"I definitely learned that word yesterday, and today it's gone." Anyone who studies regularly has had this experience. It is not a problem with your memory. It is simply how the human brain is built to work.
The first person to describe that mechanism in numbers was the German psychologist Hermann Ebbinghaus. His research, published in 1885, remains a starting point for memory research 140 years later.
What Ebbinghaus actually did
Ebbinghaus's experiments were remarkably painstaking. He created large numbers of meaningless three-letter syllables such as "WID" and "ZOF", memorized them himself, then relearned them after a delay, and he repeated this work for years.
What he measured was not "what percentage was forgotten." It was "how much effort was saved when relearning, compared with the first time" (known as the savings score). Material you have learned once can be relearned faster than the first time, even if you cannot recall it. He treated that saved effort as the amount of memory that remained.
Here is what he found.
- Memory starts to fade rapidly right after learning
- The loss is not steady: it is steepest at the start and then levels off
- Even after a long time, the memory trace never drops fully to zero
This curve, a sharp early drop followed by a gradual slope, is the forgetting curve. In 2015, a Dutch research team (Murre & Dros) replicated the experiment and confirmed that the 130-year-old results were largely reproduced.
A common misconception: "42% forgotten after 20 minutes"
Articles about the forgetting curve often quote specific figures such as "42% is forgotten after 20 minutes and 67% after one day." Taking those numbers at face value is risky.
- The original experiment measured the savings score described above, not the amount you can actually recall
- The study material was meaningless syllables. Meaningful content, such as foreign-language vocabulary with example sentences, cause and effect in history, or the purpose behind a statute, is known to be forgotten much more slowly
- The only subject in the experiment was Ebbinghaus himself
So those figures are not "the universal speed of human forgetting." They are closer to "a rough guide under worst-case conditions (rote memorization of meaningless material)." What matters is not memorizing the numbers but the two facts that the shape of the curve tells you.
- Forgetting begins right after learning and is fastest at the start
- Left alone, most of your effort is gone within a few days
The speed of forgetting varies with the material and with how well you know it, so there is no way to measure exactly what percentage you remember right now. The retention rates that study tools display are also estimates based on a model. For the purpose of planning reviews, though, it is safer to plan around a conservative curve that assumes you forget sooner than around an optimistic one. Reviewing too early costs little, while reviewing too late costs a lot.
Forgetting is not a defect of the brain
There is one more important point. Forgetting is not a breakdown of the memory system. It is a function.
Your brain is exposed to an enormous amount of information every day. If it held on to everything it encountered with equal strength, it would struggle to retrieve the information that really matters. So the brain actively lets go of information that goes unused and prioritizes keeping information that is used repeatedly.
Turn that around and you can see how to make memories stick: tell your brain, "this information keeps getting used, so it's important." That is what reviewing does.
Forgetting is not a failure. It is a signal that tells you when to review.
Reviewing changes the shape of the curve
This is where the forgetting curve gets truly interesting. When you recall something just as you are starting to forget it, the memory does not merely return to where it was. The curve until you next forget becomes flatter than before.
- After the first review: forgetting slows down
- After the second review: it slows down further
- With each review, the memory changes in nature, from a short-term trace that fades quickly into knowledge that lasts
That is why the key to efficient learning is not how many hours you study, but whether you can systematically keep recalling things just as you are starting to forget them. And managing that timing is exactly what people are worst at. Keeping track of when each of dozens or hundreds of items is about to be forgotten would itself become a huge memorization task.
It makes sense to hand this part over to a system. With a system that calculates review dates along the forgetting curve and shows you only the reviews due today, you can focus on the work itself instead of on when to do it.
Summary
- Forgetting begins right after learning and is fastest at the start. This holds for everyone
- Frequently quoted figures such as "67% forgotten in one day" come from experiments with meaningless syllables. The shape of the curve matters more than the numbers
- Forgetting is not a defect of the brain but a function for picking out important information
- Reviewing just as you start to forget flattens the curve and turns memories into long-term knowledge
- Managing review timing is not a job people are suited to. Leave it to a system and focus on the learning itself