Spaced Repetition (Anki) Interval Predictor Calculator

The SM-2 algorithm (SuperMemo 2) by Piotr Wozniak calculates optimal review intervals based on your feedback. When you review a card, you rate it on a 4-point scale: Again (1) — forgot, card returns to step 1. Hard (2) — recalled with difficulty, interval increases slightly. Good (3) — recalled correctly, interval increases normally. Easy (4) — very easy, interval increases significantly. The algorithm adjusts intervals based on: (1) Ease Factor (EF): starts at 2.5, increases with Easy/Good, decreases with Hard/Again. (2) Interval calculation: first success: 1 day, second: 6 days, third: EF × previous interval (e.g., 2.5 × 6 = 15 days), subsequent: interval × EF. (3) Minimum interval: 1 day (relearning). Our calculator uses a simplified model of this algorithm to predict daily workload. Real Anki intervals will vary based on your individual recall performance.

Recommendations by workload: Light study (30 min/day): 10-15 new cards/day — sustainable long-term, good for language learning. Moderate study (1 hr/day): 20-30 new cards/day — Anki default is 20, most common. Heavy study (2+ hrs/day): 30-50 new cards/day — medical students often do 100+ but this is intense. The formula for setting new cards: NewPerDay = SustainableMinutes / ( ReviewTimePerCard × AverageReviewsPerCard × 1.5 ). A good rule: your total daily review time (new + review cards) should not exceed your available study time. Most Anki users find that their daily review count stabilizes at 5-10× their new card count after 1-2 months. Starting too high leads to review overload and abandonment. Better to start at 15 and increase after 2 weeks if workload feels light.

The mathematically optimal retention rate is a trade-off between retention and efficiency. Anki's default target is 90% — research and Anki creator interviews confirm this as the sweet spot for most learning. Here is the trade-off: 95% retention: requires ~40% more reviews than 90% for only 5% more retention. Useful for: medical students (lives depend on recall), law (exact terminology), language learners (high frequency words). 90% retention: most efficient — best balance of retention vs time investment. Recommended for most users. 85% retention: requires ~25% fewer reviews. Acceptable for: concepts you can reconstruct, lower-stakes material, pre-exam cramming. 80% retention: minimizes review time but only 80% of cards retained. Only suitable for: temporary decks, low-priority material, or familiar subjects. Default to 90% unless you have a specific reason to change.

The number of daily reviews depends on: (1) Total cards in deck, (2) New cards per day, (3) Retention rate, (4) Time since starting. The daily review count follows a predictable curve: Initial phase (first 2 weeks): very few reviews — mostly learning new cards. Growth phase (weeks 2-6): reviews accumulate as intervals are short (1-7 days). Stabilization phase (week 6+): reviews plateau at 5-10× daily new card count as intervals lengthen. Actual numbers for 20 new cards/day at 90% retention: Week 1: ~30 reviews/day, Week 4: ~100 reviews/day, Week 8: ~150 reviews/day, Stabilized: ~120-180 reviews/day (about 15-25 min at 8 sec/card). For 50 new cards/day (med school level): Week 1: ~75 reviews, Week 4: ~250 reviews, Stabilized: ~300-450 reviews/day (40-60 min). The key insight: reviews stabilize — they do not grow indefinitely because mature cards reach long intervals (30-365+ days).