A spaced-repetition engine from 1995, rebuilt for the browser
Learn kanji the way metal cools.
Every kanji carries a temperature. Miss it and it heats up; get it right and it cools. K2 keeps drawing the hot ones until they go cold, then brings in new ones on its own. No due dates. No review pile.
No account needed · 1,006 kyōiku kanji, grades 1–6 · Progress saves in your browser
700
kanji in 12 hours.
Erik Bethke, one weekend in 1995, using the first version of K2: two six-hour sessions, from 100 kanji known to 800.
Numerical Recipes, a stack of flashcards, and a pair of dice.
In 1995 Erik was a software engineer stuck at the flashcard wall. There are over 2,000 kanji in common use, and scheduled reviews felt like shoveling sand with a teaspoon.
On his desk was Numerical Recipes, open to Monte Carlo methods. So he gave every kanji a probability of being picked and let the dice decide: a miss made a kanji more likely to come back, a right answer let it fade.
Thirty years later he rebuilt it for the browser. The engine is still about fifty lines.
Read the full story →The whole engine fits on an index card.
No intervals, no ease factors, no calendar. Each kanji holds one number, and three rules move it.
Roll the dice
Pick a random kanji from your pool and roll between 0 and 1. It's shown only if the roll lands under its number, so hot kanji come up often and cold ones rarely.
show if roll < pHalve or double
Right answer: the number halves. Miss: it doubles, up to 1. Three right in a row takes a kanji from 0.5 to 0.0625, and it all but disappears.
right → p ÷ 2 · miss → p × 2Grow the pool
When the kanji you're still working on average below 0.15, five to ten new ones arrive at 0.5. You never set a daily quota.
avg p < 0.15 → +5–10 newOne kanji, eight answers
selection probability p, log scaleField guide
Kanji you'll meet on day one.
All eight are in the deck. Learn them at home and they stop being decoration the moment you land.
- 01stations出口Exitdeguchi
Every station, every mall, every stairwell.
- 02shops入口Entranceiriguchi
Shops, museums and car parks.
- 03stations東口East exithigashiguchi
Where your friend says to meet you.
- 04streets駅Stationeki
Street signs, maps and every train line.
- 05stations改札Ticket gateskaisatsu
Where your IC card beeps.
- 06transit乗り場Boarding areanoriba
Trains, buses and the taxi rank.
- 07shops営業中Openeigyōchū
The sign hanging on the shop door.
- 08menus生Draftnama
As in 生ビール, draft beer, top of every izakaya menu.
Past the kanji: the Japanese you'll actually say.
Seven drills on the same engine, so the grammar you trip on keeps coming back until it doesn't.
Powered by Bike4Mind
The engine picks the card. Bike4Mind gives it a voice.
K2 is powered by Bike4Mind. Sign in with your Bike4Mind account and the same drills open up: natural voices, live AI practice in every language, and your progress on every device.
The core drills stay free without an account. AI features run on your Bike4Mind credits.
Hear it in HD
Natural, studio-quality voices on every card, so 出口 sounds like the station announcement and not a robot.
Live AI practice
Dynamic reading, listening, speaking and dialogue, generated fresh across every language instead of drawn from a fixed script.
Your progress, everywhere
Every kanji's temperature syncs across your laptop and phone. Drill on the train, pick it up at home.
Same engine, other things worth knowing
Questions people ask first
How is it different from Anki?
Anki asks what's due. K2 asks what you need right now.
A calendar can't tell whether you're bored or drowning, so it serves what's due either way. K2 re-reads you after every single answer. Miss a kanji and it's back within moments; nail it and it steps aside. You stay right at the edge of what you know, where every card is a real challenge and none is a chore. That's flow state, and K2 holds you in it for as long as you keep going. It’s how six hours felt like one in 1995.
Is K2 free?
Yes. Every K2 drill is free and works without an account. Sign in with Bike4Mind for sync, HD voices and live AI practice.
Why am I getting new kanji already?
Because the ones you have are cooling. K2 adds new material when your working set averages below 0.15, which keeps you at the edge of what you know.
Do I need to know any Japanese?
No. K2's kanji start with 日, 月 and 火, and every card shows its readings and meaning as you go.