Spaced repetition is a study method in which you review the same material several times with gaps of days or weeks between reviews, usually by testing yourself, instead of repeating it many times in one sitting. It rests on the spacing effect, one of the most reliable findings in memory research: the same amount of study, spread out, is remembered far longer than it is when crammed. The research also answers the practical question most guides skip, which is how long the gaps should be.
Spaced repetition at a glance
- What it is: short reviews of the same material, spread over time, each one a chance to recall it.
- Why it works: each review comes after some forgetting has begun, and recalling it then strengthens the memory more than repeating it while it is still fresh.
- Best gap: longer when you need to remember for longer. Roughly a day or two for a test next week; about a month for something you need next year.
- Best format: self-testing (flashcards, practice questions, explaining from memory), not rereading.
- Not a shortcut to IQ: it makes what you learn last longer; it does not raise general reasoning ability.
Where spaced repetition comes from
Hermann Ebbinghaus noticed in the 1880s that relearning a list later took less time than learning it had, and that repetitions spread over several days held up better than the same number packed into one. His measurements of how quickly we forget are covered in our article on long-term memory, which also explains why the often-quoted figure for how much we forget in a day misreads his data.
The idea of a deliberate schedule came later. In 1967 the linguist Paul Pimsleur proposed a fixed “memory schedule” of steadily lengthening intervals for language learners. In 1972 the German writer Sebastian Leitner described a box of flashcards in which cards you know move to compartments you review less often. Software took over from there: modern flashcard apps estimate when you are likely to forget each card and schedule the review for about then.
What the evidence shows
The spacing effect is large and robust
Nicholas Cepeda and colleagues (2006) pooled 839 comparisons from 317 experiments in 184 articles. Spaced study beat massed study, and the analysis showed that the gap producing the best retention grew as the time until the final test grew. In the 2013 review of ten common study techniques by John Dunlosky and colleagues, distributed practice was one of only two techniques rated high utility; the other was practice testing. Rereading and highlighting were rated low.
The best gap depends on when you need it
To find the best gap, Cepeda and colleagues (2008) taught more than 1,350 people a set of facts, gave them a review after a gap of up to 3.5 months, and tested them after a further delay of up to a year. Lengthening the gap first improved final scores, then slowly reduced them. Measured as a share of the time until the test, the best gap fell from about 20–40% for a test one week away to about 5–10% for a test one year away.
| When you need to remember it | Rough first review gap (from the 2008 ratios) |
|---|---|
| In one week | 1–3 days |
| In one month | About 1 week |
| In one year | About 3–5 weeks |
The table is a rule of thumb derived from those ratios, not a tested prescription for every subject. The month row is an interpolation between the study’s one-week and one-year results.
It holds up over years
In a nine-year study, Harry Bahrick and three family members learned and relearned 300 English–foreign word pairs, with sessions 14, 28 or 56 days apart, then were tested up to five years after training ended. Longer gaps slowed learning slightly but produced much better retention: 13 sessions spaced 56 days apart gave retention comparable to 26 sessions spaced 14 days apart. Twice the spacing saved half the sessions.
Learners feel the opposite
Nate Kornell (2009) had students learn word pairs with flashcards, either in one large stack (spacing) or in small stacks studied separately (massing). Spacing was more effective for 90% of participants, yet after the first session 72% believed massing had worked better. Cramming feels fluent because the answer is still fresh; that fluency is exactly what fades by the exam.
Spacing plus testing: why the reviews should be recall
Spacing tells you when to review; retrieval practice tells you how. Jeffrey Karpicke and Henry Roediger (2008) taught students foreign vocabulary and then dropped words from further study or from further testing once they were learned. Repeated studying after that point had no effect on recall a week later; repeated testing had a large effect. The students’ own predictions of how well they would do were unrelated to how well they actually did.
Combining the two works best. A 2021 meta-analysis by Alice Latimier, Hugo Peyre and Franck Ramus gathered 29 studies of spaced retrieval practice. Spacing out the self-tests gave a strong benefit over massing them (an effect size of about g = 0.74 in the authors’ preprint). But expanding gaps (1, 3, 7, 14 days) were not reliably better than equal gaps (an effect close to zero), except when items were tested many times. The popular belief that each interval must be longer than the last is not what the evidence shows.
