What is the SM-2 algorithm used in Anki?

Anki uses a modified SM-2 algorithm. After each review, the interval is multiplied by the Ease Factor (EF), which starts at 2.5 and adjusts based on recall quality. A ‘Hard’ response multiplies interval by 1.2; ‘Good’ by EF; ‘Easy’ by EF × 1.3. EF never falls below 1.3.

How much time can spaced repetition save?

Cepeda et al. (2006) showed that optimal spacing reduces study time by 50–75% compared to massed practice for equivalent retention. For a 10,000-item deck, spaced repetition typically requires only 15–20 minutes of daily review at steady state.

Spaced Repetition Optimal Review Interval

Formula Definitions

The Spaced Repetition Optimal Review Interval works by applying a well-defined formula to the values you enter. Understanding the formula behind the calculation helps you interpret the result and check that your inputs are correct. Below we break down the key components that drive the Spaced Repetition.

The core formula used by this calculator is:

Result = f(inputs, settings)

Each variable in the formula has a specific meaning:

inputs — Values entered into the Spaced Repetition.
settings — Options, units, or parameters that adjust how the calculation is performed.
Result — Computed output for the chosen inputs and settings.

How to Use This Calculator

Enter the required values. Enter the required values into the Spaced Repetition Optimal Review Interval input fields.
Adjust settings. Adjust any settings, select applicable options, or choose units as needed.
Calculate the result. Press Calculate to compute the result using the underlying formula.
Interpret the output. Review and interpret your result using the provided context, reference ranges, or explanatory notes.

Glossary and Definitions

Input: A value entered into the calculator to be processed.
Output: The computed result returned by the calculator for the given inputs.
Setting: An option that adjusts how the calculation is performed, such as unit selection or mode.
Spaced repetition: A key concept referenced by the Spaced Repetition: Calculate optimal review intervals using the SM-2 spaced repetition algorithm. Interval₁ = 1 day, interval₂ = 6 days, intervalₙ₊₁ = intervalₙ × EF, where Ease Factor (EF) adjusts based on recall quality (0–5 scale).
SM-2 algorithm: A key concept referenced by the Spaced Repetition: Calculate optimal review intervals using the SM-2 spaced repetition algorithm. Interval₁ = 1 day, interval₂ = 6 days, intervalₙ₊₁ = intervalₙ × EF, where Ease Factor (EF) adjusts based on recall quality (0–5 scale).
Anki interval: A key concept referenced by the Spaced Repetition: Calculate optimal review intervals using the SM-2 spaced repetition algorithm. Interval₁ = 1 day, interval₂ = 6 days, intervalₙ₊₁ = intervalₙ × EF, where Ease Factor (EF) adjusts based on recall quality (0–5 scale).
Review schedule: A key concept referenced by the Spaced Repetition: Calculate optimal review intervals using the SM-2 spaced repetition algorithm. Interval₁ = 1 day, interval₂ = 6 days, intervalₙ₊₁ = intervalₙ × EF, where Ease Factor (EF) adjusts based on recall quality (0–5 scale).
Active recall: A key concept referenced by the Spaced Repetition: Calculate optimal review intervals using the SM-2 spaced repetition algorithm. Interval₁ = 1 day, interval₂ = 6 days, intervalₙ₊₁ = intervalₙ × EF, where Ease Factor (EF) adjusts based on recall quality (0–5 scale).
Memory efficiency: A key concept referenced by the Spaced Repetition: Calculate optimal review intervals using the SM-2 spaced repetition algorithm. Interval₁ = 1 day, interval₂ = 6 days, intervalₙ₊₁ = intervalₙ × EF, where Ease Factor (EF) adjusts based on recall quality (0–5 scale).