The Complete Guide to Spaced Repetition

What is Spaced Repetition?

Spaced repetition is a learning technique where you review information at increasing intervals over time, timed precisely when your brain is about to forget it.

It's not a new idea — German psychologist Hermann Ebbinghaus described the "forgetting curve" in 1885. But software now makes it practical for everyday learning.

The Forgetting Curve

Ebbinghaus's research showed memory decays predictably:

Without review, nearly 80% of what you learn is gone within a month.

How Spaced Repetition Fights Forgetting

Each time you successfully recall information, the forgetting curve resets — but at a slower decay rate. Review it again at the right moment, and the curve gets even shallower.

Optimal review schedule (example):

After 5–6 reviews with this spacing, the information moves into long-term memory.

The SM-2 Algorithm

Modern spaced repetition apps (Anki, SuperMemo) use algorithms to calculate exact review times. The SM-2 algorithm — developed in 1987 — calculates intervals based on:

You don't need to understand the math — the app handles it. Your job is to rate your confidence honestly after each recall attempt.

Spaced Repetition + Active Recall

The real power comes from combining spaced repetition with active recall — not passively re-reading, but actively testing yourself.

The workflow:

Tools like SimpleQuizMaker generate quiz questions from your materials, which you can then use as review material on your spaced schedule.

What to Study with Spaced Repetition

High value:

Lower value:

Getting Started with Anki

Anki is the gold standard free spaced repetition app:

Pro tip: Import question sets from SimpleQuizMaker into Anki for instant study decks.

Frequently Asked Questions

How long should I study with spaced repetition each day?

20–30 minutes of focused review is more effective than 2-hour cramming sessions.

What if I miss a day?

Don't panic — just resume where you left off. The algorithm adapts. Consistency over perfection.

Can I use spaced repetition for essays or complex topics?

Not directly. Spaced repetition excels at discrete facts. For complex topics, use concept mapping and practice problems alongside.

The math behind spaced repetition intervals

The interval growth in classic spaced repetition isn't arbitrary; it tracks the forgetting curve. Ebbinghaus's 1885 finding (still broadly accurate): without review, recall drops to ~50% within 24 hours, ~30% within a week, ~20% within a month. Each successful retrieval flattens that curve.

The intervals most modern apps use (FSRS, SM-2, Anki) follow roughly:

The exact intervals adapt per-card based on how confident you were on the previous review. A card you "easily" recalled gets a longer next interval; a card you "barely" recalled gets a shorter one.

Why interval timing matters so much

The optimal interval for retrieval practice is the moment just before you would have forgotten. Too soon = easy retrieval, weak strengthening. Too late = total failure, you have to re-learn. The sweet spot is what cognitive psychologists call "desirable difficulty" — effortful but achievable.

Modern algorithms estimate that moment based on your past behavior. Older systems (Leitner boxes from the 1970s) used fixed intervals; they were better than nothing but worse than personalized scheduling.

When spaced repetition fails

A few common failure modes:

Spaced repetition for different content types

Combining with other study techniques

Spaced repetition is one input to a strong study system, not the whole system:

A study plan using only flashcards plateaus at recall. Pair it with practice problems, teaching back, and concept mapping for the deeper levels.