In the forgetting curve, Hermann Ebbinghaus found that memory fades fast at first and then slowly — most of what you learn today is gone within a day or two if you never touch it again. The calculator above lets you play with the other half of that finding: what happens when you do touch it again, at spaced intervals.
What the calculator shows
The dashed line is the honest bad news — learn something once and never review it, and retention falls off a cliff. The solid orange line is your intervention. Each time you add a review, two things happen: retention jumps back to 100%, and the next stretch of the curve decays more slowly than the last. After four or five well-spaced reviews, the day-45 number that started near single digits is holding above 90%. That widening gap between the two lines is the entire case for spaced repetition, in one picture.
The "easy ↔ hard" slider changes how quickly the untouched memory decays. Meaningful, well-understood material fades slower than a random phone number, and the slider is a rough stand-in for that. Notice that wherever you set it, the shape of the win is the same: spacing the reviews out beats piling them up.
Why spacing beats cramming
Cramming pours every review into one night. On the curve, that's a single reset — steep as ever — and the knowledge is gone by the weekend. Spacing the same number of reviews across days and weeks resets the curve again and again, each time onto a gentler slope. Same effort, radically different retention. This is the spacing effect, one of the most replicated findings in learning science.
Two things make each review actually count:
- Retrieve, don't re-read. The gain comes from pulling the answer out of your head — that's active recall, and the win it produces is the testing effect. Re-reading your notes feels productive and does almost nothing for the curve.
- Review when it's slightly hard. If recall feels effortless, you waited too little; if it comes up blank, too long. The productive zone is right at the edge of forgetting — which is exactly where a good schedule aims each review.
An honest note on the model
This is an illustrative model, not a fitted one. Real memory doesn't follow a single tidy equation — it depends on the material, your sleep, how well you understood it the first time, and how hard each retrieval was. The curve here uses a smooth power-law decay whose stability roughly doubles with each spaced review, which reproduces the shape the research keeps finding without pretending to predict your particular Tuesday. Treat the numbers as directional, not clinical.
Where notes fit
Spaced repetition is the right tool for things that must live in your head — vocabulary, anatomy, exam facts. But most of what you want to keep just needs to be findable: decisions, ideas, the reason behind a choice. For those, a note is the cheaper mechanism — an external memory with a flat curve — as long as you capture it before the steep part does its work. Writing one idea per note keeps each one reviewable later, the same way atomic notes stay linkable, and a habit of revisiting what you read quietly runs spaced repetition without a flashcard deck. It's the principle behind the way Clair Mind resurfaces older notes beside new ones — a forgetting curve, gently interrupted.