Prep
What the Leitner system is
The Leitner system is a way of running flashcards so that the cards you know well come up rarely and the cards you struggle with come up often. It is named after Sebastian Leitner, a German science writer who described it in a popular book on learning in the 1970s.
The rules fit on a sticky note:
- All new cards start in box 1.
- Each box has a review interval. Box 1 is reviewed most often, box 5 least often.
- When you review a card and get it right, it moves up one box.
- When you get it wrong, it goes back to box 1, however high it was.
That's it. Paper cards and shoeboxes work fine. Most flashcard apps use some descendant of the same idea, with the box logic hidden behind an algorithm.
| Box | Review how often | Card arrives here when | If you answer correctly | If you answer wrong |
|---|---|---|---|---|
| Box 1 | Every study day | New, or missed in any box | Move to box 2 | Stay in box 1 |
| Box 2 | Every 2 days | Correct once from box 1 | Move to box 3 | Back to box 1 |
| Box 3 | Twice a week | Correct from box 2 | Move to box 4 | Back to box 1 |
| Box 4 | Once a week | Correct from box 3 | Move to box 5 | Back to box 1 |
| Box 5 | Every 2 weeks | Correct from box 4 | Stay in box 5 (or retire after 2 correct reviews) | Back to box 1 |
Is the Leitner system backed by research?
Directly, not much. We found little controlled research testing the Leitner system itself against other flashcard schedules, and no evidence that its particular intervals are optimal. Be wary of anyone who says "studies prove the Leitner box".
What the system does is enforce two principles that have strong evidence behind them.
Retrieval practice: every review is a test
A flashcard only works if you answer before you flip it. Done that way, each review is retrieval practice. A review by Roediger & Butler (2011) concluded that retrieval practice often produces large gains in long-term retention compared with repeated studying, and a meta-analysis by Adesope et al. (2017) found practice tests beat restudying and every other comparison condition. The review by Dunlosky et al. (2013) rated practice testing as high utility.
Spacing: easy cards wait longer
Moving known cards to less frequent boxes spreads their reviews out. The meta-analysis by Cepeda et al. (2006) (317 experiments) showed spaced study beats massed study, and that the best gap grows with the time until the test. A 2025 classroom meta-analysis by Mawson et al. found a moderate benefit of distributed practice, d = 0.54: the typical learner who spaced practice scored about half a standard deviation higher than one who massed it.
The most relevant study: flashcards specifically
Kornell (2009) tested spacing with flashcards directly. Studying one large stack, which spaces each card's repetitions, beat studying four small stacks one after another, and spacing also beat cramming on the last day. Spacing won for 90% of participants, but after the first session 72% believed massing had worked better. Leitner boxes protect you from that wrong intuition by making the schedule decide, not your feelings.
A 5-box Leitner schedule you can copy
The table below gives a common 5-box setup for a certification exam four to twelve weeks away. The intervals are a sensible convention, not research-derived values. If your exam is closer, shrink them; if it is further away, stretch the top boxes.
A simple way to run it with a calendar:
- Every study day: review box 1.
- Every second day: also review box 2.
- Twice a week (for example Monday and Thursday): also review box 3.
- Once a week (for example Sunday): also review box 4.
- Every second Sunday: also review box 5.
Always review the higher boxes first on days they are due, so the cards you are close to mastering get your fresh attention.
Worked example: one card's journey
Take one Security+ card: "Which concept means a sender cannot later deny having sent a message?" (Answer: non-repudiation.)
- Day 1, box 1: you write "integrity". Wrong. It stays in box 1.
- Day 2, box 1: you answer "non-repudiation". Right. It moves to box 2.
- Day 4, box 2: right again. Box 3.
- Day 7, box 3: you hesitate, then get it. It counts as right, but note the hesitation. Box 4.
- Day 14, box 4: you answer "authentication". Wrong. Straight back to box 1.
- Days 15 to 30: it climbs again, this time faster, because you now know exactly what you confused it with.
Two things happened that rereading would not show you. First, the card dropped back after two weeks, which is the forgetting the system is designed to catch. Second, the wrong answer told you something useful: you mix up non-repudiation with integrity and authentication. That is a concept gap, not a fact gap. Write a short explanation of how the three differ, then keep the card in rotation.
Notice also how little time the card cost once it was learned: a few seconds every week or two, instead of a few seconds every day.
Paper boxes or an app?
Most popular flashcard apps schedule cards with their own algorithms, which descend from the same idea as Leitner boxes but adjust intervals card by card. We are not aware of strong evidence that one app algorithm beats a well-run paper system for exam prep. Choose based on what you will actually use daily. Whichever you pick, the same two rules decide whether it works: answer before you look, and grade yourself honestly.
Rules that make it work
- Answer out loud or on paper before flipping. Recognising the answer after you see it is not retrieval.
- Grade yourself strictly. Half right is wrong; the card goes back to box 1. In Dunlosky & Rawson (2012), students practised recalling definitions and judged their own answers; those whose judgements were more accurate retained more two days later. Lenient grading moves cards up before they are learned.
- One idea per card. "Port 22: SSH" is a card. "All remote access protocols and their ports" is a list that needs splitting.
- Keep the stack large rather than many tiny decks. Kornell's results suggest small stacks reduce spacing between repetitions.
- Write cards as questions you might be asked. "Which protocol provides encrypted remote command-line access?" beats "SSH = ?" because it matches how the exam asks.
Limits: what flashcards cannot do
The Leitner system is excellent for discrete facts: ports, acronyms, definitions, formulas, process names. It is weak for the scenario questions that make up much of Security+, CCNA, PMP and AWS exams, where you must apply ideas to a situation you have not seen.
For those, pair your flashcards with practice questions and explanation-based methods such as the Feynman technique or elaborative interrogation. Mixed practice questions also bring in interleaving, which flashcards sorted by topic do not.
One more limit: the system only spaces the cards you remember to put in it. Topics that never make it onto a card get no review at all. Spacing research stresses that reviews should be cumulative; Cepeda et al. (2009) found cumulative reviews matter for retention over months. So once a week, also answer a mixed set of practice questions from every domain you have covered so far, not just your flashcards.
How to use this for a certification exam
- Build the deck from the objectives. Go through the official exam objectives and make cards only for facts the exam can test. Aim for quality over volume.
- Add cards from your misses. Take a free practice test, such as the CCNA practice test or the AZ-900 practice test. Read each explanation and turn any fact you did not know into a box-1 card.
- Run the 5-box schedule daily, 15 to 20 minutes, before your main study block.
- Every two weeks, retest. Retake a practice test. Facts behind any miss go back to box 1, even if their cards were in box 5.
- Final week: review boxes 1 to 3 daily and do one pass through boxes 4 and 5. Do not add hundreds of new cards in the last days.
To understand the timing logic in more depth, read our guides to spaced repetition and the forgetting curve, or go back to the study science overview.