How the scheduling works

The forgetting curve

Memory research going back to Ebbinghaus shows that memorized material fades on a predictable curve. Reviewing a word right before it fades is dramatically more efficient than reviewing it on a fixed calendar — the whole idea behind spaced repetition. The catch: the fade point differs for every word and every person.

FSRS-6, the algorithm inside

WordIsle schedules with FSRS-6 (Free Spaced Repetition Scheduler), the open algorithm also used by Anki. After every answer it updates two numbers per word:

• Stability — how many days until the memory is expected to drop to 90% retrievability.
• Difficulty — how hard this particular word is for you, which changes how much each answer moves stability.

Your ratings (Forgot, Hard, Good, Easy) feed both numbers. A word you keep answering Good on gets longer and longer intervals; a word you forget comes back sooner, at the front of the queue. The target recall is set to 90%.

Why not just review randomly?

Random flashcard apps treat every word the same. That wastes time on words you know cold and starves the ones you keep losing. FSRS-6 maintains a per-word schedule, so a 5-minute session is always the 5 minutes that matter most — reviews that are due today, sorted most-starved first, then a capped batch of new words.

What mastered means here

A word counts as mastered once it reaches review state with a stability of 21 days or more — the scheduler expects you to still know it three weeks from now without seeing it. You can watch that number grow on your progress page.

Your data

Every rating and schedule lives in your browser only. Export a JSON backup anytime from the progress page — it restores fully on any device.