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Spaced Repetition Calculator

Plan optimal flashcard review intervals with the SM-2 algorithm

How well you recalled the answer (SM-2 quality grade)

Default 2.5, min 1.3

Number of reviews to plan (max 20)

Total Review Time

3.3h

8 sessions over 2476 days

Final EF

2.50

Last Interval

1488 days

Review Schedule

#DateIntervalEF
1Mar 27, 20261d2.50
2Apr 2, 20266d2.50
3Apr 17, 202615d2.50
4May 25, 202638d2.50
5Aug 28, 202695d2.50
6Apr 23, 2027238d2.50
7Dec 8, 2028595d2.50
8Jan 4, 20331488d2.50

Time Breakdown

Cards per session50
Time per session25m
Total review time200m (3.3h)

Frequently Asked Questions

Q

How does the SM-2 spaced repetition algorithm work?

SM-2 calculates review intervals by multiplying the previous interval by an Easiness Factor (EF). The first review is always after 1 day, the second after 6 days, and subsequent intervals follow interval(n) = interval(n-1) * EF. The EF adjusts based on recall quality (0-5 scale), starting at 2.5 and decreasing for poor recall.

  • Repetition 1: interval = 1 day (always fixed)
  • Repetition 2: interval = 6 days (always fixed)
  • Repetition 3+: interval = previous interval * EF
  • EF starts at 2.5 and adjusts after each review
  • Quality 4-5 maintains or increases intervals; 0-2 resets to repetition 1
Review #Interval (EF=2.5)Cumulative DaysApproximate Date
11 day1Tomorrow
26 days71 week
315 days223 weeks
438 days602 months
594 days1545 months
Q

What does the Easiness Factor (EF) mean?

The Easiness Factor is a multiplier (minimum 1.3) that determines how quickly intervals grow. An EF of 2.5 means each interval is 2.5x the previous one. The EF adjusts after each review using the formula: EF' = EF + (0.1 - (5-q) * (0.08 + (5-q) * 0.02)), where q is your recall quality.

  • EF = 2.5: Default for new cards, intervals grow quickly
  • EF = 1.3: Minimum value, intervals grow slowly (difficult card)
  • Quality 5 (perfect): EF increases by 0.10
  • Quality 4 (hesitation): EF stays unchanged
  • Quality 3 (difficult): EF decreases by 0.14
  • Quality 0-2: Card resets to repetition 1
QualityEF ChangeExample EF (from 2.5)Effect on Schedule
5 (Perfect)+0.102.60Longer intervals
4 (Good)+0.002.50No change
3 (Hard)-0.142.36Shorter intervals
2 (Failed)-0.322.18Card resets
Q

How many flashcards can I realistically review per day?

Most learners can review 100-200 flashcards per day spending 30-60 minutes, assuming 15-30 seconds per card. New cards take longer (45-60 seconds each). A deck of 50 new cards requires about 25-30 minutes for the first session and progressively less as intervals lengthen.

  • 50 cards at 30 sec each = 25 minutes per session
  • 100 cards at 20 sec each = 33 minutes per session
  • New cards take 2-3x longer than review cards
  • Aim for 20-30 new cards per day to avoid burnout
  • Total daily load decreases as cards reach longer intervals
Q

How does SM-2 compare to Anki's algorithm?

Anki uses a modified SM-2 with additional features like separate learning/relearning steps, fuzz factors to spread reviews, and customizable intervals. The core interval multiplication is the same, but Anki adds graduating intervals, easy bonuses, and interval modifiers that SM-2 does not have.

  • Both use EF-based interval multiplication at their core
  • Anki adds "learning steps" before SM-2 scheduling kicks in
  • Anki applies random fuzz (up to 5%) to prevent clustering
  • Anki has an "Easy" button that gives a 1.3x bonus multiplier
  • SM-2 is simpler and more predictable for manual planning
Q

What recall quality should I honestly rate myself?

Rate yourself 5 only if the answer came instantly with zero effort. Rate 4 if you recalled correctly but paused to think. Rate 3 if you got it right but it was a struggle. Rate 2 if you saw the answer and thought "I knew that." Rate 0-1 if the answer was completely unknown.

  • Quality 5: Instant recall, no hesitation at all
  • Quality 4: Correct after brief thinking (1-3 seconds)
  • Quality 3: Correct but required significant effort
  • Quality 2: Failed but recognized the answer immediately
  • Quality 0-1: Complete failure, no recognition

Example Calculations

1Language Vocabulary Deck (Quality 4)

Inputs

Total Cards50
Recall Quality4 (correct after hesitation)
Starting EF2.5
Minutes per Card0.5

Result

Total Review Time (8 sessions)200 min (3.3h)

With quality 4, EF stays at 2.5. Intervals: 1, 6, 15, 38, 94, 235, 588, 1469 days. Each of the 8 sessions takes 50 * 0.5 = 25 min, totaling 200 min. The last review is over 4 years out.

2Difficult Medical Terms (Quality 3)

Inputs

Total Cards100
Recall Quality3 (correct with difficulty)
Starting EF2.5
Minutes per Card0.5

Result

Total Review Time (6 sessions)300 min (5h)

Quality 3 decreases EF each time: 2.36, 2.22, 2.08, 1.94, 1.80, 1.66. Intervals grow more slowly: 1, 6, 14, 29, 57, 94 days. Each session: 100 * 0.5 = 50 min. Total: 300 min across 6 sessions spanning about 201 days.

3Easy Review Cards (Quality 5)

Inputs

Total Cards30
Recall Quality5 (perfect recall)
Starting EF2.5
Minutes per Card0.3

Result

Total Review Time (5 sessions)45 min

Quality 5 increases EF: 2.60, 2.70, 2.80, 2.90, 3.00. Intervals: 1, 6, 16, 45, 130 days. Each session: 30 * 0.3 = 9 min. Total: 45 min over 5 sessions spanning about 198 days.

Formulas Used

SM-2 Interval Calculation

I(n) = I(n-1) * EF (for n > 2); I(1) = 1; I(2) = 6

Calculates the next review interval by multiplying the previous interval by the Easiness Factor, with fixed intervals for the first two reviews.

Where:

I(n)= Interval in days for the nth review
I(n-1)= Interval from the previous review
EF= Easiness Factor (minimum 1.3, default 2.5)
n= Review repetition number (1, 2, 3, ...)

Easiness Factor Adjustment

EF' = EF + (0.1 - (5 - q) * (0.08 + (5 - q) * 0.02))

Adjusts the Easiness Factor after each review based on the recall quality rating, with a minimum floor of 1.3.

Where:

EF'= Updated Easiness Factor after the review
EF= Current Easiness Factor before the review
q= Recall quality rating (0-5 integer scale)

Understanding Spaced Repetition and the SM-2 Algorithm

Spaced repetition is the most evidence-backed study technique in cognitive science. Instead of cramming all material at once, you review each item at increasing intervals timed to catch it just before you forget. This leverages the spacing effect discovered by Hermann Ebbinghaus in 1885.

The SM-2 algorithm, created by Piotr Wozniak in 1987, automates this process. It assigns each flashcard an Easiness Factor (EF) that determines how quickly review intervals grow. Cards you find easy get longer intervals; difficult cards get reviewed more frequently. The system adapts to your actual memory performance.

To use this calculator effectively, start with the default EF of 2.5 and rate your recall honestly. Over 8-10 review sessions, a typical 50-card deck will grow from daily reviews to intervals measured in months, requiring progressively less total study time while maintaining strong retention.

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Last Updated: Mar 25, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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