What Is the Forgetting Curve? Measured Across 299,120 Reviews
The forgetting curve describes how memory decays over time. We measured it across 299,120 reviews: 94.6% still recalled clearly after three days, 79.1% after two months — and the same gap means completely different things for different words.
What Is the Forgetting Curve? Measured Across 299,120 Reviews
How much of what you learn today will you still know a month from now?
We ran the numbers on 299,120 reviews from Tomaru. The result: for words that are already well established, 94.6% are still recalled clearly after three days; after two months, that drops to 79.1%.
This article explains what the forgetting curve is, and what these numbers actually mean for learning vocabulary.
What the forgetting curve is
The forgetting curve describes how memory decays over time. It was proposed by the German psychologist Hermann Ebbinghaus in 1885. Its shape has two features: the loss is steepest in the first few days, then it flattens out.
This curve is the theoretical basis for spaced repetition. If memory decays at predictable points in time, review it once before it falls past the point of retrieval, and the curve gets pushed back up.
Ebbinghaus measured how much you forget when you never review. We are filling in the other half: how much memory actually survives when you do.
Where this data comes from
| Item | Value |
|---|---|
| Period | 13 April – 23 August 2026 |
| Reviews | 299,120 |
| Users | 764 |
| Cards | 32,121 |
| Scheduler | FSRS |
Every review records two things: how many days passed since the previous review, and which of four options the user chose — Forgot, Fuzzy, Clear, or Know it. "Recalled clearly" in this article means the last two. Fuzzy does not count as a success.
All figures are anonymised aggregates and contain no personal data.
Finding 1: the longer the gap, the less you remember
Looking only at cards that are already well established and in stable review — 32,134 reviews.
| Days since last review | Recalled clearly | Sample |
|---|---|---|
| 1–3 days | 94.6% | 2,224 |
| 4–7 days | 92.8% | 9,599 |
| 8–14 days | 88.5% | 14,212 |
| 15–30 days | 82.1% | 4,876 |
| 31–60 days | 79.1% | 1,223 |
The curve falls monotonically, and it falls faster later on: the first two weeks cost only 6 percentage points, while the stretch from week three to month two costs another 9.
In practical terms: leave a set alone for two months and roughly one word in five will be gone.
Finding 2: the same number of days means completely different things for different words
The table above only covers cards that are already established. Bring in every card and group them by memory strength, and the same interval produces completely different outcomes.
"Memory strength" is the scheduler's estimate for each card, measured in days — roughly how much longer that card can hold.
| Memory strength | Gap of 1–7 days | Gap of 8–30 days |
|---|---|---|
| Weak (under 3 days) | 66.5% | 54.5% |
| Medium (3–7 days) | 82.4% | 68.3% |
| Strong (7–15 days) | 93.2% | 87.6% |
| Very strong (over 15 days) | 93.1% | 85.6% |
Look at the first column. For the same within-a-week gap, weak cards come back only 66.5% of the time while strong cards come back 93.2% of the time — a spread of nearly 27 percentage points.
Which means "how many days should I wait?" is the wrong question. The same three-day gap is too late for a difficult word you just met and far too early for one you already know. What should set the interval is the state of that individual card, not the calendar.
Three practical conclusions
1. Forgetting right after you learn something is normal. Forgetting is the default behaviour of memory, not a failure of studying. What decides the outcome is not whether you forget but whether you retrieve it before you do.
2. Weak memories need frequency; strong memories need distance. A weak card is down to 66.5% after a single week — that word needs to come back within days. A strong card is still at 87.6% after a month, so reviewing it weekly is wasted effort. Move those spare repetitions onto the weak words and the total time stays the same while the result changes considerably.
3. Intervals should not be set in fixed days. A blanket "review every three days" over-reviews the easy words and under-reviews the hard ones. Both ends waste time.
One more point deserves its own line: re-reading is not reviewing. Seeing a word and feeling that you recognise it is a different state from being able to produce its meaning. Every number above measures active recall — try first, then check. That is also the only action that earns a longer next interval.
FAQ
What is the forgetting curve?
The forgetting curve describes how memory decays over time, and was proposed by Hermann Ebbinghaus in 1885. It shows that material you stop encountering is lost quickly over the first few days, then decays more slowly. It is the basis of spaced repetition: review before memory falls past the point of retrieval and you can hold onto it with far fewer repetitions.
Is the forgetting curve real?
Yes. We measured it across 299,120 reviews. On cards that are already well established, the share recalled clearly falls monotonically as the gap grows: 94.6% at 1–3 days, 88.5% at 8–14 days, and 79.1% at 31–60 days. The shape matches what Ebbinghaus described.
How much will I remember after a month?
It depends on the current memory strength of that particular word. Our data shows weak cards drop to 54.5% over a gap of 8–30 days, while strong cards hold at 87.6%. Cards already in stable review sit at 82.1% for a 15–30 day gap and 79.1% for 31–60 days.
Why do I still forget things I have reviewed?
Because a single review only pushes forgetting further out; it does not fix a memory permanently. A card whose strength is still weak returns at only 66.5% even when reviewed within a week. What matters is that every review lands shortly before you would have forgotten, rather than long after you already have.
How does the forgetting curve relate to spaced repetition?
The forgetting curve is the phenomenon; spaced repetition is the response. Since memory decays at predictable points, schedule the review just before the decay reaches the threshold and you maintain the memory with the fewest repetitions. Algorithms like FSRS go further and estimate the decay rate for each card individually, so the timing tracks the moment you are actually about to forget.
How often should I review?
There is no single number of days that works for every word. Our data shows that for the same within-a-week gap, weak cards return at 66.5% while strong cards return at 93.2% — nearly 27 percentage points apart. The interval should follow each card current memory strength rather than a uniform rule.
Is more review always better?
No. What works is one review shortly before you forget, not repetition while the memory is still sharp. Strong cards still return at 87.6% after 8–30 days; reviewing inside that window buys very little memory for the time spent, and it crowds out the words that genuinely need the slot.
Does re-reading a word list count as reviewing?
It is far weaker. Recognising a word is a different memory state from producing its meaning. Every number in this article measures active recall — attempt first, then check the answer. Active recall is what earns a longer next interval; passive re-reading does not.
Can I apply the Ebbinghaus curve directly?
Not really. Ebbinghaus used nonsense syllables and no review at all, so what he measured is decay without intervention. Real vocabulary study involves words of differing strength that all get reviewed, so copying a fixed table of "20 minutes, 1 hour, 1 day" adds little. Use the shape of the curve as background; let the individual card state set the timing.
The data in this article comes from anonymised review records on Tomaru. Tomaru uses the FSRS algorithm to schedule every card. Further reading: What is active recall, What is the testing effect, What is FSRS.