You have a stack of 200 flashcards for your biology exam. Reviewing all 200 every single day is tedious and wasteful — you already know most of them, and the few you struggle with get lost in the pile. What if your cards could sort themselves so you spent the most time on the ones you need the most help with?
That is exactly what the Leitner system does. Invented in the 1970s, it is one of the simplest and most effective flashcard methods ever devised. All you need is a set of cards and five boxes (or dividers, or envelopes). The system handles the rest — automatically giving you more practice on difficult cards and less on easy ones.
What Is the Leitner System?
The Leitner system is a flashcard-based study method that sorts cards into groups based on how well you know them. Cards you struggle with get reviewed every day. Cards you know well get reviewed once a week or less. The result is a self-adjusting review schedule that focuses your limited study time where it matters most.
The system was developed by Sebastian Leitner, a German science journalist, and published in his 1972 book So lernt man lernen (Learning to Learn). Leitner was not a psychologist or a neuroscientist — he was a journalist who studied the research on memory and designed a practical system that anyone could use with index cards and a shoebox.
His insight was elegant: instead of treating all flashcards equally, sort them by difficulty. Give difficult cards the most review time and easy cards the least. This simple rule, applied consistently, produces a spaced review schedule that closely mirrors what cognitive scientists now know is optimal for long-term retention.
How the Leitner System Works
The system uses five boxes (or compartments), each with a different review frequency:
- Box 1 — Review every day (new and difficult cards)
- Box 2 — Review every 2 days
- Box 3 — Review every 4 days
- Box 4 — Review every week
- Box 5 — Review every 2 weeks (well-known cards)
The rules are simple:
- All new cards start in Box 1.
- When you answer a card correctly, move it to the next box. A card in Box 1 moves to Box 2. A card in Box 3 moves to Box 4.
- When you answer a card incorrectly, move it back to Box 1 — regardless of which box it was in.
- Only review the box that is scheduled for that day. On most days, you review Box 1. Every second day, you also review Box 2. Every fourth day, Box 3. And so on.
The Leitner System: Five-Box Method
Cards move forward when answered correctly, back to Box 1 when answered wrong
Key insight: You spend most time on cards you find difficult, and barely any time on cards you already know.
A quick example: You are learning Spanish vocabulary. The word biblioteca (library) starts in Box 1. You review it today and recall it correctly, so it moves to Box 2. Two days later, Box 2 comes up for review. You get biblioteca right again — it moves to Box 3. Four days later, you review Box 3 but stumble on biblioteca. It goes all the way back to Box 1, where you will see it again tomorrow.
This cycle continues until cards reach Box 5 and stay there. A card that consistently lands in Box 5 is one you truly know — you have recalled it successfully at every interval.
Why the Leitner System Works
The Leitner system is effective because it naturally incorporates three principles that cognitive science has identified as critical for memory:
1. The Spacing Effect
The spacing effect is the well-established finding that spreading study sessions over time produces far stronger memory than cramming. The Leitner system builds spacing directly into its structure: cards in higher boxes are reviewed at longer intervals, giving your brain time to consolidate each memory between sessions.
2. Active Recall
Every flashcard review is an active recall exercise. You see the prompt and must produce the answer from memory before checking. This retrieval effort is what strengthens the memory — not passively re-reading the answer. The Leitner system ensures you perform retrieval practice on every single card, every single session.
3. Difficulty-Based Focus
The system automatically allocates more practice time to the cards you find hardest. Cards that keep returning to Box 1 get reviewed daily. Cards that progress quickly through the boxes get reviewed less frequently. This means your study time is spent where it has the highest return — on the material you have not yet mastered.
4. Self-Correcting Feedback
The penalty for getting a card wrong — moving it back to Box 1 — is deliberately harsh. This prevents cards from drifting into higher boxes before you truly know them. It also provides immediate feedback: if a card keeps bouncing back to Box 1, that is a signal to spend extra effort understanding it, not just memorising the answer.
