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SM-2 Next Review Interval Calculator

Q: What does the E-Factor (Ease Factor) represent in SM-2?

The E-Factor is a per-card multiplier reflecting how easy the card is to recall. It ranges from **1.3** (very hard) to **2.5** (very easy). In Anki and other SM-2 apps, it adjusts automatically: 'Easy' raises it, 'Hard' or 'Again' lowers it. All new cards start at 2.5 by default.

Q: How is the next SM-2 interval calculated step by step?

For reviews after the first two: **Next interval = round(Current interval × E-Factor)**. The first review is always 1 day; the second is always 6 days. From the third review onward, the formula applies. Example: interval = 15 days, EF = 2.0 → next interval = round(15 × 2.0) = **30 days**.

Q: What are typical starting intervals in SM-2?

SM-2 prescribes **1 day** for the first review and **6 days** for the second (these are fixed). From the third review onward, the formula interval × EF applies. Anki slightly modifies this (1 day and 4 days by default), and users can adjust these in deck settings.

Q: What happens if I forget a card (rate it 'Again' in Anki)?

When a card is failed (quality < 3), SM-2 resets its interval to **1 day** and penalizes the E-Factor by roughly 0.2 (down to a minimum of 1.3). The card restarts the learning cycle. This calculator computes the success case — it assumes you recalled the card correctly.

Q: Does Anki use exactly the SM-2 algorithm?

Anki uses a modified SM-2. The core interval formula (interval × ease factor) is the same, but Anki adds extra modifiers: an Interval Modifier per deck, a Hard interval multiplier, and an Easy bonus. Anki 23.10+ also introduced **FSRS**, a machine-learning-based algorithm that can replace SM-2 entirely and generally outperforms it.

Q: What is the difference between SM-2 and FSRS?

SM-2 uses a fixed ease-factor-based multiplication. **FSRS (Free Spaced Repetition Scheduler)**, introduced in Anki 23.10, uses a machine-learning model trained on real review data to predict the optimal next review date, taking into account estimated memory stability and retrievability. FSRS generally outperforms SM-2 on retention efficiency, but SM-2 is simpler to understand and implement manually.

Q: What is a good E-Factor target for most cards?

A well-known rule of thumb in the SRS community: keep your E-Factor above **2.0** for most cards. If a card's E-Factor keeps dropping toward 1.3, it usually means the card is too complex and should be broken into smaller, simpler cards ('card splitting' or 'atomic notes').

Q: Can I use this calculator for subjects other than language learning?

Yes. The SM-2 algorithm is content-agnostic — it schedules anything you want to memorize: vocabulary, formulas, historical dates, music theory, anatomy. As long as you can rate your recall on a card, SM-2 applies.

Q: Why does this calculator not ask for a quality score (0–5)?

This calculator computes the next interval assuming a **successful recall** (score ≥ 3). The E-Factor update formula requires a quality rating, which changes the E-Factor for the next cycle. If you want to simulate a score-adjusted E-Factor, first compute: New EF = EF + (0.1 − (5 − q) × (0.08 + (5 − q) × 0.02)), then enter that updated EF into the calculator.

Calculate your next flashcard review interval using the SM-2 spaced repetition algorithm (used by Anki and SuperMemo). Enter your current interval and ease factor to get the exact number of days until your next review — plus a full reference table.

Data verified · June 2026
Edited by Martín Rodríguez
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Ebbinghaus Forgetting CurveEnter days elapsed and memory stability to determine whether it's time to review — and estimate your current retention.How many flashcard reviews per day to hit your goalTotal reviews needed ÷ days available. Plan your Anki or SRS workload with spaced repetition benchmarks.Calculate Your Daily Anki CardsCalculate daily Anki cards based on your learning goal. Optimize spaced repetition with our free calculator.Optimal Exam Review SchedulePlan your spaced exam reviews to peak before test day (SAT, GRE, MCAT, USMLE). Maximize retention with science-backed timing. Try free online now.

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Follow this tool’s steps, then review its formula, assumptions, and limits below.

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Spaced repetition is one of the most evidence-backed techniques in learning science. The SM-2 algorithm, developed by Piotr Woźniak at SuperMemo in 1987, schedules flashcard reviews at precisely the right moment — just before you forget. The core mechanic: each time you recall a card correctly, the next review is pushed further into the future by multiplying your current interval by your ease factor (E-Factor). This calculator implements that exact formula so you can plan or verify your review schedule without needing a full SRS app.

