What Is Spaced Repetition? The Science of Never Forgetting (2026)
The Forgetting Curve
In 1885, German psychologist Hermann Ebbinghaus documented the forgetting curve — one of the most replicated findings in psychology. After learning new information, people forget it at a predictable rate: roughly 70% within 24 hours, 90% within a week, without any review.
But the forgetting curve has a property that Ebbinghaus also discovered: each time you successfully retrieve information from memory, the curve flattens. The next time you review that information, you'll remember it for longer before it starts to fade.
This is the mechanism behind spaced repetition: by reviewing material just before you'd forget it, you strengthen the memory trace and extend the time before the next review is needed. Over time, well-retained information requires review only once every few months.
Why Spaced Repetition Works
The scientific explanation involves several mechanisms:
1. The testing effect (retrieval practice) Attempting to retrieve information from memory — even if you fail — strengthens the memory trace more than re-reading the same information. Spaced repetition forces retrieval practice at each review session.
2. Distributed practice Memory consolidation happens during rest and sleep, not during study. Multiple short study sessions across time give your brain more consolidation cycles than one long session.
3. Desirable difficulty Reviewing material when it's getting hard to remember (just before forgetting) produces stronger memory encoding than reviewing when it's easy to remember. Spaced repetition systems time reviews to hit this optimal difficulty point.
4. Reduced interference Studying the same subject in one long block creates interference between similar memories. Spaced repetition breaks up sessions and reduces this effect.
Spaced Repetition vs. Cramming
Most students cram — studying intensively for short periods before an exam. Cramming produces adequate immediate recall (which is why it feels like it works), but the forgetting is rapid: most of what's crammed is gone within 48 hours.
Research comparing spaced practice to massed practice (cramming) consistently shows:
- Spaced practice produces 10-30% better performance on immediate tests
- Spaced practice produces 50-100%+ better performance on delayed tests (weeks later)
- Total time spent studying is typically lower with spaced practice for equivalent retention
The paradox: cramming feels more efficient because the information is fresh when you test yourself immediately. Spaced practice feels less efficient because retrieval is harder (the information is partially forgotten between sessions). That difficulty is the mechanism that makes it work.
How Spaced Repetition Systems Work
Modern spaced repetition apps (Anki, Knowt, RemNote) automate the scheduling using algorithms. You review a card and rate how easily you recalled it. The algorithm uses your response to calculate when to show you that card again.
The SM-2 algorithm (used by Anki for decades) assigns each card an "ease factor" based on your performance and schedules reviews accordingly. Cards you know well are shown less frequently; cards you struggle with appear more often.
The FSRS algorithm (Free Spaced Repetition Scheduler, now default in Anki) is more sophisticated. It models your memory mathematically, predicting the exact probability that you'll remember each card at any given time. It schedules reviews to maintain a target retention rate (typically 90%), adapting to your individual memory profile.
For practical purposes: both algorithms work dramatically better than manual review schedules.
How to Implement Spaced Repetition
Option 1: Use Anki (Most Powerful, Free)
Anki is the most widely used spaced repetition app and the gold standard for serious learners.
Setup:
- Download Anki desktop (free, ankiweb.net)
- Create decks organized by subject/course
- Add cards (front: question, back: answer)
- Review daily — Anki shows you only the cards due for review each day
For card creation without manual work: Use ChatGPT to generate cards, then import them:
Create 25 Anki flashcards on [topic].
Format: Front: [question requiring recall] / Back: [concise answer, 2 sentences max]
One concept per card. Include cards for commonly confused pairs.
Import via: File → Import → select your text file.
Daily review target: Complete all due cards each day. Don't skip days — the algorithm assumes you're reviewing regularly, and skipping creates large backlogs.
Option 2: Use Knowt (Easiest Setup, Free)
Knowt includes spaced repetition on the free tier with a much simpler interface than Anki. Upload your notes or paste text, and it auto-generates cards.
Best for students who want spaced repetition without learning Anki's interface.
Option 3: Manual Spaced Repetition (No App Required)
If you don't want to use an app, a simple manual system still outperforms cramming:
The Leitner Box system:
- Sort cards into 5 boxes
- Box 1: review daily
- Box 2: review every 2 days
- Box 3: review weekly
- Box 4: review bi-weekly
- Box 5: review monthly
When you get a card right, move it to the next box. When you get it wrong, move it back to Box 1.
Minimum viable spaced repetition: Study new material → review from memory the next day → review again in a week → review again in a month. Even this simple schedule dramatically outperforms cramming.
