The forgetting curve describes how quickly you lose newly learned information when you do nothing to keep it. It falls steeply in the first hours after learning and then levels off, so most of what you are going to forget disappears early.
Hermann Ebbinghaus first charted it in 1885, and its shape has held up for well over a century. If you have ever opened your lecture notes a week later and found them strangely unfamiliar, you have met the curve in person.
The same research tradition that measured forgetting also found reliable ways to slow it down, and none of them depend on having a better memory. They depend on better timing. This guide covers what the Ebbinghaus forgetting curve actually measures, what a modern replication found, where the popular version goes wrong, and the three habits with the strongest evidence for bending the curve.
What Is the Forgetting Curve?
In psychology, the forgetting curve is a graph of retention plotted against time since learning. Time runs along the bottom, memory up the side, and the line drops fast before settling into a long, shallow tail.
Two features make it useful. First, forgetting is not linear: you lose far more in the first hours than across the whole following week. Second, whatever survives the steep early drop tends to stay, which is why scattered facts from a course you took years ago can still surface.
You will also see it called the curve of forgetting, and it shows up as a standard definition in introductory and AP psychology courses. The idea fits in one line: memory decays fastest right after you learn something, unless something interrupts that decay.
How Ebbinghaus Measured Forgetting
Ebbinghaus ran his experiments on himself, and he was 29 when he did the core work in 1879 and 1880. To keep prior knowledge out of the results, he memorized lists of nonsense syllables, short letter combinations that meant nothing and connected to nothing he already knew.
He did not simply count how many syllables he could recall. He used a subtler measure called the savings method. He learned a list until he could recite it perfectly, waited a set interval, then relearned the same list and measured how much less effort the second round needed.
Here is how the arithmetic works. Say a list takes 25 repetitions to learn, and after one day it takes 20 to relearn. You saved 5 of the original 25 repetitions, a savings score of 20%. That worked example comes from the 2015 replication study discussed below, which also tabulated Ebbinghaus's original numbers.
Ebbinghaus's Original Numbers
He relearned lists after intervals ranging from roughly 20 minutes to 31 days. His savings scores looked like this:
- About 20 minutes: 58.2%
- About 9 hours: 35.8%
- 1 day: 33.7%
- 2 days: 27.8%
- 6 days: 25.4%
- 31 days: 21.1%
Read the list from top to bottom and the curve draws itself. The drop between 20 minutes and 9 hours is larger than the drop between 1 day and 31 days. Most forgetting happens before you have even slept on the material, and after that the decline slows to a crawl.
That pattern is the practical core of the whole idea. If you want to keep something, the first day is where the most is at stake.
The Curve Has Been Replicated
A single self-experiment from the 1880s would normally deserve some caution. In 2015, Jaap Murre and Joeri Dros at the University of Amsterdam repeated it as closely as they could. Dros spent about 70 hours learning and relearning lists at the same intervals Ebbinghaus used, and the results were published in PLOS ONE.
The curve came back looking very similar to the original. Dros showed 47.2% savings at 20 minutes, 31.7% at 1 day and 16.8% at 6 days. An earlier German replication with two subjects, Mack and Seitz, produced the same general shape.
The outlier was the 31 day point, where Dros scored just 4.1%. The authors point to one likely reason: those particular lists had been learned in a compressed block near the start of the experiment, a form of massed learning. In other words, cramming the original learning may have made long-term forgetting worse, which previews the first fix further down.
Why There Is a Bump at 24 Hours
The replication also confirmed something Ebbinghaus noticed and waved away. The curve is not perfectly smooth. Savings after one or two days sit a little higher than a smooth curve predicts, a small upward jump that starts around the 24 hour mark.
Ebbinghaus did not trust that data point, so he retested it later and got a very similar score. Murre and Dros treat it as a valid measurement and found that adding a boost at one day improved how well their equations fit the data.
As early as 1924, the psychologists Jenkins and Dallenbach read the bump as an effect of sleep, and went on to test that idea directly. Murre and Dros are careful on this point. They say current sleep research would predict a jump like this, but that proving sleep causes it in this specific kind of experiment still needs more work.
What the Popular Version Gets Wrong
The forgetting curve is one of the most shared graphs in education, and it often travels without its caveats. Three of them matter if you want to use it well.