When to use this calculator

Language learners using Anki, Mnemosyne, or custom SM-2 decks who want to verify or predict future review dates.
Developers building or testing an SRS application who need to confirm their interval calculation logic matches the SM-2 spec.
Students learning about the SM-2 algorithm in courses on cognitive science or educational technology.
Teachers designing paper-based spaced repetition systems and calculating manual review schedules.

SM-2: Cambio del E-Factor según calidad de respuesta (q)

Calidad (q)	Descripción	Δ E-Factor	Ejemplo (EF inicial 2.5)
5	Perfecto, sin duda	+0.10	→ 2.50 (tope)
4	Correcto, pequeña duda	+0.00	→ 2.50
3	Correcto, dificultad notable	−0.14	→ 2.36
2	Fallo leve (respuesta recordada al ver)	−0.32	→ 2.18
1	Fallo claro	−0.54	→ 1.96
0	Olvido total	−0.80	→ 1.70

Fuente: Woźniak, P. — SM-2 Algorithm, SuperMemo.com (1990). Fórmula: Nuevo EF = EF + (0.1 − (5 − q) × (0.08 + (5 − q) × 0.02)). EF mínimo: 1.3. Con q < 3 el intervalo se reinicia a 1 día además de reducir el EF.

How it works

How SM-2 Calculates the Next Review

The SM-2 algorithm (SuperMemo 2), published by Piotr Woźniak in 1987, underlies apps like Anki and Mnemosyne. Its key insight is the forgetting curve: memory decays exponentially after learning, but each successful review extends the interval further.

The formula

// From the 3rd review onward:
Next interval = round(Current interval × E-Factor)

// First two reviews (fixed by SM-2 spec):
1st review: 1 day
2nd review: 6 days

// E-Factor update after each review (quality q from 0 to 5):
New EF = EF + (0.1 − (5 − q) × (0.08 + (5 − q) × 0.02))
Min EF: 1.3   // never drops below this

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SM-2 Interval Growth Table (by E-Factor)

How the interval grows over successive reviews with different E-Factors:

Review #	EF = 1.3 (hard)	EF = 1.8 (average)	EF = 2.5 (easy)
1	1 day	1 day	1 day
2	6 days	6 days	6 days
3	8 days	11 days	15 days
4	10 days	20 days	38 days
5	13 days	36 days	94 days
6	17 days	65 days	235 days
7	22 days	117 days	588 days (~1.6 yr)

Key insight: With a high E-Factor (2.5), cards quickly reach months-long intervals. With a low E-Factor (1.3), intervals grow very slowly — a signal the card may need to be split into simpler pieces.

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Quick Reference: Interval × E-Factor → Next Review

Current Interval	EF 1.3	EF 1.8	EF 2.1	EF 2.5
1 day	1 day	2 days	2 days	3 days
6 days	8 days	11 days	13 days	15 days
10 days	13 days	18 days	21 days	25 days
21 days	27 days	38 days	44 days	53 days
30 days	39 days	54 days	63 days	75 days
60 days	78 days	108 days	126 days	150 days
90 days	117 days	162 days	189 days	225 days

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Why the E-Factor matters

Cards with a high E-Factor (close to 2.5) grow into very long intervals quickly — eventually appearing only once every few months or years. Cards that remain difficult stabilize at low E-Factors and appear frequently until your recall improves.

Standard SM-2 E-Factor update formula

After a review with quality score q (0–5):

New E-Factor = E-Factor + (0.1 − (5 − q) × (0.08 + (5 − q) × 0.02))

q = 5 (perfect): EF increases by +0.1

q = 4 (correct, small hesitation): EF stays roughly the same

q = 3 (correct, significant difficulty): EF decreases by −0.14

q < 3 (fail): EF decreases by −0.32 or more, interval resets to 1

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Common Mistakes

1. Manually editing the E-Factor: EF is computed by the algorithm based on your ratings. Editing it manually breaks the system.
2. Always rating 'Easy' (5): Intervals become so long you start forgetting. Rate honestly.
3. Using weeks or months instead of days: SM-2 works exclusively in whole days.
4. Failing a card without updating EF: When failing (quality < 3), SM-2 resets the interval to 1 day AND reduces EF. Only resetting the interval without updating EF causes the card to reappear with long intervals too soon.