What to Put in Your Spaced Repetition System
Not everything belongs in Anki. Spaced repetition works best for:
High-yield content:
- Definitions and key terms
- Formulas and equations
- Dates, names, and specific facts
- Vocabulary (especially for language learning)
- Drug mechanisms and dosages (medical)
- Case names and rules (law)
Lower-yield content (use other methods):
- Conceptual understanding that requires explanation (use Feynman technique or ChatGPT)
- Complex processes with many steps (use diagrams)
- Arguments and essay structure (use writing practice)
The rule: if it can be expressed as a clean question-answer pair, it's a good Anki card. If explaining it properly requires a paragraph, use a different method.
Spaced Repetition for Different Subjects
Language Learning
Spaced repetition is most transformative for language learners. Vocabulary acquisition — the largest bottleneck for most language learners — is exactly the kind of content that benefits from systematic spaced review.
Pre-built community decks (JLPT Japanese, HSK Chinese, TOPIK Korean, DELE Spanish) are available on AnkiWeb and contain thousands of cards with audio, example sentences, and images. Most serious language learners use these rather than building from scratch.
For grammar and conversation, spaced repetition of specific sentence patterns works well:
- Front: English sentence with a grammatical challenge
- Back: Correct translation in the target language
See: How to Learn a Language with AI for a complete language learning system.
Medical School
Anki with the AnKing deck is the standard approach for USMLE Step 1 and Step 2. The deck contains 30,000+ cards, maintained by the medical school community, organized by First Aid and Pathoma.
Most medical students doing Anki properly review 200-500 cards per day. This sounds like a lot but takes 1-2 hours — and the retention it produces means much less re-studying of the same material before exams.
Undergraduate Courses
For typical college courses, a modest Anki system works well:
- Add 15-20 new cards per lecture
- Review due cards daily (takes 15-20 minutes once the system is running)
- Don't add cards for material you understand conceptually — only for specific facts you need to memorize
By the end of a semester, you'll have 500-1,000 cards per course with most of the semester's material actively maintained.
Professional Certifications
Bar exam, CPA, CFA, NCLEX — any high-stakes certification with large factual knowledge bases benefits from Anki. The long study timelines (6-12 months) mean the algorithm's compounding effect is pronounced.
Common Mistakes with Spaced Repetition
Making cards too complicated: One fact per card. Cards that require long explanations to answer correctly produce inconsistent results and are hard to review efficiently.
Making cards too simple: "What is the capital of France?" is not a useful Anki card if you already know it. Cards should test things you're actually uncertain about.
Not reviewing daily: The algorithm assumes daily review. Missing days creates large backlogs and disrupts the scheduling. Short daily sessions (15-30 minutes) beat long weekly sessions.
Adding too many cards at once: Adding 100 new cards in a day means 100 cards coming due for review a day or two later, then cascading backlogs. Add 15-30 new cards per day maximum.
Confusing recognition with recall: When reviewing a card, always attempt to answer before seeing the back. Peeking defeats the purpose — the algorithm needs your honest self-assessment of how well you recalled it.
Frequently Asked Questions
How long does spaced repetition take to set up? Anki takes 2-3 hours to learn properly. Knowt takes 15 minutes. Both are worth the investment if you'll use them consistently for more than a few weeks.
How many cards should I review per day? For most college students: 50-150 cards per day takes 20-40 minutes. For medical school: 200-500 cards per day. The exact number depends on how many cards you've added and how long you've been using the system.
Is spaced repetition useful for all subjects? Most useful for content that can be expressed as discrete question-answer pairs: vocabulary, definitions, formulas, dates, names. Less useful for conceptual understanding, argument construction, and skills requiring practice. Most subjects benefit from a combination of spaced repetition (for facts) and other methods (for understanding).
What's the difference between Anki and Quizlet for spaced repetition? Anki's FSRS algorithm is significantly more sophisticated than Quizlet's spaced repetition. For short-term exam prep (1-4 weeks), the difference is smaller. For long-term retention over months, Anki's algorithm produces meaningfully better results. For a full comparison, see: Anki vs Quizlet.
Can I use spaced repetition without an app? Yes — the Leitner Box system works manually. Any schedule that distributes review over time beats cramming. Apps automate the scheduling optimization, but the core mechanism works without technology.
How do I know if my spaced repetition system is working? Track your retention rate in Anki (aim for 85-90% correct on review). If you're below 80%, your cards may be too complex or you may need to add more context. If you're above 95%, you may be reviewing too frequently — the algorithm will adjust over time.
Ready to test whether you've retained what you just read? Try the How to Learn Effectively Quiz — it includes questions specifically on spaced repetition and retrieval practice. The Critical Thinking Quiz is also a good test of whether you can apply these principles rather than just recall them.