Work out spacing effect interval instantly with clear inputs, formula shown and shareable results.
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.
Next interval
next = previous interval x ease factor, modified by recall quality
Failure
reset the interval to one day and reduce the ease factor
Because a failed item has effectively lost its stability. Rebuilding from a short interval is faster than guessing a partial reduction.
How easy the item is for you. It starts near 2.5 and adjusts with performance, so difficult items get reviewed more often.