Ever shelved a vocabulary book with the first thirty pages black with notes and the rest still spotless?
The problem isn't willpower. It's order. Nobody can tell by feel what to review and when. KotsuKotsu is a Japanese vocabulary app that does that math for you. It holds 18,557 Japanese words, from hiragana to JLPT (Japanese-Language Proficiency Test) N1 and beyond, and all you have to do is today's cards.
“What should I study today?”
The app picks for you. It starts with the 208 hiragana and katakana of the gojūon chart, then runs through JLPT N5, N4, N3, N2, and N1 to words beyond the levels: 18,557 words in days of 50 cards. No need to remember where you stopped yesterday. Open the app and today's cards are ready, and when you're done, it tells you that's it for today.
“I saw it once. Why didn't it stick?”
That's simply how memory works.
In 1885, Hermann Ebbinghaus tested himself to measure how fast memories fade, and drew the forgetting curve. Close to half of something new is gone within an hour, and two thirds or more within a day. A word you clearly learned yesterday slipping away today has nothing to do with willpower.
But the curve has a loophole. Recall something just before you forget it, and the curve flattens. Seeing a word once a day for ten days sticks far longer than seeing it ten times in one day. This is called the spacing effect, confirmed by a 2006 meta-analysis of 317 experiments from 184 papers. The same research found that the longer you want to remember something, the wider the gaps should be.
So the real question is when “just before you forget” is. That's not something anyone can guess by feel.
“So when does it ask me again?”
The moment your chance of remembering drops to 90%.
KotsuKotsu draws a separate forgetting curve for every card and asks again at the point where the memory has started to blur but isn't gone yet. Meet a word after you've fully forgotten it and you're learning it from scratch. Meet it too early and it's still so fresh that nothing gets warmed up. Memories that take effort to recall last longer, so it aims for the point where a single review is worth the most.
Trying to recall the meaning before you see it strengthens memory on its own, far more than reading it again. Those few seconds you pause before flipping a card are the most important part of this app.
Each time you get a word right, the curve flattens and the next meeting moves further out. A word you just learned comes back in 10 minutes, one you've gotten right a few times in a few days, and one you truly know in a few months. Miss it and the gap shrinks again. Under each grade button you can see when the card will come back, so you know what your answer changes before you press it.
A paper vocabulary book makes you look at the words you already know as often as the ones you don't. KotsuKotsu moves that time to the words you don't know. That's why the same 30 minutes leaves you with more.
The math is done by FSRS, the spaced repetition algorithm built into Anki. It's a model refined on 700 million reviews from 10,000 real learners, and KotsuKotsu uses its latest version as is. You can also change the 90% target to 85% or 95%. Raise it and you'll see words more often; lower it and you'll see them less, at the cost of forgetting a little more.
“How much do I know right now?”
Every word has a temperature: where it sits on its forgetting curve right now, turned into a color.
A word you just learned is a warm green. One you haven't seen in a while cools through yellow to orange, and one you've missed three or more times sinks to red. Reviewing is warming cold words back up.
We could have used numbers, but didn't. “73%” has to be read and then interpreted; a color takes only a glance. Scroll through a word list and the gaps show themselves. The kana charts are colored the same way, so the characters you haven't touched yet stand out right away.
“How far have I come?”
Your streak, accuracy, daily average, and time per card add up every day. Look back over past days on a single heatmap, and see ahead how many reviews are waiting in the coming days. It even tells you the date you'll finish if you keep your current pace.
“What can I do on the website?”
Open the same vocabulary as the app right in your browser. Search all 18,557 words by kanji, reading, or meaning, and follow the levels and days down to one page per word. In the lists you can hide readings and meanings and shuffle them to test yourself, and grading cards here gives words a temperature too.
Temperatures set on the website stay in that browser only. Picking today's cards and bringing words back right before you forget is the app's job. The website is for learning, on the spot, a word you found through search; the app is for keeping at it every day.
“Why no example sentences?”
Because they don't help you memorize words.
Every time we considered a feature, we asked: “Could you still memorize words without it?” Example sentences didn't make the cut. Neither did rankings, friend lists, or logins. What's left on the card is the kanji, the hiragana reading, the meaning, the part of speech, and the word's origin. That's all.
“Why no rankings?”
Because someone else's pace shouldn't become your yardstick.
How many new words a day, the review limit, your target retention, the time your day starts over, the order new words appear in, and how many answer buttons you get: you decide all of it.
There are no notifications either. Piled-up badges and nagging alerts only make you feel chased and anxious, and an app you open that way doesn't last. Your streak is a record, not a punishment, so the app won't warn you when it breaks or beg you to keep it going. Open it when you want to.
There are no accounts. All your study history stays on your device, and if you need to, you can export it to a file and move it to another device.
And more
- Native audio plays automatically when you flip a card
- Choose from four voices, two female and two male, and download them once to listen offline
- Search by word, reading, or meaning
- Save the words you like and add your own notes
- Words you're forgetting and words you often miss gather in their own lists
- Light and dark themes, with background effects and blur you can turn off
It's a vocabulary app for complete beginners starting from hiragana, test takers organizing words level by level for the JLPT, and working adults teaching themselves Japanese without classes.
Smart is the app's job. Steady is yours.
Just finish today's cards.