How WaniKani SRS works (and why 4h, 8h, 23h, 47h)
WaniKani's knowledge base calls the Apprentice III wait "1 day" and the Apprentice IV wait "2 days". The API stores 82,800 and 169,200 seconds: 23 hours and 47 hours. The whole ladder is like this. In hours it runs 4, 8, 23, 47, then 167, 335, 719, 2,879, and every wait of a day or longer is exactly one hour short of its round number.
That missing hour changes your day. Do a "1 day" review at 5 PM and it comes back at 4 PM tomorrow, so reviewing at the same time each day means your items are always already waiting for you, and a late-night catch-up session returns at a more civilised hour next time. The popular explanation is that the hour is shaved off on purpose to stop your daily slot drifting later. That is community inference: no staff post says it, and neither the knowledge base nor the API docs give a reason. The effect is verified. The reasoning is WaniKani's secret.
The interval ladder
Wait before the next review at each stage, sqrt-scaled so the 4-hour bar survives next to the 4-month one. Labels use the everyday lengths; the exact waits are an hour shorter (167h, 335h, 719h, 2,879h).
Reading the ladder
The four Apprentice stages are the crunch. An item you learn this morning comes back four times in its first three and a half days, and you have to pass each review before the next timer starts. Clear Apprentice IV and it reaches Guru. Guru I is the stage the level-up rule counts and the point where the demotion penalty doubles. After that the system mostly leaves you alone: a week to Guru II, two more to Master, a month to Enlightened, then one final review four months out. Pass that and the item is Burned, out of the queue for good.
Add every wait together and a perfect item takes 4,182 hours from lesson to burn, a hair under six months. Eight passing reviews spread over half a year is the entire price of keeping an item forever.
Just before you forget
The entire published rationale for the ladder is one sentence in WaniKani's glossary: "Our timing intervals are meant to fall just before you'll forget the item." No numbers come with it, and nobody at WaniKani has published where 4-8-23-47 came from.
The psychology behind it is old. Ebbinghaus measured memory decaying exponentially in 1885, and every spaced-repetition system since makes the same bet: recall an item just before it fades and the forgetting curve gets shallower, so the next review can wait longer. WaniKani never names Ebbinghaus or publishes a target retention rate, and the gaps in its table grow by between double and quadruple per stage. The staircase below is the idealised model the ladder is betting on.
The fixed table also means WaniKani never adapts to you. Anki nudges each card's ease as you rate it, and its FSRS scheduler models what you personally are likely to forget. WaniKani hands every item the same eight waits: predictable enough to plan a level around, and rigid enough that there is no way to tell it 一 is easy.
Passing takes two answers
Most items ask for a meaning and a reading, and both must be correct in the same session for the item to climb one stage. (Radicals and kana vocabulary are meaning-only.) One right plus one wrong is a fail, and the item drops by a published formula.
The pairing surprises people: getting a review wrong twice costs the same as getting it wrong once. And because the penalty doubles at Guru I, drops from Guru and above always come in steps of two. A Master item you miss once or twice lands on Guru I and returns in a week; miss it three or four times in one session and it falls to Apprentice III. Apprentice IV is unreachable from Master.
Items that keep falling through that formula are leeches, and a handful of them can crowd a review queue more than a whole level of fresh lessons.
The 90% level-up rule
You advance a level when 90% of your current level's kanji reach Guru I or higher. Radicals never count toward the threshold, since their job is unlocking kanji, and vocabulary can lag levels behind without blocking anything.
The floor under any level is the Apprentice ladder itself: a fresh kanji lesson cannot reach Guru I in less than 82 hours.
In practice most levels add another 3-4 days on top, because a typical level's kanji sit locked behind its radicals, and the radicals need their own 82-hour walk to Guru first.
Levels 1 and 2 run a shifted ladder
The first two levels use WaniKani's accelerated SRS, with Apprentice waits of 2h, 4h, 8h and 23h. People call it the normal ladder halved, but half of 23 is not 8. It is the normal ladder shifted down one rung: each wait replaced by the next-shorter one, with Guru onward identical. The real table starts at level 3, which is why the pace feels like it hit the brakes around then. It did not. The first two levels were the anomaly.