Spaced repetition, explained
Review something right before you’d forget it, and the memory comes back stronger, so the next review can wait longer. Repeat, and a fact can last years on a few minutes of total effort.
Follow one card through its life
This is the same scheduler the Cardsmith study session uses. Pretend you’re reviewing one card and choose how the recall went each time. Watch the gap to the next review grow on the timeline.
Day 0: the card is in front of you. How did recall go?
Press a button to see what happens to the gap before the next review.
Why spacing beats cramming
After you learn something, the chance you can recall it slides downhill: the forgetting curve. If you review while it’s still easy (cramming), you get little benefit because the memory didn’t need rescuing. If you wait until it’s nearly gone and then successfully pull it back, the memory is rebuilt more durably. Cognitive scientists call that the spacing effect, and it has been replicated in hundreds of studies since Ebbinghaus first noticed it in 1885.
Cramming does work for tomorrow morning. It just doesn’t last: in Cepeda and colleagues’ 2006 meta-analysis, spaced practice consistently beat massed practice on later tests, and the longer you need to remember, the bigger the advantage.
The schedule, in plain numbers
With the classic SM-2 algorithm and a card you keep answering correctly, the gaps look like this:
| Review | Gap before the next one | Card last seen on |
|---|---|---|
| 1st | 1 day | day 0 |
| 2nd | 6 days | day 1 |
| 3rd | 15 days | day 7 |
| 4th | 38 days | day 22 |
| 5th | 95 days | day 60 |
| 6th | ~8 months | day 155 |
Six reviews carry a card past a year. Hard cards get shorter gaps, easy ones longer. You can plan it by date with the spaced repetition schedule calculator.
How to use spaced repetition well
- Test, don’t reread. Every review should be an attempt to recall before you look (active recall).
- Study a little every day. The schedule assumes you show up; a missed week piles up reviews.
- Keep cards small. One fact per card, so a single “Again” doesn’t reset five things at once (how to make flashcards).
- Understand first, then memorise. Spaced repetition keeps knowledge; it doesn’t explain it. If a card makes no sense, the “Explain with AI” button in the study session can help.
Spaced repetition apps
Paper works (the Leitner system), but software does the bookkeeping. Cardsmith is a free spaced repetition app that runs in your browser: no account, no install, decks stored on your device. Make a deck on the flashcard maker, or generate one from notes, and your first session takes a minute to set up.
Questions people ask
What is spaced repetition?
A study method where you review information at increasing intervals, timed for just before you would forget it. Each successful review makes the memory more durable, so the gaps grow: a day, a few days, a week, a month.
Does spaced repetition actually work?
The spacing effect is one of the most replicated findings in learning research. A 2006 meta-analysis by Cepeda and colleagues (Psychological Bulletin) pooled hundreds of experiments and found spaced practice beat massed practice for long-term retention, and Dunlosky et al. (2013) rated distributed practice as one of only two “high utility” study techniques.
What is the best spaced repetition interval?
There is no single fixed schedule because easy cards can wait longer than hard ones. A classic starting sequence is 1 day, 6 days, then multiply by about 2.5 each time. Research by Cepeda et al. (2008) suggests the ideal gap grows with how long you need to remember: for an exam in a month, reviews every few days to a week; for a year, gaps of weeks.
Which is the best spaced repetition app?
Anki is the most powerful and SuperMemo the original; both are desktop-first with a learning curve. Cardsmith is a free, no-signup option in the browser that uses the same SM-2 idea, plus AI card generation. The method matters more than the app: any tool you’ll open daily will work.
Can I do spaced repetition without an app?
Yes. The Leitner system does it with paper index cards and a few boxes. An app just saves you the bookkeeping of working out what is due each day.