Spaced Repetition: How It Works, Plus a Schedule Calculator

Spaced repetition is a way of studying in which you review what you have learned several times, leaving a longer gap before each review. Each review makes you recall the material from memory before it has faded, and after each one it fades more slowly.

A simple schedule to start with is to review something on the day you learn it, then 1, 3, 7, 14, 30 and 90 days later. Enter what you learned below to turn that schedule into dates you can copy into a calendar or a planner.

Why Spaced Repetition Works#

Three findings from memory research explain why a few well-timed reviews beat many hours of rereading.

The forgetting curve#

In experiments he published in 1885, the German psychologist Hermann Ebbinghaus memorized lists of nonsense syllables and measured how much of each list he had kept after delays from twenty minutes to a month. He found that most of the forgetting happened soon after learning, and that the rest of the list faded much more slowly after that. In 2015, Jaap Murre and Joeri Dros repeated his experiment and found a curve of the same shape. The exact amounts depend on the material, the person and how memory is tested, so the drawing above shows only the shape.

The spacing effect#

Ebbinghaus also noticed that repetitions spread over several days fixed a list in his memory better than the same number of repetitions on a single day. That is the spacing effect, and later research has confirmed it many times. A 2006 review of hundreds of experiments found that spaced study led to better long-term memory than massed study, which is the same amount of study crammed into one session. A large study in 2008 found that the best gap before a review grows with how long you need to remember something. A gap of a day or two suits a test next week, while material you want to keep for a year is better reviewed weeks apart.

The testing effect#

A review works best when it is a test. Trying to recall an answer strengthens a memory more than reading the answer again. In a 2006 study, students who read a passage once and then tried to write down what they remembered recalled more of it a week later than students who spent the same time rereading it, although the rereaders did better on a test five minutes later. That is why spaced repetition uses flashcards and questions, and why it goes so well with active recall. A 2013 review of ten common study techniques rated spaced practice and practice testing as the two most useful.

Spaced Repetition Schedules Compared#

Every schedule leaves longer gaps as you go, but they differ in how fast the gaps grow and in what they were designed for.

The schedule on this page and O’Brien’s five reviews are both expanding schedules, with gaps set in advance. Pimsleur described his schedule in 1967 as an example for language teachers, and said no single schedule suits every word. Its first reviews come within minutes of each other. In a Leitner box, and in most flashcard apps, each card gets its own schedule from your answers, so the cards you find easy drop out of sight while the hard ones keep coming back.

The research on the spacing effect does not point to one perfect schedule, so treat any of these as a starting point. If you keep forgetting an item before its next review, shorten its gaps. If every review feels easy, you can stretch them.

Doing It on Paper With a Leitner Box#

Sebastian Leitner described a box for paper flashcards in his 1972 book So lernt man lernen (“How to learn to learn”). You don’t need a special box. Five envelopes or a shoebox with four dividers will do. Here is one way to set it up.

1. Label five sections 1 to 5. Decide how often you will study each one: section 1 every day, section 2 every two days, section 3 every four days, section 4 once a week and section 5 once every two weeks.
2. Write each new fact on a card, with a question on the front and the answer on the back, and put it in section 1.
3. On each study day, go through the sections that are due. Read the question, say or write the answer, and then turn the card over.
4. If you got it right, move the card up one section. If you got it wrong, move it back to section 1, however far along it was.
5. When a card in section 5 is right again, take it out of the box and keep it in a pile for an occasional check.

Cards you know well climb quickly and stop taking up your time, while the difficult ones stay in section 1 until they stick. Small cards make the habit easier to keep, because you can carry a day’s worth in a pocket. See mini flash cards for one way to make them.

Making Cards From What You Read#

Much of what is worth remembering turns up while you are reading, in an article, a textbook or a page of data. A few habits make it easy to turn that reading into cards.

