Spaced Repetition Interval Calculator
Calculate your next optimal review interval using the SM-2 spaced repetition algorithm — the same method used by Anki and SuperMemo.
How many times you have successfully reviewed this item (1 = first review)
Difficulty multiplier (1.3 = hardest, 5.0 = easiest; default 2.5)
How well you recalled the item (0–5 scale)
Number of days since the last review (0 if this is the first review)
Formula — SM-2 Algorithm
1. Update Easiness Factor (EF):
EF' = EF + [0.1 − (5 − q) × (0.08 + (5 − q) × 0.02)]
Minimum EF = 1.3
2. Calculate Next Interval I(n):
If q < 3 → reset: I = 1 day (start over)
If n = 1 → I(1) = 1 day
If n = 2 → I(2) = 6 days
If n > 2 → I(n) = I(n−1) × EF
Where q = response quality (0–5), EF = easiness factor, n = repetition number.
Assumptions & References
- Implements the SM-2 algorithm by Piotr Woźniak (SuperMemo, 1987), the most widely used spaced repetition algorithm.
- The default Easiness Factor is 2.5; it adjusts after each review based on response quality.
- EF is clamped to a minimum of 1.3 to prevent intervals from collapsing.
- A response quality of q < 3 (failed recall) resets the repetition counter to 1 and schedules a next-day review.
- Intervals are rounded to the nearest whole day.
- The algorithm assumes reviews happen on the scheduled day; early or late reviews may affect accuracy.
- Reference: Woźniak, P.A. (1990). Optimization of Learning. SuperMemo World. supermemo.com/sm2
- Used by: Anki, SuperMemo, Mnemosyne, and many other flashcard applications.