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Spacing Effect Interval Calculator

Work out spacing effect interval instantly with clear inputs, formula shown and shareable results.

About Spacing Effect Interval Calculator

Spaced repetition multiplies the previous interval by an ease factor when recall succeeds and resets it to one day when it fails. A six day interval at ease 2.5 with good recall becomes fifteen days; an easy recall stretches it to 19.5 days and raises the ease. This is the SM-2 logic behind Anki and similar systems.

Formulas

Next interval

next = previous interval x ease factor, modified by recall quality

Failure

reset the interval to one day and reduce the ease factor

Worked example

With the default values below, Spacing Effect Interval Calculator returns Next Review Interval of 15.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Previous intervalminimum 0.1 days
6 days
Ease factoraccepts 1.3–3
2.5
How well you recalled itone of Perfect, effortless, Correct with some effort, Correct but difficult, Failed to recall
Correct with some effort

Results

Next Review Interval
15.0 days
Updated Ease Factor
2.500
Interval Multiplier
2.500
Reviews Per Year At This Interval
24.35

Step by step

  1. 1Values used
  2. 2Next interval
  3. 3Failure
  4. 4Next Review Interval= 15.0 days
  5. 5Updated Ease Factor= 2.500
  6. 6Interval Multiplier= 2.500
  7. 7Reviews Per Year At This Interval= 24.35

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why reset to one day on failure?

Because a failed item has effectively lost its stability. Rebuilding from a short interval is faster than guessing a partial reduction.

What does the ease factor represent?

How easy the item is for you. It starts near 2.5 and adjusts with performance, so difficult items get reviewed more often.

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Spacing Effect Interval Calculator | Calcrivo