Forgetting Curve Memorization Time Calculator

The Ebbinghaus forgetting curve

In 1885, German psychologist Hermann Ebbinghaus published Über das Gedächtnis. He had memorized long lists of nonsense syllables and tracked how much he could still recall as the days passed. The data fit an exponential decay, R = e^(-t/S), where R is the fraction retained, t is time elapsed and S is the relative strength of the memory trace. Leave new information alone and you lose roughly 70% of it within 24 hours; a week later only about 25% is still there.

Review the trace right as it starts to fade and the curve flattens. The memory's S value grows, so the next forgetting interval can stretch out further. That is what spaced repetition is built on. Algorithms such as SuperMemo SM-2, used by Anki and Mnemosyne, work out when each item should come back so you keep studying near the edge of forgetting. Sleep helps too: consolidation during slow-wave (N3) and REM sleep strengthens the trace, which is why cutting your sleep wrecks long-term retention.

Applications

You'll find it behind language learning (Anki decks, Duolingo), medical and law school exam prep, corporate training, software-skill onboarding, and pretty much any flashcard-driven workflow. Even public-school curricula are leaning more on retrieval practice and distributed review, which trace straight back to Ebbinghaus's findings.

FAQ

How long is the "optimal" spacing? A classic schedule puts the first review around 24 hours, then 3 days, 7 days, 21 days and 60 days. SM-2-style algorithms go a step further and adjust each interval to how hard the recall actually was for you.

Does the curve apply to skills as well as facts? Not in the same way. Motor skills fade far more slowly than declarative memory, because things like cycling and typing run on procedural memory, which lives in different neural circuits (basal ganglia, cerebellum).

Why is sleep so important? Memory consolidation moves information from the hippocampus to the neocortex, and most of that happens during slow-wave sleep. Skimp on sleep and you essentially reset part of the curve.