FSRS scheduling

FSRS (Free Spaced Repetition Scheduler) is the algorithm that decides when each atom comes back to you. It's what turns a handful of captures 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 rate Forgot, stability drops and the atom comes back soon.

The goal is simple. Show you each atom right at the moment you were about to forget it. That's where spaced repetition earns its name.

What each rating does

With the default two buttons:

• Forgot (1) resets stability partially. The atom comes back in minutes or hours.
• Remember (2) grows stability based on difficulty and the previous interval. Easy remembers push further than hard ones.

With the full FSRS scale (enable in Settings → Labs → Review buttons):

• Again (1) resets stability fully. You'll see it very soon.
• Hard (2) grows stability less than Good.
• Good (3) is the standard "I got it" answer.
• Easy (4) grows stability fastest and extends the interval more aggressively.

Two buttons are enough for almost everyone. The four-rating scale gives you more precision if you want to tell the difference between a shaky recall and a confident one, but the resulting schedules are surprisingly close.

Why not fixed intervals

Older spaced repetition systems used fixed intervals (1 day, 3 days, 7 days, and so on). They worked for some atoms and broke down for others. Atoms that are hard for you got the same schedule as atoms you've memorized in your sleep.

FSRS fits a curve to each atom's history. An atom you keep forgetting stays close. An atom you've nailed four times in a row drifts out to weeks or months. Same algorithm, different pace per atom.

This is what keeps the effort constant as your knowledge grows. The hard atoms demand your time. The easy ones fade into the background without ever being dropped.

You don't have to tune it

The defaults in Atomus are chosen to work for almost everyone. You don't need to set retention targets, schedulers, or weights. If you want to dig into the parameters, they're in Settings → Advanced → FSRS, but most people should leave them alone.

If you're curious about the math, the FSRS algorithm is open source and well documented. Atomus uses the standard implementation with defaults that target ~90% retention.