Spaced Repetition Intervals: The Science Behind the Numbers (2026)

Why does Anki show you a card after 1 day, then 4 days, then 10 days? Here's the science behind spaced repetition intervals — and how to know if yours are optimized.

Why Intervals Grow: The Stability Model

Each time you successfully recall a memory, you don't just remember it — you strengthen it. The memory becomes more stable. A more stable memory decays more slowly. This is why the intervals grow exponentially:

These are approximate. Your actual intervals depend on which buttons you press and the algorithm used (SM-2 vs FSRS). FSRS produces more accurate intervals by modelling each card individually.

What Happens When You Press Again

When you press Again (forgot the card), two things happen:

1. The interval resets. The card goes back to the learning queue, typically with a short interval (1–10 minutes). It won't be scheduled for days — it needs to be re-learned first.
2. The card's difficulty increases (in SM-2) or its stability decreases (in FSRS). This means it will progress more slowly in the future — future intervals will be shorter than they would have been otherwise.

This is why good card design matters so much. A card that you consistently fail because it's ambiguous or tests two things at once will never have a long interval — it'll perpetually reset, wasting review time. See how to create Anki cards for how to fix this.

The Ease Hell Problem (SM-2)

In Anki's old SM-2 algorithm, every Again press reduces the card's "ease factor" by 20%. Ease factor determines how fast intervals grow. After several Again presses, a card might have an ease factor of 130% — meaning it barely grows at all, and you'll be seeing it every few days forever.

This is called "ease hell." Cards stuck in ease hell consume a disproportionate amount of your daily reviews. The fix is either:

• Switch to FSRS, which doesn't use ease factors at all
• Use the Reset Card Progress option on cards that have become permanent low-ease cards
• Delete or suspend cards that are genuinely too hard or poorly designed

How FSRS Calculates Intervals Differently

FSRS replaces the ease factor with a stability variable. Stability measures how long it will take for your recall probability to drop to your target retention (e.g., 90%). The next interval is simply calculated as the time until you're expected to drop below 90% recall.

This means:

• Hard cards naturally get shorter intervals (their stability grows slowly)
• Easy cards get longer intervals (their stability grows quickly)
• No ease hell — stability never permanently deteriorates from forgetting
• Intervals are calibrated to your specific retention target, not a generic schedule

Stop Worrying About Your Intervals

The most common mistake is obsessing over whether your intervals "look right." If you're using FSRS with a 90% retention target, your intervals are already optimized for your personal review history. Trust the algorithm.

What actually matters: reviewing every day (consistency beats optimization), having well-designed atomic cards, and not pressing Easy to inflate intervals artificially. See the Anki settings guide and FSRS algorithm explanation for the full setup.