SM-2 Calculator: Simulate Spaced Repetition Intervals

Rate a flashcard review from 0 to 5 and watch the SM-2 algorithm calculate the new ease factor, the next interval, and the next review date. It uses exactly the same formula WeaveWords uses to schedule your words, and the same one Anki was built on.

How to read the results

• Ease — the multiplier SM-2 applies to the interval. It starts at 2.5, rises slightly when you rate a card 5, drops when you rate it 3 or lower, and never goes below 1.3.
• Interval — days until the next review. The first two successful reviews are always 1 and 6 days; after that, the previous interval is multiplied by the ease.
• A rating below 3 counts as a failure: the card restarts at a 1-day interval, and its ease drops, so it keeps coming back more often even after you relearn it.

The formula

With q the rating, n the number of successful reviews in a row, I the interval in days, and EF the ease factor:

if q >= 3:
    n == 0 → I = 1
    n == 1 → I = 6
    else   → I = round(I × EF)
    n = n + 1
else:
    n = 0, I = 1
EF = max(1.3, EF + (0.1 − (5 − q) × (0.08 + (5 − q) × 0.02)))

Want the why behind each step? Read The SM-2 Algorithm Explained.