FSRS vs SM-2: Which Spaced Repetition Algorithm Should You Use?
If you've set up a serious flashcard app, you've met a choice: FSRS or SM-2. Both decide when to show you each card. Both are far better than guessing. But they work differently, and one is meaningfully more modern. Here's a plain-English comparison.
First, what they have in common
Both are spaced-repetition schedulers: after you review a card and say how well you knew it, they choose the next review date — pushing well-known cards further out and bringing struggling cards back sooner. The goal is the same: review each card right before you'd forget it, and no more often than necessary.
SM-2: the classic
SM-2 comes from SuperMemo and is the algorithm Anki used by default for years. It tracks an ease factor per card. Rate a card well and its interval multiplies by the ease; rate it badly and the interval collapses and the ease drops.
- Strengths: simple, predictable, battle-tested over decades.
- Limits: it doesn't really model your memory — it applies fixed multipliers. It can be needlessly conservative (showing you cards more than necessary) and it has well-known quirks, like "ease hell," where repeatedly-failed cards get stuck with a low ease factor.
FSRS: the modern successor
FSRS (Free Spaced Repetition Scheduler) is a newer, open, research-backed algorithm. Instead of a single ease number, it models three things for every card: how stable the memory is, how difficult the card is for you, and your current probability of recall. From that it predicts when your recall will drop to a level you choose (your "desired retention," e.g. 90%) and schedules the review for then.
- Strengths: it adapts to your actual performance, typically achieves the same retention with fewer reviews, and lets you dial retention up or down to trade workload against safety.
- Limits: slightly more of a black box; it benefits from some review history to personalise (though sensible defaults work from day one).
Which should you use?
For most people, most of the time: FSRS. It's the more efficient, more modern choice — fewer reviews for the same retention, and a retention target you control. SM-2 remains a perfectly good, simpler option, and if you've got years of SM-2 history you're happy with, there's no urgency to switch.
The honest truth: either one, used daily, beats no spaced repetition by a mile. The algorithm is a rounding error compared to the habit. Don't let the choice become a reason to procrastinate.
What this looks like in practice
StudyTab schedules with both FSRS and SM-2, configurable per deck, with retention presets and a workload forecast — the same modern approach you'd get from Anki, without having to make cards by hand. (More on how that compares in our Anki alternative breakdown.)
Bottom line
SM-2 is the reliable classic; FSRS is the smarter, more efficient default. Pick FSRS if you want fewer reviews and a retention target you control — then stop tweaking and start reviewing.
Want spaced repetition without the setup? Generate a deck and let the schedule run.