Flashcard Spaced Repetition Calculator
Spaced repetition stretches the gap between reviews each time you recall a flashcard, so you study just before forgetting and need far fewer sessions than cramming. This calculator models the common geometric schedule: each interval equals the one before it multiplied by an ease factor you choose. Enter the starting interval, the ease factor, and how many repetitions you plan, and it returns the final interval, the cumulative study span, and the average gap. Adjust the ease factor to compare aggressive and gentle schedules.
Spaced repetition formula
interval(n) = starting interval * ease ^ (n - 1)
final interval = starting interval * ease ^ (reps - 1)
total span = sum of all intervals (geometric series)
total span = start * (ease^reps - 1) / (ease - 1) when ease != 1
average interval = total span / reps
The schedule is a geometric series: each interval is the previous one times the ease factor. When the ease factor is exactly 1, every interval equals the starting interval and the span is start times reps.
Spaced repetition facts
- Expanding intervals exploit the spacing effect, a long-studied memory phenomenon.
- Popular software often starts cards at an ease factor near 2.5.
- Correct recalls increase the interval; lapses reset or shorten it.
- Spaced review needs far fewer sessions than massed cramming for the same retention.
- The Leitner box system is a physical card-based form of spaced repetition.
Spaced repetition: frequently asked questions
What is spaced repetition?
Spaced repetition is a study method where the gap between reviews of a flashcard grows each time you recall it correctly. The expanding intervals fight the forgetting curve, so you review material just before you would forget it. It is the principle behind tools like Anki and Leitner box systems.
How is the next interval calculated?
In a simple geometric scheme, each new interval equals the previous interval times an ease factor. If the starting interval is 1 day and the ease is 2.5, the intervals run 1, 2.5, 6.25, 15.6 days, and so on. This calculator uses that geometric growth and reports each interval plus the cumulative study span.
What is a typical ease factor?
Popular spaced-repetition software often starts cards with an ease factor of around 2.5, meaning each successful review roughly multiplies the gap by 2.5. The exact value rises or falls based on how easily you recall each card. This calculator takes the ease factor as an input so you can model different schedules.
Why do intervals grow exponentially?
Memory of well-learned material decays more slowly over time, so each successful recall lets you safely wait longer before the next review. Multiplying the interval by a fixed factor each time produces exponential growth, which keeps the number of reviews low while maintaining retention.
How many reviews do I need?
It depends on your retention target and the card's difficulty, but spaced repetition needs far fewer reviews than massed cramming for the same long-term recall. Enter the number of repetitions to see how the intervals stretch out and how many total days the schedule spans.
Official sources
- U.S. National Library of Medicine (PubMed): Spacing effect and distributed practice research.
- U.S. National Center for Education Statistics: Learning and study research resources.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.