Retention after a study session, hour by hour. Enter the time since you studied, your number of reviews, the material type and how you studied it, and see what you still hold.
How much do you remember after 48 hours?
Retention after a study session, hour by hour. Enter the time since you studied, your number of reviews, the material type and how you studied it, and see what you still hold.
Wondering how much you still remember 48 hours after studying? On the Ebbinghaus forgetting curve, abstract material learned once from notes drops fast on the first day and then flattens out. The calculator above shows the share you are holding at 48 hours, what the same session looks like with one to five reviews behind it, and where the next review should land.
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The forgetting curve, from twenty minutes to three months
A week after a lecture you are holding 20.5 percent of abstract material studied once from notes. Same material, same week, but reviewed twice: 50.7 percent. Nothing else changed. Not the hours, not the highlighter, not the effort in the room. Two passes back over the same pages, and the number more than doubles. This calculator draws that curve for your own session and shows both lines side by side.
Reading your retention step by step
- Hours since you studied - measured from the last time you looked at the material, not from the first. One day is 24, a week is 168, a month is 720.
- Number of reviews - how many times you went back after the first pass. Each one halves the decay constant, so this is the field that moves the result most.
- Type of material - abstract for vocabulary, dates and formulas; partly logical for definitions and rules; logical for narratives and procedures.
- How you studied it - shallow for reading, medium for notes and highlighting, active for flashcards, self testing and explaining it to somebody.
- Number of items - optional. Give it and the percentage is also reported as a count, which is usually more sobering than the percentage.
- Read the results - the headline is what you hold now, the three tiles compare it against the no review line, and the table walks the whole curve from twenty minutes to three months.
The basics: what the curve actually describes
Hermann Ebbinghaus spent the 1880s memorising lists of nonsense syllables and then measuring how much effort it took to relearn them later. He deliberately chose syllables with no meaning, because meaning is exactly the thing that makes forgetting unpredictable. What he found was a decay that is brutally fast at the start and then flattens: most of what is going to be lost is lost in the first day, and what survives a week tends to survive considerably longer.
The shape matters more than the exact numbers. Later work, in particular the review of retention data by Rubin and Wenzel, found that power functions fit forgetting across a wide range of tasks better than a simple exponential does. This calculator uses a power-law form, R = 1 / (1 + k × √t), where t is hours since study and k is a decay constant. It is a fit to the shape of forgetting, not a law of nature and not a measurement of your particular memory.
Three things move k. Material type sets the base: abstract material carries the full constant, partly logical material carries half of it, logical material just over a quarter. Study method scales it: shallow reading multiplies k by 1.5, active retrieval multiplies it by 0.6. Then each review halves whatever is left, which is why the model is far more sensitive to the number of passes than to how hard any single pass felt.
That last point is the practical one. Studying abstract material actively rather than shallowly takes 24 hour retention from 31.2 to 53.1 percent, which is a real gain. Reviewing that same material twice takes it from 40.5 to 73.1 percent. Method helps. Repetition helps more, and it is cheaper.
How much you forget in the first day, week and month
Abstract material, studied with notes and highlighting. The first column is a single pass; the second is the same material reviewed twice. Read the gap in the last column rather than either number on its own.
| Time since studying | One pass | Two reviews | Difference |
|---|---|---|---|
| 20 minutes | 85.3% | 95.9% | +10.6 |
| 1 hour | 76.9% | 93.0% | +16.1 |
| 9 hours | 52.6% | 81.6% | +29.0 |
| 1 day | 40.5% | 73.1% | +32.6 |
| 2 days | 32.5% | 65.8% | +33.3 |
| 1 week | 20.5% | 50.7% | +30.2 |
| 1 month | 11.0% | 33.2% | +22.2 |
| 3 months | 6.7% | 22.3% | +15.6 |
Notice where the gap peaks. It is widest at 2 days, not at three months, and it is almost nothing after twenty minutes. Reviews do not add a fixed bonus; they buy you time in the window where the curve is steepest.
What you still remember after 24 hours, by material and method
All nine combinations after a single day, with no reviews at all. The spread runs from 31.2 to 80.8 percent, which is the whole argument for not treating a study hour as a study hour.
| Material | Shallow reading | Notes and highlighting | Active recall | Reading |
|---|---|---|---|---|
| Abstract (dates, formulas, vocabulary) | 31.2% | 40.5% | 53.1% | Needs reviews within a day |
| Partly logical (definitions, rules) | 47.6% | 57.6% | 69.4% | Holds a few days |
| Logical (narratives, procedures) | 62.7% | 71.6% | 80.8% | Survives a week reasonably |
How much each review actually adds at the one week mark
Abstract material, notes and highlighting, read at the 168 hour mark. The decay constant halves each time, and the returns diminish in an instructive way.
| Reviews | Decay constant k | Retention at 1 week | Added by this review |
|---|---|---|---|
| 0 | 0.3000 | 20.5% | baseline |
| 1 | 0.1500 | 34.0% | +13.5 |
| 2 | 0.0750 | 50.7% | +16.7 |
| 3 | 0.0375 | 67.3% | +16.6 |
| 4 | 0.0187 | 80.4% | +13.1 |
| 5 | 0.0094 | 89.2% | +8.8 |
The largest single jump is the second review, not the first, and the fifth adds barely half of what the second did. If you only ever do one thing with this table, do the second review.
Five study sessions, five outcomes
Result: 57.6% against 40.5% with no review. That is 69 words held instead of 49.
Result: 63.2% against 30.0% for the same active study with no reviews.
Result: 86.0%, band Strong retention. The same material with no reviews sits at 43.4%.
Result: 39.1%, band Weak retention. Reading twice is not reviewing twice.
Result: 4.6%, band Critically low. Effectively a fresh start.
The review schedule the curve implies
The practical rule that falls out of the model is to review when retention has dropped to somewhere around 70 to 80 percent, and to let the interval grow each time because the curve keeps flattening. That produces roughly this ladder.
Less obvious relationships
How to interpret your result
| Retention | Band | What to do about it |
|---|---|---|
| 80 percent and above | Strong retention | Let the interval grow. Reviewing now spends time on material you already hold. |
| 60 to 79 percent | Good retention | The efficient window. A short review here costs little and resets the curve. |
| 40 to 59 percent | Moderate retention | Review soon. Past this point the session starts to feel like relearning. |
| 20 to 39 percent | Weak retention | Budget real time, not a glance. Most of the material has to be rebuilt. |
| Below 20 percent | Critically low | Treat it as new material and plan reviews from day one this time. |
This is an estimate of the shape of forgetting under a published model, not a measurement of your memory. Individual recall varies with sleep, prior knowledge, interference from similar material and how the test is asked. Use it to choose when to review, not to decide whether you are ready.
Questions about the forgetting curve
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Calculator verified by the LiczGrupa.pl team
Content, formulas and results have been reviewed for accuracy and relevance by our team of specialists.
Reviewed by: Patryk Matyjasik