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Field note · Household money book

yen-yen: a money book for two that never touches your bank

A money book you fill in by hand, with no bank link. Four categories (need, want, culture, unexpected) and one book you can share. Free for one person, no ads.

Kenji Sakuramoto · ｋｅｎｓａｕｒｕｓ合同会社 ·

Connect a money app to your bank and you hand it your bank login. When it sorts a purchase into the wrong category, you are the one who fixes it later.

So I made yen-yen a money book you fill in by hand, with no bank link at all. Two people, or a whole family, can keep the same book.

Entering it yourself is the point

When you type the amount yourself, you notice the money at the moment you spend it. Since every entry is manual, I made each one take a few seconds: type the amount, pick the account and the category, save.

Four categories, and a four-colour bar at the end of the week

Every expense goes into one of four categories: need, want, culture, or unexpected. It is the household-book method Motoko Hani started in Japan in 1904, carried over to the phone as it is. At the end of the week you look at one bar in four colours, and you can see where the money went.

The home screen: net worth, and this week's spending as one bar split into four colours for need, want, culture and unexpected, under a banner saying the data is a sample
The week in four colours
The expense form: an amount of 1,280 yen in JPY, a payee field, account buttons for a bank account, a wallet and a shared card, the four category buttons, and Save
Logging an expense
The accounts screen: a bank account, a wallet, a shared card and a savings account for a Sapporo trip, each with its balance
Accounts (sample)
Screenshots from the Japanese Google Play listing, updated 2026-10-08. They show the app's built-in sample household, not a real user's data.

You can record yen or dollars in the same book, at the rate from the day you spent it (28 currencies). Budgets and upcoming bills live in the same book. The app is in Japanese, English, Thai and Chinese.

One book for the two of you

Before this I used an app called Expense IQ. Keeping one book with my family in real time was hard, and nothing in it used AI to help check what we'd entered. So I made one.

Invite your partner or family, and you each record into the same book and look at the same numbers. It is for couples and families who want to keep a money book going together.

What's free, and what isn't built yet

The book for one person is free, with no ads. Manual entries, the four categories, budgets, up to three accounts, a CSV and JSON export of the whole book, and deleting your account are all free.

Sharing with your household, and pasting in statement text for AI to sort (you check each line before it is saved), are free for the first 100 households. Spots were still open on 16 October 2026. After those spots fill, the book for one person stays free.

You can try it before signing up: the app opens on a sample household you can look around. Nothing in it is saved.

Not built yet: yen-yen does not pull in bank or card statements automatically.

See it in the exhibit hall