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The forgetting curve: what it actually shows, and how to schedule reviews around it

Published 8 October 2026 · Updated 8 October 2026 · by the Paced team

What Ebbinghaus actually did

Hermann Ebbinghaus learned lists of meaningless syllables, waited, and then relearned them, measuring the time he saved compared with the first learning (the “savings” method). He ran the experiment on himself over several years and published it in 1885. Savings fell quickly over the first hours, more slowly over the following days, and the curve flattened without reaching zero: something was always left (Ebbinghaus, 1885).

In 2015, Murre and Dros repeated the experiment with one participant, 70 hours of learning and relearning, and delays from 20 minutes to 31 days. Their results were similar to Ebbinghaus's: the famous shape is not a historical accident (Murre & Dros, 2015).

What the shape is, and why it matters

• A power function, not a straight line. Across three experiments, with words, faces and even pigeons, and in a reanalysis of Ebbinghaus's own data, forgetting was best described by a power function of time: steep at first, then flatter (Wixted & Ebbesen, 1991).
• Part of what you learn stays. Measuring recall from one minute to 28 days after study, Averell and Heathcote (2011) found that every analysis supported above-chance retention at the end: even brief study left some memories effectively permanent.
• The numbers are not universal. The curve depends on the material, on how it was learned and on how memory is tested; the 1885 percentages describe nonsense syllables learned by one person. Keep the shape, not the figures.

From the curve to a review schedule

The curve explains why rereading a chapter the night before works for the morning and not for the month after. Two further findings turn it into a schedule. First, the spacing effect: for the same total time, learning spread over separate occasions is retained better, and the best gap between occasions grows with how long you need to remember (Cepeda et al., 2006). Second, the testing effect: a review in which you recall, rather than reread, does more for delayed memory (Roediger & Karpicke, 2006). Put together: review early, review by recalling, and let the gaps grow.

How much should the gaps grow? In a study of more than 1,350 learners, with gaps of up to 3.5 months and tests up to a year later, the best gap was about 20–40% of the time to the test when the test came a week after the review, but only 5–10% when it came a year after (Cepeda et al., 2008). The further the exam, the longer the gaps you can afford.

This is what an expanding schedule does in practice. On the Standard pace of Paced, a topic first studied for 2 h 30 comes back on these days, each time for a shorter recall:

These days are a practical choice consistent with the findings above, not values taken from one study; you can see the dates for your own start date in the schedule calculator, and the reasoning behind each pace in which intervals, and how to plan them.

Three mistakes the curve invites

1. Treating a percentage as a law. “You forget 70% in a day” is one reading of one experiment. Your lecture notes are not nonsense syllables; the shape holds, the figure does not.
2. Reviewing by rereading. Rereading flattens the curve for a few days and feels like learning; recall does the long-term work (Roediger & Karpicke, 2006).
3. Planning reviews without planning time. Every topic you add brings its own reviews, and they pile up under the new material. See what to do when they do.

Questions

Sources

1. Ebbinghaus, H. (1885). Memory: A Contribution to Experimental Psychology (English translation by H. A. Ruger & C. E. Bussenius, 1913). Classics in the History of Psychology, York University. The original forgetting-curve experiments, measured by the time saved when relearning.
2. Murre, J. M. J., & Dros, J. (2015). Replication and Analysis of Ebbinghaus' Forgetting Curve. PLoS ONE, 10(7), e0120644. One subject, 70 hours of learning and relearning after 20 minutes to 31 days: results similar to Ebbinghaus's.
3. Wixted, J. T., & Ebbesen, E. B. (1991). On the form of forgetting. Psychological Science, 2(6), 409–415. Three experiments (word recall, face recognition, pigeons) and a reanalysis of Ebbinghaus's 1885 savings data: forgetting follows a power function of time — fast at first, then slower.
4. Averell, L., & Heathcote, A. (2011). The form of the forgetting curve and the fate of memories. Journal of Mathematical Psychology, 55(1), 25–35. Recall measured from 1 minute to 28 days after study; every analysis supported above-chance retention at the end — part of what was learned stayed.
5. 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. Meta-analysis of 839 assessments in 317 experiments: the gap that maximises retention grows with how long you need to remember.
6. 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. More than 1,350 learners, review gaps up to 3.5 months, final tests up to a year later.
7. Roediger, H. L., & Karpicke, J. D. (2006). Test-enhanced learning: Taking memory tests improves long-term retention. Psychological Science, 17(3), 249–255. On tests two days and one week later, practising recall beat restudying the same text.

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