The Science of Spaced Repetition
How FSRS-5 uses memory research to help you learn faster and remember longer.
The Forgetting Curve
In 1885, Hermann Ebbinghaus discovered that memory decays exponentially over time. Without review, we forget roughly 50% of new information within an hour, and up to 80% within 24 hours.
However, each time you actively recall information, the rate of forgetting slows down. This is called the spacing effect — and it's the foundation of modern spaced repetition systems.
Key Insight
The best time to review a card is just before you would forget it. This maximizes the learning benefit per review while minimizing total study time.
What is FSRS?
FSRS (Free Spaced Repetition Scheduler) is a modern algorithm developed by researchers Jarrett Ye and Piotr Wozniak. Unlike older systems (like Anki's SM-2) that use fixed intervals, FSRS models memory with three dynamic parameters:
Stability
How long a memory will last, measured in days. Each successful review increases stability.
Difficulty
How inherently hard a card is. Cards you consistently struggle with stay at higher difficulty.
Retrievability
The probability you'll remember a card today. Reviews trigger when R drops below a threshold.
FSRS-5, the latest version, uses a neural network trained on millions of real review logs. It can predict your recall probability with remarkable accuracy and schedule reviews at the optimal time.
FSRS vs. Traditional Systems
| Traditional (SM-2) | FSRS-5 | |
|---|---|---|
| Scheduling | Fixed multipliers (1, 2.5, 4, 7 days...) | Dynamic intervals based on your memory |
| Adaptation | One-size-fits-all parameters | Learns from each review you make |
| Accuracy | Heuristic-based intervals | Neural network trained on 50M+ reviews |
| Reviews needed | Baseline | ~30-50% fewer for same retention |
Real result: FSRS users typically achieve the same retention rate with roughly half the daily reviews compared to SM-2. More learning, less time.
Why Active Recall Matters
Spaced repetition leverages two well-established cognitive science principles:
The Testing Effect
Actively retrieving information from memory strengthens neural pathways more than passive review. Every time you flip a card and recall the answer, you reinforce that connection.
Desirable Difficulty
Retrieval that requires some effort produces stronger long-term memory than easy recall. FSRS targets the "sweet spot" — hard enough to be effective, but not so hard that you fail.
Tips for Best Results
- 1 Study daily. Even 5-10 minutes of daily review beats cramming for an hour once a week. Consistency builds stable memories.
- 2 Be honest with your ratings. If you barely remembered, tap "Hard." If it was instant, tap "Easy." Accurate ratings help FSRS schedule optimally.
- 3 Keep cards simple. One concept per card is ideal. Complex cards are harder to remember and harder for the algorithm to schedule.
- 4 Use mnemonics. Enable AI-powered visual mnemonics for tricky words. A memorable image can boost retention dramatically.
Put science to work
Start learning with an algorithm that adapts to your memory.
Start Learning with FSRS