What actually works
The short answer: stop re-reading, start retrieving, and space your reviews over days instead of cramming the night before. Hermann Ebbinghaus measured in 1885 that without review, memory of learned material falls steeply in the first days — down to about 34 percent after one day and 21 percent after a month. A schedule of spaced reviews plus active recall is what keeps the curve from collapsing.
| Time after study | Material retained |
|---|---|
| 20 minutes | about 58% |
| 1 hour | about 44% |
| 1 day | about 34% |
| 6 days | about 25% |
| 31 days | about 21% |
Without review, memory of learned material falls steeply in the first days — from about 58 percent at 20 minutes to about 21 percent at a month.
Hermann Ebbinghaus measured the forgetting curve in 1885 and put numbers on what every student experiences: the drop is sharpest right after study, then flattens. His figures put retention near 58 percent after 20 minutes, about 44 percent after an hour, about 34 percent after one day and about 21 percent after a month.
The practical reading of the curve is that the first review matters most. If you can retrieve the material once within the first day, you interrupt the steepest part of the decline. Every subsequent spaced review resets the curve at a higher starting point, so each interval can be longer than the last.
This is why studying the same hour on three different days beats studying three hours in one sitting: the spacing effect says the same study time spread over days produces better long-term retention than massed cramming.
Two dated milestones anchor the whole approach: the measurement of the drop and the publication of the box system.
Compare re-reading, highlighting, cramming and retrieval practice side by side, and retrieval practice with spacing wins on retention weeks later.
The testing effect shows that active recall — closing the book and pulling the answer out of memory — strengthens memory more than re-reading the same material. Re-reading produces familiarity, which feels like knowing but does not survive a blank-page test.
Interleaving adds a second lever: mixing related topics within one session instead of finishing one topic before the next improves later discrimination between problem types, which is what exams actually test.
The table below lines up the common methods by what they do to long-term retention, so you can see where your current habit falls.
A practical first spacing is to review new material after one day, then after about three days, a week and two to four weeks.
The interval schedule below is the default starting point: first review after 1 day, then 3 days, 7 days, 14 days and 30 days. Each successful retrieval pushes the card or topic to the next interval; each failure sends it back to the shortest one.
The Leitner system, published by Sebastian Leitner in 1972, formalizes this with boxes: a correct answer promotes a card to a less frequent box, an error sends it back to the first box. The box number is the interval, so the system schedules itself.
You do not need an app to run this. Five piles of index cards on a desk, labeled by day count, reproduce the same logic — what matters is that the interval grows only when you retrieve correctly.
| Time after study | Material retained | What it means for your schedule |
|---|---|---|
| 20 minutes | about 58% | A quick closed-book check within the hour catches the first drop |
| 1 hour | about 44% | Under half the material survives a single hour unreviewed |
| 1 day | about 34% | Book the first review for the next day, not the weekend |
| 6 days | about 25% | The second review at day 3 or day 7 lands here |
| 31 days | about 21% | A month later, one fifth remains without any review |
Primary references for the figures and schedules cited on this page.