Worked Example: A Spanish vocabulary card

Current interval = 10 days (you reviewed the card 10 days ago).

E-Factor = 2.5 (Anki rated it as Easy several times).

Apply SM-2: Next interval = round(10 × 2.5) = 25 days.

Enter: Current Interval = 10, E-Factor = 2.5.

Next review in 25 days (new interval: 25d)

Frequently asked questions

What does the E-Factor (Ease Factor) represent in SM-2?

The E-Factor is a per-card multiplier reflecting how easy the card is to recall. It ranges from 1.3 (very hard) to 2.5 (very easy). In Anki and other SM-2 apps, it adjusts automatically: 'Easy' raises it, 'Hard' or 'Again' lowers it. All new cards start at 2.5 by default.

How is the next SM-2 interval calculated step by step?

For reviews after the first two: Next interval = round(Current interval × E-Factor). The first review is always 1 day; the second is always 6 days. From the third review onward, the formula applies. Example: interval = 15 days, EF = 2.0 → next interval = round(15 × 2.0) = 30 days.

What are typical starting intervals in SM-2?

SM-2 prescribes 1 day for the first review and 6 days for the second (these are fixed). From the third review onward, the formula interval × EF applies. Anki slightly modifies this (1 day and 4 days by default), and users can adjust these in deck settings.

What happens if I forget a card (rate it 'Again' in Anki)?

When a card is failed (quality < 3), SM-2 resets its interval to 1 day and penalizes the E-Factor by roughly 0.2 (down to a minimum of 1.3). The card restarts the learning cycle. This calculator computes the success case — it assumes you recalled the card correctly.

Does Anki use exactly the SM-2 algorithm?

Anki uses a modified SM-2. The core interval formula (interval × ease factor) is the same, but Anki adds extra modifiers: an Interval Modifier per deck, a Hard interval multiplier, and an Easy bonus. Anki 23.10+ also introduced FSRS, a machine-learning-based algorithm that can replace SM-2 entirely and generally outperforms it.

What is the difference between SM-2 and FSRS?

SM-2 uses a fixed ease-factor-based multiplication. FSRS (Free Spaced Repetition Scheduler), introduced in Anki 23.10, uses a machine-learning model trained on real review data to predict the optimal next review date, taking into account estimated memory stability and retrievability. FSRS generally outperforms SM-2 on retention efficiency, but SM-2 is simpler to understand and implement manually.

What is a good E-Factor target for most cards?

A well-known rule of thumb in the SRS community: keep your E-Factor above 2.0 for most cards. If a card's E-Factor keeps dropping toward 1.3, it usually means the card is too complex and should be broken into smaller, simpler cards ('card splitting' or 'atomic notes').

Can I use this calculator for subjects other than language learning?

Yes. The SM-2 algorithm is content-agnostic — it schedules anything you want to memorize: vocabulary, formulas, historical dates, music theory, anatomy. As long as you can rate your recall on a card, SM-2 applies.

Why does this calculator not ask for a quality score (0–5)?

This calculator computes the next interval assuming a successful recall (score ≥ 3). The E-Factor update formula requires a quality rating, which changes the E-Factor for the next cycle. If you want to simulate a score-adjusted E-Factor, first compute: New EF = EF + (0.1 − (5 − q) × (0.08 + (5 − q) × 0.02)), then enter that updated EF into the calculator.

Sources & references

Methodology & trust

Editorial

idiomas calculator with its formula verified automatically against Woźniak, P. — Optimization of Learning: SM-2 Algorithm (SuperMemo.com, 1990), per our editorial policy and methodology.

Updates

Updated: June 2026. Parameters are verified periodically against the cited sources.

Privacy

Calculations run 100% in your browser. We do not store or transmit your data.

Limitations

Indicative results. For critical decisions, consult a professional.

📌 How to cite this calculator

Rodríguez, M. (2026). SM-2 Next Review Interval Calculator. Hacé Cuentas. https://hacecuentas.com/en/srs-next-review-calculator

Content licensed under CC-BY 4.0 — reuse it citing the source with a link to Hacé Cuentas.

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