FSRS scheduling

FSRS (Free Spaced Repetition Scheduler) is the algorithm that decides when each atom comes back to you. It turns a handful of atoms into memory that holds over weeks and years, without you tracking intervals by hand.

The short version

Every atom has two hidden numbers.

• Stability: how long the atom is likely to stick before you forget it
• Difficulty: how hard it is for you, specifically

Each time you rate an atom, FSRS updates both. When stability goes up, the next review is further out. When you forget, stability drops and the atom comes back soon.

The goal is to show you each atom right around the moment you were about to forget it.

What each rating does

• Forgotten (1) drops stability. The atom comes back soon.
• Took a while (2) grows stability a little. The next interval is shorter than a confident answer would get.
• Remembered (3) grows stability based on difficulty and the time since the last review. The interval stretches further each time.

Why not fixed intervals

Older spaced repetition systems used fixed intervals (1 day, 3 days, 7 days, and so on). Atoms that are hard for you got the same schedule as atoms you know cold.

FSRS fits each atom to its own history. An atom you keep forgetting stays close. An atom you've answered well four times in a row drifts out to weeks or months.

This keeps the effort roughly constant as your knowledge grows. The hard atoms get your time. The easy ones fade into the background without being dropped.

You don't have to tune it

The defaults work for almost everyone. The one setting worth knowing is desired retention in Settings → Review → Scheduling: the share of atoms you want to still remember when they come due. The default is 90%. Higher means more frequent reviews, lower means fewer.

If you're curious about the math, FSRS is open source and well documented.