Memory

Spaced repetition: how to remember new words for good

Review each new word several times, with the gaps between reviews spread out over days and weeks, and try to recall the word before you check it. Spaced practice beats massed practice in hundreds of experiments, and how far apart the reviews are matters much more than the exact pattern of the gaps.

By the Readlang Mobile team · Updated 28 September 2026

What spaced repetition is

Spaced repetition means reviewing something several times with gaps between the reviews, usually longer gaps as the memory gets stronger, instead of going over it many times in one sitting. Flashcard apps automate it: each card comes back when the app predicts you are close to forgetting it.

The idea is older than any app. In 1885 Hermann Ebbinghaus, testing his own memory for lists of nonsense syllables, reported that 38 repetitions spread over three days were as effective as 68 repetitions made in a single day (Ebbinghaus, 1885). His measurements of how quickly memories fade gave us the forgetting curve.

The evidence for spacing

The spacing effect is one of the most consistent findings in the psychology of memory.

• A large review. Cepeda, Pashler, Vul, Wixted and Rohrer (2006) collected 839 assessments of spaced practice from 317 experiments in 184 articles. Of 271 direct comparisons between spaced and massed practice, only 12 showed no benefit of spacing or a negative one. Average final recall was 47.3% after spaced practice and 36.7% after massed practice.
• Foreign-language words over nine years. Four psychologists, the Bahricks, learned 300 English–foreign word pairs each and relearned them in 13 or 26 sessions spaced 14, 28 or 56 days apart, then tested themselves up to five years later. Longer gaps made learning slightly slower but retention substantially higher: 13 sessions 56 days apart gave about the same retention as 26 sessions 14 days apart (Bahrick et al., 1993). Half the sessions, the same result.
• Flashcards. In Kornell's (2009) experiments with GRE-style vocabulary, studying one large stack of cards, so that each card came back only after many others, beat studying the same cards in small stacks, and beat cramming on the last day. Spacing was more effective for 90% of participants, yet after the first session 72% believed the small stacks had worked better. Cramming feels productive because the answers are fresh; the benefit of spacing shows up only later.

How far apart should reviews be?

The best gap depends on how long you need to remember. In the largest study of this question, 1,354 people learned 32 obscure facts, reviewed them after a gap of up to 105 days, and took a final test up to 350 days later (Cepeda et al., 2008). The gaps that gave the best recall were:

As a share of the time to the test, the best gap falls from about 20–40% for a week to about 5–10% for a year. With the same study time, the best gap improved final recall by 64% compared with reviewing immediately. The practical reading: to keep a word for a year, you need reviews that are weeks apart, not a burst of reviews in the first few days.

Do the gaps need to grow?

Most schedules make each gap longer than the last. Paul Pimsleur (1967) called this graduated interval recall and proposed gaps that grow roughly fivefold: 5 seconds, 25 seconds, about 2 minutes, about 10 minutes and so on, reaching about four months by the tenth recall. He also stressed that no single schedule suits every word. Sebastian Leitner's card-box system, published in So lernt man lernen (1972), does something similar on paper: a card you answer correctly moves to a box that is reviewed less often, and a card you miss goes back to the first box.

Expanding gaps are sensible, but the evidence that they beat equal gaps is weak:

• Cepeda and colleagues' review found 62.0% recall with expanding gaps and 58.6% with fixed gaps across 22 comparisons, a difference that was not statistically significant (2006).
• Karpicke and Bauernschmidt (2011) found that longer spacing between three recall tests improved long-term retention by about 200% compared with no spacing, with no difference between expanding, equal and contracting schedules.
• Nakata (2015), with 128 Japanese college students learning 20 English words, found a small but statistically significant advantage for expanding gaps, the first such result for second-language vocabulary. The amount of spacing had a far larger effect.

So do not worry about the exact pattern. What matters is that reviews are spread out, and spread further for words you want to keep for a long time.

Recall, do not reread

Spacing works best combined with retrieval: trying to produce the answer from memory before seeing it. Karpicke and Roediger (2008) had students learn 40 Swahili–English word pairs. Students who kept being tested on all the words recalled about 80% a week later. Students whose words were dropped from testing after one correct recall remembered 36% or 33%, depending on the condition, and extra rounds of simply studying those words made no measurable difference. All groups predicted they would remember about half.

