I built a spaced repetition flashcard template in Notion using the actual SM-2 algorithm (the one behind Anki), not a simplified guess. Along the way I hit a real platform constraint worth knowing about if you're building anything similar.
The algorithm, actually implemented correctly
SM-2 tracks three numbers per card: an ease factor (starts at 2.5), an interval, and a repetition count. Rate your recall 0-5 after each review, and the algorithm decides how many days until you see that card again -- longer each time you get it right, reset to 1 day the moment you forget it. I cross-checked the exact 1987 Wozniak formula against a working implementation before writing a single formula, rather than reconstructing it from memory.
The wall: Notion's Automations are a paid feature
The obvious design: a database automation that updates ease factor, interval, and repetitions the instant you rate a card. Clean, one-click, no manual math. Except Notion's "when property changes, edit another property" automations sit behind a plan upgrade -- I only found out by actually opening the automations panel, not by reading pricing docs in advance.
The free-plan workaround
Formula properties don't need Automations -- they just compute and display a value from other properties, live, on any plan. So instead of auto-updating the real fields, I added three formula columns -- Suggested Ease Factor, Suggested Interval, Suggested Repetitions -- that calculate the correct next values the moment you set a rating. You copy those three numbers into the real fields yourself. Two extra clicks per review, zero dollars a month, same correct math.
Checked, not assumed
I hand-calculated two scenarios before trusting the formulas: a perfect recall (ease factor 2.5 -> 2.6, interval -> 1 day) and a total failure (ease factor 2.5 -> 1.7, interval and repetitions both reset). Notion's live formula preview matched both by hand, exactly. Small thing, but it's the difference between a template that looks right and one that actually is.
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