Memory Retention Estimate Calculator

Work out memory retention estimate instantly with clear inputs, formula shown and shareable results.

About Memory Retention Estimate Calculator

This uses the exponential forgetting curve R = e^(-t / S), where S is the stability of the memory in days; half-life follows as S x ln 2. The curve is steepest immediately after learning, so a review on day one buys far more retention than the same review a fortnight later.

Formulas

Forgetting curve

R = e^(-t / S), with t the elapsed days and S the memory stability

Half-life

time to 50% recall = S x ln(2) = 0.693 x S

Worked example

With the default values below, Memory Retention Estimate Calculator returns Estimated recall now of 54.9%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Time since studyingminimum 0 days: 3 days
Memory stabilityminimum 0.1 days: 5 days

Results

Estimated recall now: 54.9%
Estimated forgotten: 45.1%
Time to fall to 50% recall: 3.47 days
Review by, to stay above 90%: 0.53 days

Step by step

1Values used
2Forgetting curve
3Half-life
4Estimated recall now= 54.9
5Estimated forgotten= 45.1
6Time to fall to 50% recall= 3.47 days
7Review by, to stay above 90%= 0.53 days

Assumptions and limits

The result is derived only from the 2 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

How is the Memory Retention Estimate figure worked out?

R = e^(-t / S), with t the elapsed days and S the memory stability. This uses the exponential forgetting curve R = e^(-t / S), where S is the stability of the memory in days; half-life follows as S x ln 2.

Why does this matter?

The curve is steepest immediately after learning, so a review on day one buys far more retention than the same review a fortnight later.

What do I need to enter?

Enter time since studying, memory stability. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.