How spaced repetition works
Without spaced repetition, you review either too early (wasting time because memory is still fresh) or too late (memory is already gone, so you must relearn). Spaced repetition solves this by computing the optimal interval for each card.
The principle: each time you answer a card correctly, the next review interval expands. Each mistake shortens it. The system continuously models your memory state for each piece of information.
Example indicative intervals: Day 0 (creation) → Day 1 → Day 4 → Day 11 → Day 30 → Day 90 → ...
These intervals are only indicative—the algorithm adjusts them based on your actual performance per card. A card you miss often comes back much more frequently than one you master perfectly.
SM-2 vs FSRS: two algorithms, two philosophies
SM-2 — the foundational algorithm
SM-2 (SuperMemo 2) was developed by Piotr Wozniak in the late 1980s. It powers Anki and many flashcard apps. It relies on an ease factor per card: a value adjusted by your performance that determines how quickly intervals grow.
SM-2 is robust and proven on millions of users. Its main limitation is that it treats cards relatively independently and does not model interactions between memory traces or your global learning state in depth.
FSRS — modern modeling
FSRS (Free Spaced Repetition Scheduler) is a more recent algorithm, developed by Jarrett Ye and grounded in Wozniak’s memory work. It incorporates two key parameters: memory stability (S) and intrinsic difficulty (D).
FSRS computes a target retention probability for each card (90% by default) and adjusts intervals to maintain that threshold. Comparative studies show FSRS yields more precise intervals than SM-2, especially long term. It is the algorithm used in Memia.
With FSRS, cards you know well get aggressively longer intervals—fewer unnecessary reviews. Difficult cards are resurfaced more intelligently. Net effect: less time on what you already know, more on what you need.
Grading your answers: the most critical point
The algorithm is only as good as the data you feed it. Typical grading levels and what they really mean:
- Again / Forgot — You could not produce the answer. The card returns quickly, within hours or next day.
- Hard — You got it, but with hesitation or significant effort. The interval grows only slightly. Use this whenever there is real doubt.
- Good — Correct answer with normal effort. Interval grows at the standard pace. This is the most common rating.
- Easy — Immediate answer, no effort. Interval grows strongly. Reserve for cards you truly master.
Marking “Good” on a card you hesitated on pushes the next review too far out. The information may be forgotten between sessions. Be honest: if you hesitated, choose “Hard.” The goal is not to look like you are progressing—it is to actually progress.
Build a sustainable daily routine
Spaced repetition only works when it is regular. Principles for a routine that lasts:
- Do due reviews first—the cards scheduled for today. Never skip this step: postponed reviews create hard-to-clear accumulation.
- Add new cards after—10 to 20 per day at cruising pace, depending on your total volume and schedule. Do not exceed what you can sustain.
- Grade honestly on every card—this is the signal the algorithm uses for the next interval.
Duration, timing, and backlog management
15 to 20 minutes per day is enough for most learners. Evening is theoretically optimal (sleep helps consolidation), but consistency matters far more than timing.
If you miss several days, overdue cards pile up. Recommended strategy: do not clear everything at once (high discouragement risk). Resume normal pace and temporarily add zero new cards until backlog is absorbed.
Organize your decks effectively
- One deck per coherent subject or discipline—avoid mixing very different domains in a single deck
- Use subdecks by chapter once a deck exceeds 300 cards
- Use tags to filter by topic, difficulty, or status (to review, mastered)
- Archive finished decks instead of deleting—they remain useful references
Architecture example for a medical student
Deck Anatomy: subdecks Upper limbs, Lower limbs, Central nervous system.
Deck Pharmacology: subdecks Antibiotics, Cardiovascular.
Your deck structure reflects and reinforces your mental structure of knowledge. Investing in architecture early prevents painful reorganizations later.
Frequently asked questions
The algorithm accumulates overdue cards. Do not force a full catch-up in one session: resume at normal pace and temporarily pause adding new cards. The system is built to absorb occasional interruptions.
It depends on available time and card complexity. As a rough average, 100 to 150 daily reviews are manageable in 20 to 30 minutes with well-written cards. If your queue is consistently too heavy, you likely introduced new cards too quickly.
Spaced repetition is optimized for long-term retention. For an exam in two weeks, it still outperforms cramming—but its largest gains appear over weeks and months. Start as early as possible in your learning cycle.