Spaced Repetition Interval Calculator
Spaced repetition spreads reviews over growing intervals so you revisit each fact just before you would forget it. The SM-2 algorithm, the openly documented method behind many flashcard apps, computes the next interval from the current repetition number, the previous interval, the item's ease factor, and how well you recalled it this time. This calculator applies the SM-2 rules exactly: the first review waits a day, the second six days, and later intervals multiply by the ease factor, which itself adjusts each review. A recall quality below 3 counts as a lapse and resets the cycle. The formulas are fixed, so the schedule is fully deterministic.
SM-2 formulas
new EF = EF + (0.1 - (5 - q) * (0.08 + (5 - q) * 0.02))
EF is floored at 1.3
if q < 3: reset repetition to 0, next interval = 1 day
else if new rep = 1: interval = 1 day
else if new rep = 2: interval = 6 days
else: interval = round(previous interval * new EF)
q is the recall quality from 0 to 5. The ease factor updates every review and never drops below 1.3. A quality below 3 is a lapse that restarts the item's interval cycle.
Using the result
- Carry the new ease factor and repetition number forward to the next review.
- Grade quality honestly; inflating it lengthens intervals and risks forgetting.
- A lapse resets the schedule but keeps the (lowered) ease factor.
- Intervals grow geometrically, so well-known items quickly reach long gaps.
- SM-2 is the classic method; some apps use newer schedulers, but the logic is similar.
Spaced repetition: frequently asked questions
What is spaced repetition?
Spaced repetition schedules reviews of material at growing intervals so each review lands just as you are about to forget, which strengthens long-term memory efficiently. The SM-2 algorithm, the classic public method, decides how long to wait before the next review based on how well you recalled the item.
How does SM-2 set the interval?
The first successful review schedules the next in 1 day, the second in 6 days, and each subsequent interval is the previous interval times the ease factor. The ease factor starts at 2.5 and adjusts up or down based on your recall quality each time you review.
What is the ease factor?
The ease factor is a per-item multiplier, starting at 2.5, that controls how fast its intervals grow. After each review it updates by the SM-2 formula using your quality grade (0 to 5). Easy items earn a higher ease factor and longer gaps; hard items get a lower one, bottoming out at 1.3.
What does the quality grade mean?
Quality is your self-rated recall on a 0 to 5 scale: 5 is a perfect, effortless answer and 0 is a complete blank. Grades below 3 are treated as a lapse, which resets the repetition count so the item is reviewed again soon. The calculator follows this SM-2 rule.
Is the SM-2 algorithm freely documented?
Yes. SM-2 is a published, openly documented algorithm widely used in open-source flashcard software. The interval and ease-factor formulas are fixed, so any correct implementation produces the same schedule for the same inputs.
Official sources
- National Center for Education Statistics: NCES (learning and assessment context).
- U.S. Department of Education: official site.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. The SM-2 formulas are a published, fixed algorithm. See our methodology.