The Leitner System vs. Modern SRS Algorithms
The Leitner system is a manual form of spaced repetition. Modern apps use more sophisticated algorithms to achieve the same goal. Here is how they compare:
The Leitner system uses fixed intervals. All cards in Box 3 are reviewed every four days, regardless of whether a particular card is slightly easier or harder than the others in that box. The intervals are coarse-grained — a card is either in a box or it is not.
Modern algorithms use adaptive intervals. Apps like Sticky calculate a unique review interval for each individual card based on your full performance history. A card you have struggled with three times gets a shorter interval than one you have always answered correctly, even if both are in the same "difficulty tier." Algorithms like SM-2 (used by Anki) and FSRS use mathematical models to predict the optimal moment for each review.
When the Leitner system is the better choice:
- You prefer physical flashcards — the tactile experience of shuffling real cards
- You have a small deck (under 100 cards) where coarse intervals work fine
- You want a system that requires zero technology
- You are teaching the concept of spaced repetition to someone new
When algorithm-based apps are the better choice:
- You have a large collection (hundreds or thousands of cards)
- You want precision scheduling tailored to each card
- You want to track performance statistics over time
- You do not want to manage physical boxes and schedules
In practice, the Leitner system and modern SRS apps are built on exactly the same principle: review harder material more often and easier material less often. The apps are simply more precise about it.
Setting Up Your Own Leitner Box
You can build a working Leitner system in five minutes with materials you already have:
Option 1: Shoebox with dividers. Take a shoebox or any small box. Cut five dividers from cardboard and label them Box 1 through Box 5. Place all your flashcards behind the Box 1 divider.
Option 2: Labelled envelopes. Use five envelopes labelled 1 through 5. All new cards go in Envelope 1. As you review, move cards between envelopes.
Option 3: Binder with sections. A small binder with five tabbed sections works well for larger card collections.
Making your cards: Write one fact, term, or question per card. Keep the prompt on the front and the answer on the back. Do not cram multiple facts onto one card — atomic cards are easier to review and produce better retention.
Daily routine: Each day, review Box 1 completely. Check your calendar to see which other boxes are due (Box 2 every two days, Box 3 every four days, etc.). A typical daily session takes 10 to 20 minutes depending on how many cards are in Box 1.
Common Mistakes
The Leitner system is simple, but a few common errors can undermine it:
Moving cards forward when you are unsure. Be honest with yourself. If you had to guess or the answer came slowly, the card should stay in its current box or move back. Only promote cards when recall is quick and confident.
Reviewing all boxes every day. The whole point is that higher boxes have longer intervals. If you review every box daily, you lose the spacing benefit and waste time on cards you already know.
Starting with too many cards. Adding 200 cards on day one means 200 reviews tomorrow. Start with 20 to 30 cards and add 10 to 15 new cards each day as older cards progress into higher boxes.
Ignoring the back-to-Box-1 rule. When you get a card wrong, it goes back to Box 1 — even if it was in Box 4. This feels harsh, but it is what makes the system work. Skipping this rule lets shaky cards slip through to higher boxes where they will not get enough review.
Getting Started Today
The Leitner system is one of the fastest study methods to set up and one of the most effective to use. Here is how to start in the next five minutes:
Step 1: Pick one subject you need to study — a chapter, a vocabulary list, or a set of definitions.
Step 2: Make 20 flashcards with one question or term per card.
Step 3: Set up five sections (envelopes, dividers, or piles) and put all cards in Section 1.
Step 4: Review Section 1 tomorrow. Move correct cards to Section 2, keep incorrect cards in Section 1.
Step 5: Keep going. Within a week, your cards will naturally sort themselves by difficulty.
If you prefer to skip the physical setup, Sticky applies the same core principle — spending more time on what you find difficult — using an adaptive algorithm that handles the scheduling automatically. You get the benefits of the Leitner method with none of the manual sorting.
The beauty of the Leitner system is its simplicity. Five boxes, two rules, and ten minutes a day. That is all it takes to turn a pile of flashcards into a system that genuinely works.
