Flashcard Spaced Repetition Calculator

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.