Solve any sudoku with notes alone.
NOTE-KU is a free sudoku app and a method. You build a map of candidates with a few strict rules, let every note eliminate others, and when each empty cell holds exactly one candidate, you confirm the whole board in one final sweep. No guessing, at any level.
Continue playing ▸Notes aren’t a fallback. They’re the method.
Most players solve sudoku reactively: spot an obvious number, write it in, scan again. Notes come out only when they get stuck, and on harder puzzles those notes end up incomplete, out of date and impossible to trust.
NOTE-KU turns that around. Notes are the only tool you use, from the first empty cell to the last. You never write an answer until the notation has proven it, and you never guess.
Digit-first thinking. Instead of looking at a cell and asking “what goes here?”, you pick a digit and ask “where can it still go?” across the whole board. That one change reveals constraints that cell-by-cell solving hides.
Restraint by design. You only write a note when a digit is down to one or two possible cells in a box. Every note on the board carries real information, so the board gets simpler with every pass instead of more cluttered.
Three phases. Every puzzle.
The phases never change. A harder puzzle only means more passes through them.
The Notation Pass
Take digit 1. In each box, find the cells where it can still go. One or two cells: note them. Three or more: skip the box for now. Repeat for every digit.
Elimination & Iteration
Every note changes what’s possible elsewhere. Follow each consequence through its row, column and box, then return to the boxes you skipped. Repeat.
The Final Pass
When every empty cell holds exactly one candidate, the puzzle is already solved. Confirm it digit by digit in one continuous sweep.
Temp Mode
Stuck on a hard board? Test one of two candidates in a contrasting color. If it leads to a contradiction, the other one is proven. One tap clears the test.
Built around the method, free in your browser.
No download, no account. Open noteku.app/play on your phone, tablet or computer and start solving.
Five levels
★ Mini 6×6, Beginner, Intermediate, Advanced and ★★★★★ Extreme. Fewer givens at each level, more passes to reach the threshold.
Puzzles you can trust
A new puzzle every game, checked for exactly one solution and solvable by pure logic. No broken boards, no forced guessing.
Two ways to play
NOTE-KU Mode: notes only until the Final Pass unlocks. Classic Mode: regular sudoku with full note support.
Bring your own puzzle
Type in any puzzle from a book or newspaper. The app checks it has a single solution before you play.
Make it yours
25 themes and 26 digit styles, from Roman numerals to world currencies, unlocked with NoteCoins you earn by solving. No real money involved.
Private by design
Your name, coins and settings stay in your browser. Nothing to sign up for and nothing sent to a server.
The NOTE-KU Starter Pack
Thirty puzzles to learn the method on paper, from your first Mini to your first Extreme. Every puzzle has exactly one verified solution, and every level is rated by the techniques it actually needs.
- 30 puzzles: 6 Mini 6×6, 6 Beginner, 6 Intermediate, 6 Advanced, 6 Extreme
- The complete method on one page
- A digit tracker under every grid to know when the Final Pass is ready
- Full answer keys · 23 pages · made for printing at home
Orders are handled by email for now: we reply with payment details (USDT, BTC or ETH) and send your PDF as soon as the payment arrives. Card payments are coming soon.
One method, taught at every level.
NOTE-KU is becoming a series of books, each teaching the same method at a different depth. They are in preparation; the Starter Pack is available now.
First Marks
The grid, digit-first thinking, the Two-Candidate Rule and your first Final Pass, with fully guided puzzles.
Narrow It Down
Deeper elimination for boards where the first passes stall, and how to read the whole grid at once.
The Final Strike
Expert and extreme puzzles, proof by contradiction with Temp Mode, and full-board reading at scale.
Editions for children (ages 4–9) and teens (10–15) are planned as well.
Learn to solve without guessing.
How to solve sudoku without guessing
A complete, step-by-step approach that works from easy to extreme.
Read ▸The Two-Candidate Rule
The single rule that keeps your notes clean, meaningful and trustworthy.
Read ▸Stuck on a sudoku? Use Temp Mode
How to test a candidate safely and prove the answer by contradiction.
Read ▸Frequently asked.
Is NOTE-KU free?
Yes. Every level and both modes are free in your browser at noteku.app/play. Themes and digit styles are unlocked with NoteCoins you earn by solving, never with real money.
Do I have to write every possible candidate in every cell?
No, and that’s the point. NOTE-KU is not “full candidate mode”. Under the Two-Candidate Rule you only note a digit when it has one or two possible cells in a box. Why it works.
Does it work on hard and extreme puzzles?
Yes. The method is the same at every level; harder puzzles need more passes. For the hardest boards, Temp Mode lets you test one of two candidates without ever guessing.
Do I need an account?
No. There is nothing to sign up for. Your progress is saved in your browser on your own device.
A puzzle that trains how you think.
A 2026 systematic review of brain-imaging studies found that solving sudoku consistently engages the brain’s frontoparietal network: working memory, planning and error monitoring. NOTE-KU makes those processes explicit, one phase at a time.