The Leitner System: Five Boxes That Schedule Your Reviews

The Leitner system is spaced repetition you can build with a shoebox. A German science journalist, Sebastian Leitner, described it in his 1972 book So lernt man lernen (How to learn to learn). The idea is simple: cards you know well should come back rarely, and cards you keep missing should come back often. A row of boxes does the scheduling for you.

This guide gives you a five-box setup you can copy, a 16-day calendar, the two versions of the rules, and a way to fit the gaps to your exam date.

The real benefit appears gradually, over several weeks of consistent sessions, as the deck sorts itself according to difficulty. Easy cards climb quickly and stop eating your time. Hard cards stay in Box 1 and get daily practice until they stick. You spend most of your effort where your memory is weakest, which is the whole point of spaced repetition.

Leitner's original design and later versions vary. Wikipedia's summary of the Leitner system shows a three-box example with reviews every day, every three days and every five days. The doubling schedule below is one practical version for a semester-long course. Treat the numbers as a starting point, not a law.

The 16-day calendar

To avoid calculating dates every evening, number your study days consecutively and follow this repeating pattern. Box 1 every day. Box 2 on even days. Box 3 on days 4, 8, 12 and 16. Box 4 on days 8 and 16. Box 5 on day 16. Then start again at day 1. Always review the higher boxes first on days when several are due, so the long-gap cards get your freshest attention.

The strict rule is honest: if you forgot a card after a sixteen-day interval, the memory is still fragile and deserves daily attention again, even though this means Box 1 can quickly become crowded. The gentle rule keeps the workload smoother, but a card can hover in the middle boxes for weeks without ever being truly secure. A good compromise is strict for Boxes 1 to 3 and gentle for Boxes 4 and 5.

The cards themselves matter as much as the boxes. A card that asks for five facts will bounce in and out of Box 1 forever. Write one idea per card with a clear question; our guide on how to make flashcards shows the rules with examples.

A large study by Cepeda and colleagues tested gaps and delays with more than 1,350 people. As a share of the time until the final test, the best gap fell from about 20 to 40% for a test one week away to about 5 to 10% for a test one year away. For a student, that turns into a simple rule: if your exam is in two weeks, cap your longest box at a few days. If it is a semester away, the full 16-day schedule is fine.

• Exam in 1 to 2 weeks: use three boxes, every day, every 2 days, every 4 days.
• Exam in 3 to 6 weeks: use four boxes, up to every 8 days.
• Exam months away or a cumulative final: use all five boxes and keep Box 5 cards in rotation until the exam.

Also watch Box 1. If it holds more than about 30 cards, stop adding new ones until it shrinks. That is the paper version of the advice in how many flashcards per day: new cards create future reviews, and a flooded Box 1 means the system has stopped spacing anything.

One more practical consideration: the Leitner system depends entirely on your honesty when grading. Counting an approximately correct answer as correct feels harmless, but it promotes cards that are not yet reliable and gradually fills the upper boxes with material you cannot actually produce under exam conditions. Decide in advance what counts as correct, such as the exact term or every step of a definition, and apply that standard consistently.

Spacing is what matters most. In a 2009 study by Kornell, spaced flashcard practice beat massed practice for 90% of participants, even though most of them believed the opposite. Any method that gets you spacing reliably will help. If you are unsure which format suits you, compare paper and digital flashcards.