Spaced repetition, explained interactively
Spaced repetition means reviewing something just as you are starting to forget it, with the gaps between reviews growing each time. It is the most reliable way to move facts into long-term memory, and you can see why in about thirty seconds.
On exam day (day 30)96%
Two weeks later (day 45)87%
With no reviews0%
Illustrative model in the shape Ebbinghaus described: recall decays exponentially, and each review resets it and makes the next decay slower (here, memory strength grows 2.2× per review, more when the review came after some forgetting). Real numbers vary by person and material.
What the curve shows
In the 1880s Hermann Ebbinghaus memorised lists of nonsense syllables and tested himself at different delays. Most of what he learned was gone within a day or two, and the loss slowed down after that: the forgetting curve. He also found that relearning was faster each time, and that reviews spread over days held up better than the same number crammed together.
Play with the chart above: the cramming preset looks fine on exam day, but look at the second number, two weeks later, then tap “Spaced” and compare where the curve settles. Spaced reviews each start from a lower point, which is exactly what makes them strengthen the memory more. Researchers call this desirable difficulty: a review that takes some effort does more than one that feels easy.
A simple spaced repetition schedule
| Review | When | What to do |
|---|---|---|
| 1 | Same day you learn it | Make the cards and go through them once |
| 2 | Next day | Study all cards; missed ones go back to the start |
| 3 | About 3 days later | Only cards you got right twice move on |
| 4 | About a week later | Most cards should feel familiar but take a second |
| 5 | Two weeks later, then monthly | Quick check; anything missed restarts |
You don’t need to track this by hand. The Leitner system turns it into boxes, and our study mode keeps the boxes for you so missed cards come back first.
Spaced repetition vs cramming
Cramming isn’t useless: it can get you through tomorrow’s quiz. The problem is what happens after. In Cepeda and colleagues’ 2006 review of more than 250 studies on spacing, spreading study sessions out beat massing them together for long-term retention in almost every comparison. If the material will come up again (a final exam, the next unit, a language you want to speak) spacing pays for itself.
Spaced repetition apps and algorithms
Anki and SuperMemo use algorithms (SM-2 and its successors, and newer ones like FSRS) that estimate when you are about to forget each card and schedule it for then. They are powerful for large, long-term decks like medical school or a language. For a class test in a few weeks, a five-box Leitner schedule gets you most of the benefit with no setup, and pairs well with active recall.
Questions people ask
What is spaced repetition in simple terms?
Reviewing information at increasing intervals, such as one day, three days, a week and two weeks later, instead of all at once. Each review comes just as you would start to forget, which makes the memory last longer.
How long should the gaps be?
Start short and roughly double each time: next day, 3 days, a week, 2 weeks, a month. If you miss a card, restart it at the short interval.
Does spaced repetition work for exams?
Yes. Spacing is one of the two study strategies rated high utility in Dunlosky and colleagues’ 2013 review, alongside practice testing. Begin at least a week or two before the exam to get the benefit.
Is the Leitner system spaced repetition?
Yes. It is a paper-friendly way to do it: cards you know move to boxes you review less often, and cards you miss go back to the box you review every session.