How does spaced repetition work in Tomaru?

Spaced repetition calculates the exact moment to review a vocabulary word before you forget it. Tomaru uses the open-source FSRS-6 algorithm to track card retrievability, stability, and difficulty, allowing you to maximize retention while eliminating unnecessary reviews across iOS, Android, and the web.

What problem does spaced repetition solve?

Human memory decays predictably over time. If you review a vocabulary word too early, you already remember it, so the time spent adds nothing to your memory. If you review it too late, you have forgotten it entirely and must pay the full cost of learning it from scratch.

Both extremes waste time. Spaced repetition identifies the precise sweet spot between those two points—scheduling a review at the exact moment your memory is about to fade, just when testing yourself yields the highest possible retention gain.

What is FSRS?

Tomaru powers its schedule using FSRS-6, the 21-parameter generation of the open-source Free Spaced Repetition Scheduler. FSRS models human memory with far greater precision than older legacy schedulers.

Because FSRS is open source, its underlying mechanics are completely transparent and publicly verifiable. Tomaru applies this engine across all ready-made and custom vocabulary decks.

What three quantities does FSRS track per card?

FSRS maintains a mathematical model of your memory by tracking three distinct variables for every individual card:

• Retrievability: The estimated probability that you can recall the word right now, expressed as a percentage.
• Stability: How slowly your retrievability decays over time, representing the strength of your memory trace.
• Difficulty: An inherent measure of how hard the word is for you to learn and retain relative to your overall study history.

These values shift dynamically after every review, pushing the next appearance date forward or backward depending on your performance.

What is target retention, and how do you set it?

Target retention is the percentage probability you want to maintain for correct recall when a card becomes due. Tomaru lets you set this target to 85, 90, or 95 percent, with 90 percent set as the default.

This setting involves a direct trade-off rather than a difficulty adjustment:

• Higher retention (95%): Requires more frequent reviews per word, which reduces the number of new words you can introduce within your available study time.
• Lower retention (85%): Requires fewer reviews, allowing more new words into your schedule at the cost of higher rates of forgetting.

How do the four rating buttons control the schedule?

After reviewing a card, you report your recall using four buttons: Forgot, Fuzzy, Clear, and Know it. These ratings are the only input the FSRS algorithm receives to update your card history.

Honest reporting is essential for accurate scheduling. Rating a card more generously than your actual memory warrants does not help you learn faster; it misleads the algorithm into thinking the word is easier than it is, resulting in premature forgetting.

How does Tomaru personalize your schedule?

FSRS uses 21 numerical parameters to calculate memory decay. When you start, these parameters reflect a general population average rather than your personal cognitive patterns.

You can start parameter optimization from your app settings once you reach roughly 1,000 qualifying reviews. Only reviews logged at least one day after the previous review of the same card count toward this requirement, as same-day repetitions do not reflect long-term retention. Our servers fit all 21 parameters on your review history using the official FSRS optimizer, then apply the updated parameters automatically across all your devices and word lists.

To learn more about how Tomaru structures your daily study queue, read our guide on what Tomaru is and our overview of Smart Study for limited time.

Related questions

What is the difference between Forgot, Fuzzy, Clear, and Know it?

These four buttons record your exact recall state: Forgot indicates a complete blank, Fuzzy means you struggled to recall the word, Clear represents standard successful recall, and Know it denotes instant, effortless recognition.

Can I change my target retention percentage later?

Yes. You can adjust your target retention between 85, 90, and 95 percent at any time in your settings, and the schedule will adapt to your new target.

Do I start the FSRS parameter optimization myself?

Yes. You start the optimization from your app settings once you reach roughly 1,000 reviews, counting only reviews spaced at least one day apart per card. Tomaru then handles the rest: our servers run the fit using the official FSRS optimizer and apply the finished 21 parameters automatically across every device.