Work out forgetting curve retention instantly with clear inputs, formula shown and shareable results.
Ebbinghaus found forgetting follows an exponential decay, R = R0 e^(-t/S), where S is the memory's stability. With S = 2.5 days, retention falls to 6.1% after a week and the half-life is 1.73 days. Each successful review increases S, which is why spaced repetition schedules lengthen the intervals progressively.
Forgetting curve
R = R0 x e^(-t / S)
Memory half-life
half-life = S x ln2
Because retrieval practice increases stability S, flattening the curve. Each review buys a longer interval than the last.
Just before retention drops too far - typically around 85-90% recall probability, which is what spaced repetition algorithms target.