about to lose instead of the things you already know well.
How often should I review flashcards with spaced repetition?
Daily is ideal because a scheduler shows you only the cards that are due, which keeps each session short. Missing a day is fine; the cards simply stay due until you get to them.
What is the forgetting curve?
The forgetting curve is a pattern first described by Hermann Ebbinghaus in 1885: memory of newly learned material drops quickly at first and then levels off. Reviewing at the right moments flattens the curve so you forget more slowly each time.
Is spaced repetition better than cramming?
For anything you need to remember beyond a few days, yes. Cramming can help for tomorrow's test, but research on distributed practice shows that spreading study over time leads to better long-term retention.