Spaced Repetition Interval Planner
Free Spaced repetition interval tool for learning & teaching tools. Enter values to see solutions, formulas, and educational explanations.
Formula
Interval(n) = Interval(n-1) x Easiness Factor
Where the first interval is 1 day, the second is 6 days, and subsequent intervals are calculated by multiplying the previous interval by the Easiness Factor (default 2.5). Retention probability follows an exponential decay: R = 2^(-t/S) where t is time since last review and S is stability. The forgetting half-life determines how quickly memories fade without reinforcement.
Worked Examples
Example 1: Language Vocabulary Learning
Problem: A student wants to learn 500 Spanish vocabulary words, studying 15 new cards per day with 30 minutes daily. Target retention is 90% with default easiness factor 2.5.
Solution: Days to learn all cards: 500 / 15 = 34 days\nReview intervals: Day 1, Day 6, Day 15, Day 38, Day 94, Day 235\nDaily reviews (steady state): 500 x 0.15 = 75 cards\nTotal daily cards: 15 + 75 = 90 cards\nMinutes per card: 30 / 90 = 0.33 min (20 seconds)\nEstimated mastery: 34 + 94 = 128 days
Result: 34 days to introduce all cards | 90 daily reviews at steady state | ~4 months to mastery
Example 2: Medical Board Exam Prep
Problem: A medical student has 2,000 anatomy flashcards, studies 40 new cards per day with 90 minutes daily. Target retention is 95% with easiness factor 2.0 (harder material).
Solution: Days to learn all cards: 2000 / 40 = 50 days\nReview intervals: Day 1, Day 6, Day 12, Day 24, Day 48, Day 96\nDaily reviews: 2000 x 0.15 = 300 cards\nTotal daily cards: 40 + 300 = 340 cards\nMinutes per card: 90 / 340 = 0.26 min (16 seconds)\nEstimated mastery: 50 + 48 = 98 days
Result: 50 days to introduce all cards | 300 daily reviews | ~3.5 months to mastery
Frequently Asked Questions
What is the SM-2 algorithm used in spaced repetition systems?
The SM-2 algorithm, developed by Piotr Wozniak in 1987, is the foundational algorithm behind many modern spaced repetition programs including Anki. It assigns each card an easiness factor starting at 2.5 and adjusts it based on how well the learner recalls the item. The first review occurs after one day, the second after six days, and subsequent intervals are calculated by multiplying the previous interval by the easiness factor. If a learner rates recall as difficult, the easiness factor decreases, shortening future intervals. If recall is easy, the factor increases, lengthening intervals. This adaptive approach ensures optimal spacing for each individual item.
What is the forgetting curve and how does spaced repetition counteract it?
The forgetting curve describes the exponential decline in memory retention over time without review. Hermann Ebbinghaus found that roughly 50% of newly learned information is forgotten within one hour, 70% within 24 hours, and 90% within a week without reinforcement. Spaced repetition counteracts this by scheduling reviews just before the expected forgetting point, each time resetting and flattening the curve. After each successful review, the memory becomes more resistant to forgetting and the next review can be delayed longer. Through repeated well-timed reviews, information transitions from fragile short-term memory to durable long-term memory over weeks and months.
How long does it take to master a set of flashcards using spaced repetition?
Mastery time depends on the number of cards, daily study volume, and target retention rate, but a general guideline is that cards reach long-term stability after 5 to 7 successful reviews spanning 2 to 4 months. For a deck of 1,000 cards studied at 20 new cards per day, initial learning takes about 50 days, but achieving stable long-term intervals requires an additional 2 to 3 months of reviews. After this period, most cards will have intervals of 30 days or longer, requiring minimal maintenance. Some particularly difficult cards may take 6 months or more to reach stable long intervals, while easy cards may stabilize within weeks.
What types of material work best with spaced repetition?
Spaced repetition works best with discrete facts that have clear question-answer pairs, such as vocabulary words, historical dates, anatomical terms, legal definitions, and mathematical formulas. It is highly effective for language learning, medical education, law studies, and any field requiring memorization of large fact sets. However, it is less effective for conceptual understanding, procedural skills, and creative problem-solving, which require different learning approaches. The most effective flashcards follow the minimum information principle, breaking complex topics into atomic facts. Cards should test one specific piece of knowledge rather than requiring elaborate answers.
How do leeches affect spaced repetition efficiency?
Leeches are cards that a learner repeatedly fails to remember despite multiple reviews, consuming disproportionate study time without achieving retention. In Anki, a card is flagged as a leech after 8 consecutive lapses by default. Leeches typically represent about 5 to 10 percent of a deck but can consume 30 to 40 percent of total study time. The best strategy for dealing with leeches is to reformulate them using simpler wording, add mnemonic devices or images, break them into smaller sub-facts, or add context clues. Simply forcing more repetitions on leech cards is usually ineffective because the underlying encoding is flawed.
Can spaced repetition be combined with other study techniques?
Yes, spaced repetition is most effective when combined with complementary techniques rather than used in isolation. Active recall testing through flashcards pairs well with elaborative interrogation, where learners explain why a fact is true. Interleaving different topics within a study session improves discrimination between similar concepts. The Feynman technique of explaining concepts in simple terms helps create better flashcard content. Pre-study techniques like reading textbook chapters or watching lectures provide the initial understanding that flashcards then help retain. Many successful learners use spaced repetition as the retention layer on top of deeper learning activities.