Savings is not recall. Ebbinghaus's 21.1% at 31 days does not mean he could recite a fifth of a list a month later. It means relearning went 21.1% faster than learning from scratch. A memory you cannot bring to mind can still make relearning cheaper, which is part of why a second pass through old material feels quicker than the first.
Nonsense is not your coursework. Meaningless syllables were chosen precisely because they link to nothing. Lecture content links to what you already know, so treat the shape of the curve as the lesson and the exact percentages as belonging to the experiment.
No single curve fits everyone. Even the four curves in the replication paper differ from one another at the long end, from 4.1% to 25.8% savings at 31 days. How and when the material was first learned is one reason, and what you do afterwards is the part you control.
How to Beat the Forgetting Curve
You cannot stop forgetting, and you would not want to remember every detail of every day. What you can change is the slope. Three habits have strong, replicated evidence behind them: spacing your reviews, testing yourself, and sleeping after you learn.
None of them asks for more total study time. They ask you to move the time you already spend to the points where it does the most good.
Space Your Reviews
Reviewing material after a gap beats reviewing it straight away, and the research on how long that gap should be is unusually large. In a study of more than 1,350 people, Cepeda and colleagues taught a set of facts, gave a review after a gap of up to three and a half months, and then tested again up to a year later.
For the same amount of study time, the best gap produced a 64% increase in final recall compared with reviewing on the same day. The ideal gap depends on how long you need to remember. It was about 20% of the time until the test when the test was a few weeks away, falling to about 5% when it was a year away. In their data, a test 7 days out was best served by a 1 day gap, and a test 35 days out by an 11 day gap.
So for an exam five weeks away, a first review about a week and a half after the lecture beats rereading your notes that same evening. The spaced repetition guide goes deeper on building a full review schedule.
Test Yourself Instead of Rereading
The second habit is retrieval practice, pulling information out of memory rather than putting it back in. Roediger and Karpicke had students study prose passages, then either take recall tests or restudy the material the same number of times. A final test followed 5 minutes, 2 days or 1 week later.
At 5 minutes, rereading won. On the delayed tests, prior testing produced substantially greater retention than studying. The twist is that the rereading students felt more confident about remembering the material. Rereading feels like learning because the page looks familiar, and familiarity is exactly what the forgetting curve wears away.
That finding is not a one-off. Rowland's 2014 meta-analysis of 61 studies, summarized in a Frontiers in Education review of ten learning techniques, found an effect size of d = 0.50 in favor of testing over restudy, with larger effects for recall than for recognition.
The hard part is usually writing the questions. Primo Notes turns a recorded lecture, a PDF or a photographed page into a note, and its quiz and flashcards tools build the retrieval practice from that note. For more formats, see these active recall techniques.
Sleep After You Learn
The third habit is the one most often dropped during exam season. In a study by Gais and colleagues, declarative memory was enhanced when sleep followed within a few hours of learning, independent of time of day. In a second experiment, people who slept within a few hours of learning recalled more 48 hours later than people kept awake through that first night.
This lines up with the bump in the Ebbinghaus data, and with a much larger body of work. A major review by Rasch and Born concluded that more than a century of research has established that sleep benefits the retention of memory.
The practical version is short. An all-nighter spends the night that would have protected what you just studied. A study session followed by a full night of sleep keeps that first night working for you instead of against you.
Putting It Together
The three habits stack neatly into one routine you can run for any course:
- On the day: capture the material properly, then test yourself once before bed instead of rereading.
- A few days later: a short retrieval session aimed at whatever you missed the first time.
- Before the exam: further reviews with gaps that widen, sized to how far away the test is.
Explaining a topic out loud is another form of retrieval, because you cannot explain what you cannot recall. The Feynman mode in Primo Notes runs a guided spoken session where you explain the material and the app works through the gaps in your explanation.
Conclusion
The forgetting curve is real, it has been replicated, and it is steepest in the hours right after learning. Ebbinghaus measured savings rather than recall, with nonsense syllables rather than coursework, so the shape travels better than the exact numbers.
What slows the curve is well established. Space your reviews to match how long you need to remember, test yourself rather than rereading, and protect the night of sleep after you study. Each one moves effort you already spend to the point where it keeps the most.
