Forgetting Curve Calculator
Estimate how likely you are to remember a topic today, and when to review it before your exam. Free, no sign-up.
The original forgetting curve
How much time Ebbinghaus saved when he relearned lists of nonsense syllables after each delay, with the 2015 replication.
- Ebbinghaus, measured (1885)
- His formula
- Murre and Dros, measured (2015)
Time since learning, log scale
See the numbers
b = 100k ÷ ((log10 t)c + k)
Ebbinghaus's formula: b is the time saved in percent, t is minutes since learning, k = 1.84 and c = 1.25.
| Delay | Ebbinghaus, 1885 | His formula | Murre and Dros, 2015 |
|---|---|---|---|
| 20 minutes | 58.2% | 57.0% | 47.2% |
| 1 hour | 44.2% | 46.8% | 37.3% |
| 9 hours | 35.8% | 34.5% | 27.6% |
| 1 day | 33.7% | 30.4% | 31.7% |
| 2 days | 27.8% | 28.1% | 23.0% |
| 6 days | 25.4% | 24.9% | 16.8% |
| 31 days | 21.1% | 21.2% | 4.1% |
Sources: Ebbinghaus, Memory, chapter 7 (1885, translated 1913); Murre and Dros, PLOS ONE (2015), Table 3.
How it works
- 1
Enter the topic and date
Type the topic and the date you first studied it.
- 2
Add your reviews
Add each time you tested yourself on the topic, and how it went.
- 3
See your review dates
The calculator estimates how likely you are to remember the topic today, and when to review it before the exam.
Frequently asked questions
What is the forgetting curve?
It shows how memory fades after you learn something: quickly at first, then more slowly. Hermann Ebbinghaus first measured it on himself in 1879 and 1880, using lists of nonsense syllables.
Do we forget 70% of what we learn within a day?
No. That claim doesn't match Ebbinghaus's results. He measured how long it took to relearn a list, not how much of it he could recall, and a day after learning, relearning a list still took 66.3% as long as learning it had.
How is the estimate worked out?
It uses FSRS, a model fitted to flashcard reviews, with its published default settings. The day you first studied the topic counts as a first correct review, and each review you add changes the curve. Easily counts as FSRS's Good, With effort as Hard, and Couldn't recall as Again.
How far apart should reviews be?
It depends on when the test is. In one study, for tests 7, 35, 70 and 350 days after a review, the best gaps were 1, 11, 21 and 21 days, and a gap that was too long cost less than one that was too short (Cepeda and colleagues, 2008).
Read about the forgetting curveWhy does forgetting slow down over time?
Studies comparing curve shapes found that forgetting fits a power curve better than an exponential one (Wixted and Ebbesen, 1991). On a power curve, the longer a memory has lasted, the more slowly you lose it. The model behind the calculator is a power curve too.
How accurate is the calculator?
It's a rough guide. It only knows dates and how each review went. Ebbinghaus used nonsense syllables, which are very different from material you understand, and FSRS was fitted to single flashcards, while a topic is many facts at once.
Does SymbioLearn use this model?
No. SymbioLearn uses a simpler rule: your first correct answer on a concept sets a two-day gap, a correct answer two or more days after you last worked on it doubles the gap, up to 60 days, and a wrong answer halves it.
Is the forgetting curve calculator free?
Yes. There's no sign-up, and what you enter stays in your browser unless you share the link.
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Sources
- Ebbinghaus, H. (1885). Memory: A Contribution to Experimental Psychology, chapter 7. Translated by H. A. Ruger and C. E. Bussenius (1913).
- Murre, J. M. J. and Dros, J. (2015). Replication and analysis of Ebbinghaus' forgetting curve. PLOS ONE, 10(7), e0120644.
- Wixted, J. T. and Ebbesen, E. B. (1991). On the form of forgetting. Psychological Science, 2(6), 409–415.
- Cepeda, N. J., Vul, E., Rohrer, D., Wixted, J. T. and Pashler, H. (2008). Spacing effects in learning: a temporal ridgeline of optimal retention. Psychological Science, 19(11), 1095–1102.