Back to blog
Science

How spaced repetition actually works

A deep dive into the algorithm that powers Fluence's review system. We explain the Ebbinghaus forgetting curve, how we calculate optimal review intervals, and why you remember words you learned weeks ago.

Mar 20, 2026 · 12 min read

The forgetting curve

In 1885, German psychologist Hermann Ebbinghaus discovered something fascinating: we forget new information in a predictable pattern. Within 24 hours of learning something new, we forget about 70% of it. Within a week, we've lost about 90%.

This sounds depressing, but there's a silver lining: every time you review the material, the forgetting curve gets flatter. The first review might buy you 2 days. The second review buys you 5 days. The third buys you 2 weeks. Eventually, the word is in your long-term memory.

How Fluence's algorithm works

Every word you learn in Fluence gets a "memory strength" score from 0 to 100. Here's what happens:

  1. New word learned: Memory strength starts at 50
  2. Correct answer: Memory strength increases (amount depends on current strength)
  3. Wrong answer: Memory strength decreases significantly
  4. Time passes: Memory strength gradually decays based on the forgetting curve

When a word's memory strength drops below a threshold, it gets scheduled for review. Words you struggle with (low strength) appear more often. Words you know well (high strength) appear less often.

The math behind the intervals

We use a modified version of the SM-2 algorithm (the same foundation used by Anki, one of the most popular spaced repetition tools). But we've adapted it for language learning specifically:

  • Easy words (consistently correct): Intervals double each time (1 day → 2 days → 4 days → 8 days → 16 days)
  • Medium words (sometimes wrong): Intervals grow more slowly (1 day → 2 days → 3 days → 5 days → 8 days)
  • Hard words (frequently wrong): Intervals stay short until you demonstrate mastery (1 day → 1 day → 2 days → 3 days)

Why it feels like magic

After using Fluence for a few weeks, most learners have a moment of surprise: they remember a word they learned in week one, without having studied it recently. That's spaced repetition working as designed.

The beauty of the system is that you don't have to think about it. You just show up, do your lesson, and the algorithm handles the rest. It knows what you need to review and when.

Active recall: the other piece

Spaced repetition only works when combined with active recall — the act of retrieving an answer from memory rather than passively recognizing it. Every exercise in Fluence is a mini-test: we show you a word and you have to produce the translation, not just recognize it from a list.

Research shows that active recall strengthens memory 2-3x more than passive review. Combined with spaced repetition, it's the most efficient way to learn vocabulary that science has found.

Start building your memory

Every lesson you complete in Fluence feeds the algorithm with data about your memory patterns. The more you practice, the smarter it gets at scheduling your reviews. After about 2 weeks of daily practice, the system has a solid model of your memory and reviews become increasingly personalized.

Ready to start learning?

Put these insights into practice. Start your first lesson today.

Start today — it's free