How the calculator picks your dates
On day 0 you learn the topic and quiz yourself once. After that the gaps double, 1 day, then 2, then 4, up to a ceiling that depends on how long you have before the exam. That ceiling is about 30% of the time left when the exam is a week away and falls towards 7.5% when it is a year away. The last review always lands on the day before.
These proportions come from Cepeda and colleagues. In their study, the best gap between two sessions was about 20 to 40% of the time left for a test a week away, and 5 to 10% for a test a year away (Cepeda et al., 2008). The calculator takes the middle of those ranges. For an exam in 30 days it gives 7 reviews, on days 1, 3, 7, 14, 21, 28 and 29. For an exam in 90 days it gives 10, with a ceiling of 14 days between two passes.
Work out your review dates
Pick the day you learn the topic and the day of the exam. The table gives each review, the gap since the previous one and what to do that day. Nothing is sent anywhere, the maths runs in your browser.
| No. | Date | Gap | What to do |
|---|---|---|---|
| 0 | Fri 6 Nov | Learn, then quiz yourself once without notes | |
| 1 | Sat 7 Nov | 1 day | Quiz yourself without notes, then check |
| 2 | Mon 9 Nov | 2 days | Quiz yourself without notes, then check |
| 3 | Fri 13 Nov | 4 days | Quiz yourself without notes, then check |
| 4 | Fri 20 Nov | 7 days | Quiz yourself without notes, then check |
| 5 | Fri 27 Nov | 7 days | Quiz yourself without notes, then check |
| 6 | Fri 4 Dec | 7 days | Quiz yourself without notes, then check |
| 7 | Sat 5 Dec | 1 day | Short last pass, mostly your mistakes |
| Sun 6 Dec | Exam |
The longest gap is about 30% of the time left for an exam a week away and 7.5% for an exam a year away, in the middle of the ranges found by Cepeda and colleagues (2008). The first gaps double, 1 day, 2 days, 4 days.
Why spacing beats reviewing everything at once
A meta-analysis of 839 measures confirms that spacing reviews beats massing them into one session (Cepeda et al., 2006). The trouble is that the massed session feels better at the time. With vocabulary flashcards, spacing worked better for 90% of participants, yet 72% thought after the first session that massing worked better (Kornell, 2009).
A schedule written in advance keeps you from trusting that feeling. You know which day to come back to which topic, and you do not have to wonder whether it is too early or too late. If you are preparing several chapters, run the calculation for each one from the day you learn it, then gather the dates in your plan, as shown in How to make a study schedule before an exam. The page Spaced repetition, how long to wait between two reviews covers the rest of the research.
What to do on a review day
A review date only helps if you quiz yourself on that day. Close your notes, answer the questions on your cards or write down what you remember, then compare with your notes and mark what was missing. In the study by Roediger and Karpicke, between a test 5 minutes later and a test a week later, students who had reread forgot 52% of what they knew, against 14% for those who had quizzed themselves (Roediger and Karpicke, 2006).
So a spaced review session can be short. Ten minutes of questions on a chapter beat an hour of rereading. Topics you missed deserve one extra pass before the next date, topics you know well can wait. The page Active recall, why quizzing yourself beats rereading explains how to ask good questions, and How to make flashcards that actually work how to write them.
The method and its limits
Cepeda’s study used a set of facts learned by more than 1,350 people, with a review up to 3.5 months later and a final test up to a year after that (Cepeda et al., 2008). Its ranges give an order of magnitude, not a date to the day. Doubling the first gaps is a design choice, common in flashcard software, that brings a fresh topic back early.
Adjust the schedule to your life. If a topic slips on a review day, come back to it the next day before you return to the rhythm. If the exam is less than two days away, spacing has no room left to work, and the page Does cramming the night before an exam work? explains how to make the most of the last night. For a whole syllabus, combine this calculation with How to make a study schedule before an exam.
| Time before the exam | Reviews | Review days after learning |
|---|---|---|
| 7 days | 4 | 1, 3, 5, 6 |
| 14 days | 5 | 1, 3, 7, 11, 13 |
| 30 days | 7 | 1, 3, 7, 14, 21, 28, 29 |
| 60 days | 8 | 1, 3, 7, 15, 26, 37, 48, 59 |
| 90 days | 10 | 1, 3, 7, 15, 29, 43, 57, 71, 85, 89 |
Sources · Cepeda et al., 2008
Frequently asked questions
- How many times should I review a topic before the exam?
- With this calculator, 4 reviews for an exam a week away, 7 for one a month away and 10 for one three months away. Rawson and Dunlosky also advise recalling an item correctly 3 times in the first session before spacing it out (Rawson and Dunlosky, 2011).
- What interval should I leave between two reviews?
- It depends on the exam date. About 20 to 40% of the time left for a test a week away, about 5 to 10% for a test a year away, according to Cepeda and colleagues (Cepeda et al., 2008).
- What if I miss a review day?
- Do it the next day and keep the following dates. A slightly longer gap is still spacing, what matters is coming back to the topic before the exam.
Lumi does this calculation for you, card by card. It forgets at the pace of the forgetting curve, then comes back in the evening with its questions, right before you forget in turn, with at most one reminder a day. The night before the exam, the Mist rises over your sky, and every right answer pushes it back.
Sources
- Cepeda, Vul, Rohrer, Wixted, Pashler (2008). Spacing effects in learning. A temporal ridgeline of optimal retention. Psychological Science, 19(11). doi.org/10.1111/j.1467-9280.2008.02209.x
- Cepeda, Pashler, Vul, Wixted, Rohrer (2006). Distributed practice in verbal recall tasks. A review and quantitative synthesis. Psychological Bulletin, 132(3). doi.org/10.1037/0033-2909.132.3.354
- Kornell (2009). Optimising learning using flashcards. Spacing is more effective than cramming. Applied Cognitive Psychology, 23(9). doi.org/10.1002/acp.1537
- Roediger, Karpicke (2006). Test-enhanced learning. Taking memory tests improves long-term retention. Psychological Science, 17(3). doi.org/10.1111/j.1467-9280.2006.01693.x
- Rawson, Dunlosky (2011). Optimizing schedules of retrieval practice for durable and efficient learning. How much is enough?. Journal of Experimental Psychology. General, 140(3). doi.org/10.1037/a0023956
