Spaced Repetition Calculator
The Algorithm Behind Every Flashcard App's "Due Today" List
When Anki, and before it SuperMemo, decide which flashcards to show you today, they're running a variant of an algorithm published by Piotr Wozniak in 1987 called SM-2. Its core insight is simple: the interval before your next review should grow each time you recall something successfully, and it should grow by a multiplier — the "ease factor" — that reflects how easy that particular item has been for you personally. This calculator runs the same interval math used under the hood of those apps.
The Formula
I(2) = 6 days
I(n) = I(n−1) × Ease Factor, for n > 2
The first two intervals are fixed regardless of ease factor. From the third successful review onward, each new interval is the previous interval multiplied by the ease factor — which defaults to 2.5 in the original SM-2 algorithm, but adjusts upward for easy recalls and downward (with a floor of 1.3) for difficult ones.
Where This Matters
- Understanding why intervals grow so fast — with the default 2.5 ease factor, an item reviewed six times can already be scheduled for review nearly 6 months out, which is why mature flashcard decks need far less daily review time than new ones.
- Tuning ease factor by item difficulty — a lower ease factor keeps a genuinely hard word coming back more often; a higher one lets an easy word drift further into the future before its next check.
- Manually scheduling review without an app — useful for planning a review calendar for material studied outside a spaced-repetition tool.
| Repetition | Interval |
|---|---|
| 1st successful review | 1 day |
| 2nd successful review | 6 days |
| 3rd successful review | 15 days |
| 4th successful review | 38 days |
| 5th successful review | 94 days |
| 6th successful review | 235 days |
These intervals assume every review is successful with an unchanged ease factor of 2.5; a missed or difficult recall resets or reduces the interval in most real implementations.
How to Use This Calculator
- Enter the repetition number (1, 2, 3, and so on) for this item.
- Enter the ease factor (defaults to 2.5, the SM-2 standard).
- For repetition 3 or higher, enter the previous interval in days.
- Select Calculate to get the next review interval.
Related Calculations
Curious how much is forgotten between reviews? See the Vocabulary Retention Calculator, or plan daily review load with the Flashcard Review Calculator.