• Write one fact per card. A card that asks for five things is hard to grade, and you will keep missing the same one of the five.
• Phrase the front as a question you could be asked, such as “In what year did the Battle of Hastings take place?”, rather than a heading like “Hastings”.
• When a page has a table, such as a list of capitals, dates or chemical symbols, you can turn it into cards without typing it out. Copy the table into a spreadsheet, keep two columns for the front and the back of each card, and save the sheet as a CSV file. Many flashcard apps can import a file like that as a whole deck.
• Add only what you understand. A card you have memorized without understanding is harder to recall and easy to confuse with its neighbors.

Collecting vocabulary while you read#

Reading in a language you are learning is one of the best sources of new words. Keep a running list with three columns: the word, the sentence you found it in, and its meaning. The sentence reminds you how the word is used, and it makes a better card than the word alone.

Some spreadsheet programs can translate a whole column of words for you, which saves time on a long list. Check each translation before you study it, since a word on its own can have several meanings and a machine will not know which one your sentence used. Then review the list on a schedule like the one above. To make the words stick from the first review, give each one a mnemonic, as described in how to memorize vocabulary words.

Spaced Repetition and Memory Palaces#

A memory palace stores information as images placed along a route through a place you know well. Reviewing a palace means walking through it in your mind, from the first location to the last, and recalling the image at each one. That walk is a recall test, so it gains from spacing as much as a stack of flashcards does.

Use the schedule above for each new palace. Walk it on the day you build it, then again after a day, three days, a week and so on. When an image is faint or missing, make it stranger or more vivid on the spot, and give that location an extra look on the next walk. After a few walks, most palaces can be left for weeks.

We offer software that combines spaced repetition scheduling and memory palaces. You can create memory palaces, add facts, and then review them using spaced repetition.

Common Mistakes#

• Cramming the reviews. Three reviews in one evening do not take the place of reviews on three different days. The gaps are what make the reviews work, so spread them out even when you have time to do more.
• Rereading instead of recalling. Look at the question, answer it from memory, and only then check. Reading the answer first feels productive, but it skips the part that strengthens the memory.
• Adding too many new cards. Every new item brings seven reviews, and six of them fall in the following three months. If you add 20 new items every day on the schedule here, by the end of the first month you will have about 100 reviews due each day on top of the new cards. Start small and add more once the daily reviews feel easy.
• Making cards too big. A long answer is hard to recall in one go and hard to grade honestly. Split it into several cards.
• Giving up after a missed day. A late review is still a review. Catch up as soon as you can rather than starting over.

Frequently Asked Questions#

Sources#

• Cepeda, N. J., Pashler, H., Vul, E., Wixted, J. T., & Rohrer, D. (2006). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin, 132(3), 354–380.
• Cepeda, N. J., Vul, E., Rohrer, D., Wixted, J. T., & Pashler, H. (2008). Spacing effects in learning: A temporal ridgeline of optimal retention. Psychological Science, 19(11), 1095–1102.
• Dunlosky, J., Rawson, K. A., Marsh, E. J., Nathan, M. J., & Willingham, D. T. (2013). Improving students’ learning with effective learning techniques: Promising directions from cognitive and educational psychology. Psychological Science in the Public Interest, 14(1), 4–58.
• Ebbinghaus, H. (1885). Über das Gedächtnis. Duncker & Humblot. Translated by H. A. Ruger and C. E. Bussenius as Memory: A Contribution to Experimental Psychology (1913).
• Leitner, S. (1972). So lernt man lernen. Herder.
• Murre, J. M. J., & Dros, J. (2015). Replication and analysis of Ebbinghaus’ forgetting curve. PLoS ONE, 10(7), e0120644.
• O’Brien, D. (2005). How to Develop a Brilliant Memory Week by Week. Duncan Baird.
• O’Brien, D. (2014, May 20). Every child can win the memory game. Edutopia.
• Pimsleur, P. (1967). A memory schedule. The Modern Language Journal, 51(2), 73–75.
• Roediger, H. L., & Karpicke, J. D. (2006). Test-enhanced learning: Taking memory tests improves long-term retention. Psychological Science, 17(3), 249–255.