You're looking for last year's tax return. Is it in "Finance," "Taxes," "2025," or "Important Documents"? You made all four folders, on different days, and now the file could be in any of them. Johnny.Decimal exists to kill that exact moment of doubt — by giving every note and file one, and only one, correct home.
Created by Johnny Noble, the system is a strict numbering scheme borrowed from the way libraries and warehouses index physical things. You divide everything you own into at most ten areas, each area into at most ten categories, and number them. A note then gets an ID like 12.03: area 10–19, category 12, item 03. Two numbers, and you know precisely where it lives.
How the numbering works
The structure is deliberately shallow — never more than two levels deep.
| Level | Range | Example |
|---|---|---|
| Area | Grouped in tens (10–19, 20–29…) | 20–29 Finances |
| Category | A single number inside an area | 22 Tax |
| ID | Category . item |
22.04 2025 return |
The "ten and no more" limit is the whole trick. A hard ceiling forces you to decide where something belongs instead of endlessly nesting sub-folders. When you can't find a home for a new note within the existing numbers, that's a signal your map needs rethinking — not that you need a folder called "Miscellaneous."
Why people love it
The payoff is certainty. Because each ID is unique, there is exactly one right answer to "where does this go?" and "where do I find it?" — the ambiguity that sinks most folder systems simply cannot occur. The numbers are also memorable in a way folder paths never are; regular users recall that receipts live in 22 the way you recall a phone extension. It's a rigorous cousin of the ideas in personal knowledge management, and it pairs naturally with keeping each note focused on one thing, the case for atomic notes.
It also travels. The same 22.04 works as a folder on your disk, a tag in your notes app, and a label on a physical binder — one index across your whole life.
Where it's overkill
Johnny.Decimal is a filing system, not a thinking system. It tells you where things go; it does nothing to connect ideas or resurface them later. If your goal is a network of linked thoughts, this isn't that — it's the opposite instinct, a place for everything rather than a web of everything.
It also asks for real up-front work: you have to design the map before it helps, and maintain the discipline to number things as they arrive. For a large, stable archive of documents and reference material, that investment pays off handsomely. For a stream of quick personal thoughts, ideas, and journal entries, assigning a numeric ID to every fleeting note is friction that will quietly kill your capture habit. Compare it to the lighter, actionability-based PARA method, or the capture-first approach in our guide to organising notes you'll actually use — both ask far less of you day to day.
There's also a modern wrinkle: the entire premise is that you must remember where you filed something. When search is instant and AI can read across everything you've written, the cost of not filing perfectly drops sharply. That's the bet Clair Mind makes — capture in one tap, skip the numbering, and let the app connect and answer across your notes. If you love a place for everything, Johnny.Decimal is a genuinely excellent map. If filing is the part you always abandon, you may not need one at all.