Flashcard Spaced Repetition Calculator
Calculate your optimal flashcard review schedule using spaced repetition.
Get review intervals for new, learning, and mature cards based on the SM-2 algorithm.
Spaced repetition is one of the most effective learning techniques backed by cognitive science. It exploits the “spacing effect” — memories are stronger and longer-lasting when you review material at increasing intervals over time, rather than cramming it all at once.
The most widely used algorithm is SM-2 (SuperMemo 2), developed by Piotr Wozniak in 1987. This algorithm forms the basis of popular apps like Anki, Duolingo, and many others.
How SM-2 works:
Each card has two key values:
- Interval (I): how many days until the next review
- Ease factor (EF): a multiplier that adjusts the interval based on how easily you remembered the card (starts at 2.5)
Review schedule with SM-2:
- First correct answer: review again in 1 day
- Second correct answer: review again in 6 days
- Third and subsequent: next interval = current interval × EF
After each review, you rate how well you remembered it (0–5 scale):
- 5: Perfect response
- 4: Correct with slight hesitation
- 3: Correct with difficulty
- 2: Incorrect — would have remembered
- 1: Incorrect — hint helped
- 0: Complete blackout
If you rate 0–2, the card resets to day 1 (goes back to the beginning). If you rate 3–5, the interval grows according to the formula.
The ease factor updates after each review: EF_new = EF_old + (0.1 − (5 − grade) × (0.08 + (5 − grade) × 0.02))
This keeps well-known cards with long intervals and keeps hard cards appearing frequently.
Practical implications:
- A card reviewed 5 times with all grade-4 answers would have intervals of: 1, 6, 14, 35, 89 days.
- After 1 year, mature cards may appear only once every 3–12 months.
- Studying 20 new cards per day leads to ~100–150 reviews per day at steady state.
This calculator simulates future review dates given the current stage, review history, and ease factor.