🔁 Spaced Repetition Interval Calculator
Enter your current repetition number, previous interval, ease factor, and how well you recalled the card to calculate your next review interval using the SM-2 spaced repetition algorithm.
The SM-2 intervals this tool uses
Ease is updated first, then the interval is chosen from the repetition number:
EF' = EF + (0.1 − (5 − q) × (0.08 + (5 − q) × 0.02)), floor 1.3
q < 3 → interval 1 day, repetition resets to 1
repetition 1 → 1 day
repetition 2 → 6 days
repetition 3+ → round(previous interval × EF')Only quality 5 raises ease, by 0.10; a 4 leaves it exactly where it was; a 3 costs 0.14, taking 2.50 down to 2.36. Work one forward: repetition 3, a 6-day interval, ease 2.50 and a quality 4 gives round(6 × 2.50) = 15 days. Answer the next one a 5 and ease becomes 2.60, so the gap widens to round(15 × 2.60) = 39 days.
Reading the inputs
- Previous interval is ignored below repetition 3. The first two steps are fixed at 1 and 6 days whatever you type.
- A lapse costs the schedule, not the ease alone. Quality 0 to 2 drops you to a 1-day interval and repetition 1, but the reduced ease is kept, so the card climbs back more slowly than it fell.
- Ease never goes below 1.30, so even a card you keep failing has intervals that grow by 30% each time.
Frequently asked questions
Why is the second review always 6 days later?
SM-2 hard-codes the first two intervals at 1 and 6 days. Ease only starts multiplying from the third repetition, so a new card follows the same opening schedule no matter how easy it feels.
What happens to a card I get wrong?
Any quality below 3 sends it back to a 1-day interval and repetition 1. Its ease keeps whatever penalty the answer earned, so a card failed repeatedly ends up near the 1.30 floor and stays in frequent rotation.
What does an ease factor of 2.5 mean?
It is the multiplier applied to the previous gap: at 2.5 each successful review makes the next wait two and a half times as long. 2.5 is the standard starting value.