Spaced Repetition Calculator
When to review a flashcard next by the SM-2 algorithm from SuperMemo: the intervals and E-Factor for a run of answer grades from 0 to 5, or an estimate of the daily review workload from new cards a day.
Result…
How it's calculated
I(1) = 1 day · I(2) = 6 days · I(n) = I(n − 1) × EF for n > 2, rounded to whole days EF′ = EF + (0.1 − (5 − q) × (0.08 + (5 − q) × 0.02)), never below 1.3 (q = grade 0–5) q < 3: the repetitions start again from I(1) Workload: expected reviews due each day, summed over every card learned so far
SM-2 is the spaced-repetition rule Piotr Wozniak wrote for SuperMemo in 1987, and the root of the schedules in many flashcard apps. Each card has an E-Factor (easiness), 2.5 at the start. After each review you grade your answer from 0 to 5; the grade changes the E-Factor, and the next gap is 1 day, then 6 days, then the last gap times the E-Factor. A grade below 3 sends the card back to the start. The workload mode runs the rule forward for a steady flow of new cards, with a share of reviews forgotten, and adds up the reviews due each day. It is an average-case estimate: your own grades decide the real numbers.
Your numbers, step by step
About the Spaced Repetition calculator
This spaced repetition calculator applies SM-2, the scheduling rule Piotr Wozniak wrote for SuperMemo, to your own flashcard answers. Type the grades you gave a card, from 0 (complete blackout) to 5 (perfect), and it works out the gap to the next review after each one, the card's E-Factor and the day the next review falls on. Every step is shown with your numbers, so you can check where an interval of 16 or 100 days came from. A second mode runs the rule forward for a steady stream of new cards a day, with a share of reviews forgotten, and estimates how many reviews a day that adds up to. It suits language learners, medical and law students and anyone deciding how many new cards a day they can keep up with. The workload is an average-case estimate, not a promise.
How to use it
- Choose “Intervals from my answers” and type the grades for one card in “Answer grades in order (0 to 5)”, the first study included.
- Leave the “Starting E-Factor” at 2.5 for a new card, or enter the value a card already has.
- Read the next interval, the E-Factor now and the day of the next review, and follow each review in the steps.
- For planning, switch to “Daily review workload” and enter “New cards a day”, “Days of study” and the share of “Reviews you forget”.
- Pick the “Grade on a review you pass” and the “Grade when you forget”, then read the reviews a day.
Worked example
A card graded 5, 4, 3, 5, 4 from an E-Factor of 2.5. Review 1 (grade 5): interval 1 day, E-Factor 2.6. Review 2 (grade 4): 6 days, E-Factor stays 2.6. Review 3 (grade 3): 6 × 2.6 = 15.6, rounded to 16 days, E-Factor 2.46. Review 4 (grade 5): 16 × 2.46 = 39.36, so 39 days, E-Factor 2.56. Review 5 (grade 4): 39 × 2.56 = 99.84, so the next review is in 100 days, on day 162 after the first study. In workload mode, 15 new cards a day for 180 days, passing at grade 4 and forgetting 10% of reviews, averages about 108 reviews a day over the last 30 days, or about 123 cards a day with the new ones.
Updated
Questions people ask
How does the SM-2 algorithm calculate the next review?
The first gap is 1 day and the second is 6 days. After that, each gap is the previous one times the card's E-Factor, rounded to whole days. A grade below 3 sends the card back to the 1-day step.
What is the E-Factor formula in SM-2?
New E-Factor = old + (0.1 − (5 − grade) × (0.08 + (5 − grade) × 0.02)), and it never goes below 1.3. A grade of 5 adds 0.1, a 4 leaves it unchanged and a 3 takes off 0.14. The SuperMemo description of SM-2 gives the formula and the grade scale.
How many flashcard reviews a day will 20 new cards create?
It depends on how often you forget and how well you pass, so run the workload mode with your own numbers. Reviews keep rising slowly for months because the intervals keep growing, so look at the chart as well as the average.
Why does my app show a different interval from this calculator?
Most apps use modified versions of SM-2, with their own steps, ease limits and fuzz. This calculator follows SuperMemo's own source code, where the interval uses the E-Factor before the new grade and rounds to the nearest day.
What is a common mistake when grading flashcards?
Giving a 3 to answers you really got wrong. That keeps the card on long intervals you can't keep up with, while a grade below 3 brings it back the next day to relearn.
How accurate is the review workload estimate?
It is an average over many cards that assumes the same grade on every pass and a fixed share forgotten. Real decks mix easy and hard cards, so treat it as a rough guide. To fit study into your day, the reading time calculator can help plan other study tasks.