What Is the Forgetting Curve?

The forgetting curve describes how quickly new memories fade if you do nothing to keep them. Retention drops steeply in the first hours after learning, then the decline slows and levels off. Each well-timed review makes the curve flatter, which is the whole logic behind spaced repetition.

Where the forgetting curve comes from

Hermann Ebbinghaus, a German psychologist, measured his own memory between 1879 and 1885 and published the results in Über das Gedächtnis (1885). He memorized lists of nonsense syllables such as WID or ZOF, so that prior knowledge could not help, and later relearned them. He measured savings: how much less time relearning took compared with the first learning.

Two things stand out: most of the loss happens early, and something survives for a long time. Murre and Dros (2015) replicated the experiment and found a very similar curve.

A simple formula

Modern spaced repetition software often models retention with an exponential decay:

R = e^(−t / S)

Here R is the probability you can recall the item, t is the time since your last review and S is the stability of the memory. A new word might have a stability of one or two days; after several successful reviews, stability can reach months. Algorithms like FSRS estimate S for each card from your review history. You can play with these values in the forgetting curve calculator.

A vocabulary example

Why it matters for learning words

Most learners do not lack exposure to new words; they lose them before the words stick. Looking up a word once is the start of the curve, not the end. Studies of reading show that learners usually need repeated encounters with a word, often spread over days, before it is reliably known. Without a plan for re-encounters, a large share of looked-up words simply fades.

How to flatten the curve

• Review before you forget. The first review matters most; aim for the next day.
• Recall, don't re-read. Retrieving the meaning from memory (retrieval practice) builds more stability than looking at it again.
• Expand the gaps. 1 day, 3 days, a week, two weeks, a month is a reasonable start.
• Make it meaningful. Learn the word with an example sentence and connect it to words you know.
• Sleep. Consolidation during sleep helps new words stick.
• Meet it again in context. Wordie highlights saved words on any page where they reappear, so each encounter acts as a small review.

For a longer treatment, see the forgetting curve explained and how to stop forgetting words.

Common misconceptions

• "Everyone forgets at the same rate." The rate depends on the material, how well it was learned, prior knowledge and sleep.
• "There is one universal curve." Each memory has its own curve, and each review changes it.
• "Forgotten means wasted." Relearning a forgotten word is faster than learning it the first time. That is exactly what Ebbinghaus's savings measure showed.

Sources: Ebbinghaus, H. (1885). Über das Gedächtnis. Murre, J. M. J., & Dros, J. (2015). Replication and analysis of Ebbinghaus' forgetting curve. PLOS ONE.