About the Spaced Repetition Calculator
The Spaced Repetition Interval Calculator applies a simplified version of the SM-2 algorithm, the scheduling method behind flashcard tools like Anki, to figure out when a card should come back up for review. Rather than reviewing every card on a fixed daily schedule, it stretches the interval between reviews for cards recalled easily and shortens it for cards that were harder to remember. It is meant for students who want to understand or replicate spaced-repetition scheduling logic outside of a dedicated flashcard app.
How It Works
You enter the previous review interval in days, the card's current ease factor (a new card starts at 2.5), and a recall quality rating from 0 to 5, where 5 is perfect recall and 0 is a complete blackout. The calculator first adjusts the ease factor up or down based on how easy or hard that recall felt, then applies a set of interval rules to decide how many days until the next review, and finally reports both the new interval and the updated ease factor to use next time.
Formula & Methodology
Work through a card with a previous interval of 20 days, an ease factor of 2.3, and a quality rating of 5 (perfect recall). The ease adjustment term becomes 0.1 - (5-5) x (0.08 + 0) = 0.1, so the new ease factor is 2.3 + 0.1 = 2.4. Since the previous interval (20) is greater than 6, the next interval is round(20 x 2.4) = 48 days. A weaker recall of quality 3 on that same card computes a smaller ease adjustment, since the (5 - quality) term grows as quality drops, shrinking future intervals for cards found harder.
Examples
Consistent card at the 6-day step
With the default inputs (previous interval 6 days, ease factor 2.50, quality 4), the calculator returns a new ease factor of 2.50 unchanged and a next interval of 6 days, because a previous interval of 6 or less always maps to the fixed 6-day step regardless of the ease factor.
Mature card reviewed well
A card previously spaced 20 days apart, with ease factor 2.30 and a perfect quality-5 recall, produces a new ease factor of 2.40 and a next interval of 48 days (20 x 2.40 rounded), showing how intervals expand quickly once a card moves past the fixed early steps.
Advantages
- Lets you check or reconstruct exactly how a spaced-repetition scheduler arrived at a given interval, rather than treating an app's scheduling as a black box.
- Separates the two moving parts, the ease factor and the interval, so you can see how a single recall quality rating affects both.
- Useful for building a custom review system or spreadsheet, since the same formula used by mainstream flashcard apps is laid out explicitly.
Common Mistakes
- Entering a simple pass/fail rating instead of a full 0-5 quality score, which changes how much the ease factor moves compared to a nuanced quality rating.
- Assuming the interval always grows by multiplying the ease factor, when the first two post-reset reviews actually use fixed 1-day and 6-day steps first.
- Manually resetting the ease factor to 2.5 after one weak review, when the algorithm only nudges it down gradually and floors it at 1.3 rather than resetting it.
Edge Cases to Watch For
- Any quality rating below 3 resets the interval to 1 day regardless of how long the previous interval was or how high the ease factor is, since a failed recall means the card needs to be relearned soon.
- The ease factor can never drop below 1.3, a hard floor built into the calculation so a difficult card does not get scheduled with an ever-shrinking ease value.
- The first two successful reviews after a reset follow fixed steps, 1 day then 6 days, before the ease-based multiplication takes over on the third and later reviews.
- A quality value typed outside the 0-5 range gets clamped back into it before any math runs, so entering 7 is treated the same as entering 5.
Common Use Cases
- Students who want to build or verify their own spaced-repetition flashcard schedule outside of an existing app.
- Self-directed learners projecting out how many days until a specific card or fact will come up again for review.
- Anyone studying how algorithms like SM-2 work, using real numbers to see the ease-factor and interval math in action.