On a flashcard, that means saying or writing the meaning before you turn the card, and marking it as failed if you could not, however familiar the answer looks afterwards.

How apps schedule reviews

Most flashcard software descends from SM-2, the algorithm Piotr Woźniak wrote for the first SuperMemo program in 1987 (Woźniak, 1990). SM-2 lengthens each card's interval by a factor that depends on how easy you found it. Anki's original scheduler is based on SM-2, and since version 23.10, released in October 2023, Anki also offers FSRS (Free Spaced Repetition Scheduler), which models how likely you are to forget each card (Anki FAQ). Anki's manual describes FSRS as an alternative that can help you remember more material in the same amount of time.

You do not need to understand the algorithm to benefit from it. You need to review most days, grade yourself honestly, and keep the number of new cards small enough that reviews stay manageable.

Practical settings for vocabulary

• Add few new cards a day. We suggest starting at 10 to 15. Every new card brings reviews for weeks; Anki's manual warns that users who add hundreds of cards in their first days get overwhelmed by reviews.
• Take cards from what you read or hear. A word with the sentence you met it in is easier to recall and to use than a bare translation. Our guide to learning words from reading explains why context and repetition work together.
• Recognition first, production later. Start with cards that show the foreign word and ask for the meaning. Add cards in the other direction for words you want to use when speaking or writing.
• Keep going after a word feels known. In the experiments above, the benefit of spacing shows up weeks and months later. Deleting cards once you have got them right once or twice gives most of it away.
• If an exam is close, shorten the gaps. For a test next week, reviewing after about a day works best; for a year, reviews weeks apart (Cepeda et al., 2008).

Tools that schedule reviews

Readlang Mobile is made by the team that writes this site and is not affiliated with Readlang (readlang.com). The reading tools comparison covers how each tool handles lookups while reading.

References

1. Anki. What spaced repetition algorithm does Anki use? Anki FAQs. faqs.ankiweb.net; Anki 23.10 release notes, github.com/ankitects/anki
2. Bahrick, H. P., Bahrick, L. E., Bahrick, A. S., & Bahrick, P. E. (1993). Maintenance of foreign language vocabulary and the spacing effect. Psychological Science, 4(5), 316–321. doi:10.1111/j.1467-9280.1993.tb00571.x
3. Cepeda, N. J., Pashler, H., Vul, E., Wixted, J. T., & Rohrer, D. (2006). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin, 132(3), 354–380. doi:10.1037/0033-2909.132.3.354
4. Cepeda, N. J., Vul, E., Rohrer, D., Wixted, J. T., & Pashler, H. (2008). Spacing effects in learning: A temporal ridgeline of optimal retention. Psychological Science, 19(11), 1095–1102. doi:10.1111/j.1467-9280.2008.02209.x
5. Ebbinghaus, H. (1885). Über das Gedächtnis: Untersuchungen zur experimentellen Psychologie. Leipzig: Duncker & Humblot. English translation (1913), chapter 8: psychclassics.yorku.ca
6. Karpicke, J. D., & Bauernschmidt, A. (2011). Spaced retrieval: Absolute spacing enhances learning regardless of relative spacing. Journal of Experimental Psychology: Learning, Memory, and Cognition, 37(5), 1250–1257. doi:10.1037/a0023436
7. Karpicke, J. D., & Roediger, H. L. (2008). The critical importance of retrieval for learning. Science, 319(5865), 966–968. doi:10.1126/science.1152408
8. Kornell, N. (2009). Optimising learning using flashcards: Spacing is more effective than cramming. Applied Cognitive Psychology, 23(9), 1297–1317. doi:10.1002/acp.1537
9. Leitner, S. (1972). So lernt man lernen. Freiburg im Breisgau: Herder.
10. Nakata, T. (2015). Effects of expanding and equal spacing on second language vocabulary learning: Does gradually increasing spacing increase vocabulary learning? Studies in Second Language Acquisition, 37(4), 677–711. doi:10.1017/S0272263114000825
11. Pimsleur, P. (1967). A memory schedule. The Modern Language Journal, 51(2), 73–75. doi:10.1111/j.1540-4781.1967.tb06700.x
12. Woźniak, P. A. (1990). Application of a computer to improve the results obtained in working with the SuperMemo method. supermemo.com