Spaced Repetition: How to Remember What You Learn

The forgetting curve

In the late 19th century, German psychologist Hermann Ebbinghaus memorized lists of nonsense syllables and measured how long it took to relearn them after hours, days, and weeks. Published in 1885, his work revealed a pattern that became known as the forgetting curve: we lose much of what we learn within the first few hours and the first day. After that, forgetting continues, but more slowly. In his data, the savings in relearning had already dropped to about one-third after a day.

Ebbinghaus also observed the flip side: each review makes a memory last longer, and spacing repetitions over time worked better than doing the same number of repetitions all in one day. More than a century of research has confirmed this spacing effect with real-world material, including vocabulary, formulas, and concepts.

Why increasing intervals work

The idea behind spaced repetition is to review just before you’re about to forget. Reviewing too soon (the same day, while everything is still fresh) takes time without doing much to strengthen memory. Review too late, and you’re starting from scratch. At the right time, the effort of recalling information strengthens your memory and lets you wait longer before the next review.

Here’s a simple schedule if you study with a notebook:

1. Study the new material.
2. Review it the next day.
3. Review it again about a week later.
4. Then review it about a month later.
5. If you remember it well, increase the interval. If you forget it, go back to shorter intervals.

Compare that with cramming the night before: reading everything five times the night before may make you feel prepared the next day, but the material fades quickly. That’s a real problem for exams and other tests that cover months of material.

The Leitner system with boxes

In the 1970s, German journalist Sebastian Leitner popularized a way to put the idea into practice using paper flashcards and numbered boxes. Here’s a common setup:

Every new flashcard goes in box 1. Get it right, and it moves up a box. Get it wrong, and it goes back to box 1. That way, cards you know show up less often, while the ones you miss come up every day.

Apps automate the same process. With Spaced repetition flashcards, you rate each answer (Again, Hard, Good, or Easy): after the first correct answer, the card is scheduled for the next day; after the second, it’s scheduled for 6 days later. After that, the intervals keep getting longer, based on how easy you said it was.

Example: 30 biology terms in one month

Imagine you need to memorize 30 cell biology concepts for a test in four weeks. Here are two ways to spend roughly the same amount of time:

• All at once: Read the entire list several times the weekend before the test. It may seem like you know everything, but material you only reviewed once in one sitting tends to fade quickly.
• Spaced out: Create 30 flashcards in the first week (10 a day over three days) and review for 10 to 15 minutes a day. Cards you get right start showing up every few days; cards you miss come back the next day. By test week, only the concepts you find truly difficult need daily review.

In the second approach, about the same amount of study time leads to longer-lasting recall, and you can also see exactly which concepts are your weak spots.

How to make effective flashcards

• One idea per card. “Causes of the French Revolution” is a topic, not a flashcard. Break it into smaller questions.
• Ask questions that make you recall, not recognize. “Which organelle produces ATP?” is better than “Does the mitochondrion produce ATP? (yes/no).”
• Use your own words and add an example when it helps: “Example of a concessive adverbial clause.”
• Use flashcards for things you need to remember: formulas, dates, concepts, grammar rules, and legal provisions. To practice problem-solving, pair them with practice questions.
• Review every day, even if only for a little while. Ten minutes a day is more effective than an hour on Sunday because it follows the review intervals.
• Don’t add too many new cards at once. Every new card creates reviews in the days ahead, so start with 10 to 20 a day.

Sources: Ebbinghaus, Memory: A Contribution to Experimental Psychology (1913 translation of the 1885 original); Dunlosky et al. (2013), Psychological Science in the Public Interest, which rated distributed practice as one of the most useful study techniques. Updated October 2026.

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