That is also the honest answer to the 2-3-5-7 method and other fixed revision timetables found in study guides. They are sensible ways to space your reviews, but their particular numbers have not been tested against each other. Any schedule that spreads short self-tests over days, with a gap that suits your deadline, captures most of the benefit.
How to use spaced repetition
- Turn the material into questions. One fact or idea per card, phrased so you have to produce the answer, not just recognise it.
- Start soon after learning. Do the first review within a day or two, before most of the material has gone.
- Set the gaps by your deadline. Use the table above: short gaps for next week’s test, gaps of weeks for knowledge you want to keep.
- Let mistakes decide the schedule. Cards you miss come back soon; cards you know well wait longer. This is what the Leitner boxes and flashcard apps do.
- Keep the sessions short and regular. Ten or fifteen minutes a day beats a long session once a week.
- Mix in understanding. Flashcards suit facts, vocabulary and definitions. For concepts, use practice problems or explain the idea from memory at each review.
The Leitner system, step by step
Use three to five boxes (or piles). All new cards start in box 1. Review box 1 every day, box 2 every few days, box 3 once a week, and so on. A card you answer correctly moves up one box; a card you miss goes back to box 1. Over a few weeks the easy cards drift to the rarely reviewed boxes and your time goes to the hard ones.
Apps and algorithms
Flashcard apps automate the same logic by predicting when each card is likely to be forgotten. Researchers at the Max Planck Institute for Software Systems (Tabibian et al., 2019) derived a scheduling rule from memory models and tested it on data from the language platform Duolingo: learners whose reviews happened to follow the rule’s timing memorised more effectively than learners on simpler fixed schedules. The app matters less than the habit: any tool that spaces out self-tests and that you actually open every day will do.
Why spacing works
There is no single agreed explanation, but the leading accounts fit together. When a review comes straight after study, the material is still in mind, so the review takes little effort and adds little. When it comes after a gap, you have to work to retrieve it, and that effort strengthens the memory and links it to more cues. Psychologists call this a “desirable difficulty”: harder in the moment, better for retention. Spaced reviews also happen in different contexts and moods, which gives you more routes back to the memory.
At the cellular level, a 2016 review in Nature Reviews Neuroscience (Smolen, Zhang & Byrne) notes that for many types of learning, spaced training produces more robust memory than massed training, and sets out the cellular and molecular mechanisms that are beginning to explain why. This links the classroom finding to the brain’s own neuroplasticity.
Spaced repetition, memory techniques and IQ
Spaced repetition decides when you review; mnemonic devices and the memory palace shape how you encode. They combine well: build a mnemonic for a hard list, then put it on a spaced schedule. Knowing which material you actually know, rather than which feels familiar, is a skill in its own right, covered in our article on metacognition.
None of this raises IQ. Spaced repetition improves retention of what you study, which helps in exams and in tests of acquired knowledge, but there is no evidence that it improves general reasoning. Claims that a study method makes you more intelligent are claims about transfer, and transfer is the part that usually fails, as our guide on how to increase IQ explains. Knowledge you keep does feed the part of intelligence psychologists call crystallized intelligence, which our article on fluid vs crystallized intelligence separates from reasoning on new problems.
Trust and scope notes
This page summarises published research for general information, with figures taken from the studies listed below; the 2021 effect sizes are those reported in the authors’ preprint. It is not medical or educational advice: memory problems that are new, worsening or worrying deserve a conversation with a doctor, and no online test can assess them. The IQ Revealed test is an online reasoning test of 40 original items for adults aged 18 and over; it is not a clinical assessment and it does not measure memory. Our guide to how IQ tests work explains what an online test can and cannot tell you.
IQ Revealed is not affiliated with, endorsed by or connected to Duolingo, the researchers, universities, institutes, journals or publishers named on this page, or any flashcard app. Names are used only to identify the research being described.