Forgetting Curve Retention Calculator

Work out forgetting curve retention instantly with clear inputs, formula shown and shareable results.

About Forgetting Curve Retention Calculator

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.

Formulas

Forgetting curve

R = R0 x e^(-t / S)

Memory half-life

half-life = S x ln2

Worked example

With the default values below, Forgetting Curve Retention Calculator returns Retention Remaining of 6.08%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Retention immediately after learningaccepts 1–100 %: 100 %
Memory stability constantminimum 0.01 days: 2.5 days
Days since learningminimum 0: 7

Results

Retention Remaining: 6.08%
Memory Half-Life: 1.73 days
Days Until Half Is Forgotten: 1.73
Review Advice: Review now - retention has fallen below 60%

Step by step

1Values used
2Forgetting curve
3Memory half-life
4Retention Remaining= 6.08
5Memory Half-Life= 1.73 days
6Days Until Half Is Forgotten= 1.73
7Review Advice= Review now - retention has fallen below 60%

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 2 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 does reviewing help so much?

Because retrieval practice increases stability S, flattening the curve. Each review buys a longer interval than the last.

When should I review?

Just before retention drops too far - typically around 85-90% recall probability, which is what spaced repetition algorithms target.