🧠 Spaced Repetition Calculator
Understand the SM-2 algorithm, visualize the forgetting curve, and calculate how many flashcards you need per day to reach your vocabulary goal.
Interactive SM-2 Demo
Review History
| # | Rating | Interval | Ease Factor | Next Review |
|---|---|---|---|---|
| Rate the card above to see history... | ||||
Cramming vs Spaced Repetition
The forgetting curve shows how quickly memories decay without review. Spaced repetition resets the curve at optimal intervals, dramatically improving long-term retention.
Retention at Various Ease Factors
Ease Factor (EF) determines how quickly intervals grow. Higher EF = faster interval growth = fewer reviews needed.
| Ease Factor | After 5 reviews | Interval grows | Weekly reviews (1000 cards) |
|---|---|---|---|
| 1.3 (minimum) | ~30 days | Slowly | ~85/week |
| 2.0 | ~60 days | Moderately | ~55/week |
| 2.5 (default) | ~100 days | Well | ~40/week |
| 3.0 | ~180 days | Quickly | ~28/week |
Daily Cards Calculator
Your Plan
Deck Size Planner
How many new cards per day do you need to add to reach a vocabulary target by a specific date?
Language Vocabulary Benchmarks
SRS App Comparison
SRS Effectiveness by Use Case
How the SM-2 Algorithm Works
SM-2 (SuperMemo 2) was developed by Piotr Wozniak in 1987. Each card has an ease factor (EF) starting at 2.5. When you review a card, the next interval is calculated as: interval Γ EF. Rating "Again" resets the interval; "Easy" gives a bonus multiplier.
The first two reviews use fixed intervals (1 day, then 6 days for "Good"). After that, the algorithm uses the EF to grow intervals exponentially. This matches Ebbinghaus's forgetting curve research β reviewing just before you forget is the most time-efficient way to learn.