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Each gold node is the sum of the four cells around it.

Nodoku

Sudoku with a twist: gold nodes reveal the sum of the cells around them.

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Nodoku – Free Online Sudoku with Sum Nodes (Daily Puzzle)

A Sudoku variant that adds a layer of logic: gold nodes reveal the sum of the four cells around them. Daily puzzles, three difficulty levels, notes mode, hints and a timer — all in your browser.

Nodoku is a free online Sudoku variant with a unique twist. It plays like normal Sudoku — fill every row, column and box with each digit exactly once — but each gold node on the grid shows the sum of the four cells touching it. When you fill those four cells, they must add up to the number on the node. The extra constraint cuts through guessing: most solutions can be deduced logically without ever backtracking. Two board sizes (6×6 and 9×9), daily shared puzzles, three difficulty levels, notes mode, and hints — all running in HTML5.

Quick Answer: What Is Nodoku?

What is Nodoku?

Nodoku is a free Sudoku variant where gold nodes on the grid corners reveal the sum of the four cells touching them. You fill the grid using standard Sudoku rules (each row, column and box gets every digit once) and must make each node's four touching cells add up to the node value. Each puzzle has exactly one solution. Daily puzzles give every player the same grid, so leaderboard times are pure skill.

How to Play Nodoku — Step by Step

  1. Start a puzzle.

    Choose Daily (same puzzle for everyone today) or Unlimited (random each time). Then pick Easy (6×6 grid, many starting digits), Medium (6×6, fewer starting digits), or Expert (9×9, more cells and nodes).

  2. Read the board.

    Filled-in cells are given digits and cannot be edited. Empty cells need you to fill them. Gold nodes sit on cell corners and show the sum of the four cells touching them.

  3. Tap a cell, then a number.

    On touch or desktop, tap the cell then tap a number on the keypad below the board. Or use keyboard digits 1–9 on desktop.

  4. Watch for conflicts.

    If a digit conflicts in its row, column or box, the cells turn red. If a node's four cells cannot possibly sum to the node value, the node turns red.

  5. Use notes mode.

    Press N or tap Notes to pencil in candidate digits. When you place a final digit, matching candidates in the same row, column and box are automatically removed.

  6. Earn hints only when stuck.

    Tap the ? button to reveal a cell's true value. Hints are limited: 3 on Easy, 3 on Medium, 2 on Expert.

  7. Solve to win.

    When every cell holds the correct digit (checked against the unique solution), the puzzle is solved. Confetti and a share result appear.

Understanding the Sum Nodes

The gold node mechanic is the heart of Nodoku. Here is exactly how it works:

Rule: Each node sits at the corner of a 2×2 group of cells. Its number equals the sum of those four cells. When all four cells are filled, their sum must match the node. If some cells are empty but the current values already make the sum impossible, the node turns red — a signal that something is wrong.

The node constraints are how Nodoku eliminates guessing. In a normal Sudoku, hard puzzles often require "advanced techniques" or trial-and-error. In Nodoku, the sum constraint usually gives you a direct deduction: "these 3 cells already sum to X, so the fourth must be Y."

Nodoku vs Standard Sudoku vs Killer Sudoku

FeatureNodokuStandard SudokuKiller Sudoku
Rows, columns, boxes✅ Yes✅ Yes✅ Yes
Extra constraint4-cell sum nodesNoneMulti-cell sum cages
Node / cage size4 cells (fixed)—2–9 cells (variable)
Solution uniqueness✅ Guaranteed✅ Usually✅ Yes
Board sizes6×6, 9×94×4 to 25×259×9 only
Daily shared puzzle✅ Yes⚠️ Some sites⚠️ Some sites
PriceFreeFree/paidFree/paid

Difficulty Levels Explained

Easy — 6×636 cells. Many digits filled in at start. 9 nodes. 3 hints. Ideal for learning the node logic without Sudoku's usual complexity.
Medium — 6×636 cells. Fewer starting digits. 6 nodes. 3 hints. The classic Nodoku experience — solvable with pure logic, no backtracking.
Expert — 9×981 cells. Full Sudoku size. 16 nodes. 2 hints. Serious solvers only — expect 15–40 minute solves.

Winning Strategies for Nodoku

  • Fill low-info cells first. Cells on the edges of the board (which touch fewer or no nodes) are often deduced by standard Sudoku logic alone.
  • Use nodes to unlock stuck cells. If a 2×2 group has 3 cells filled but the fourth is uncertain, the node gives you the answer.
  • Watch the node color. When a node turns red, you have made a mistake in one of its 4 cells. Undo and try another path.
  • Notes are essential on Expert. Fill in candidate digits (press N) for the whole board before committing to any value.
  • Scan for hidden singles. If a digit appears in only one cell of a row, column or box after notes are filled, that digit must go there.
  • Cross-reference nodes. Sometimes a node is only solvable when a specific cell in its 2×2 group gets one particular value. Look for that constraint.
  • Use hints sparingly. Every hint is a cell the puzzle has solved for you. Good players finish a Medium without using any.
  • Play the daily for skill tracking. Since everyone gets the same puzzle, comparing completion times is a genuine skill benchmark.
Pro tip — "sum first, sudoku second": On Expert, fill in nodes' 2×2 groups before doing full-board Sudoku scanning. Each node essentially gives you one free cell per 2×2 group once the other three are filled. This creates a cascade of deductions that unlocks the rest of the board.

Why Sudoku Variants Are Good for Your Brain

Logical deductionEvery Nodoku move is a theorem: "If cell A is 4, then the node sum is 3+4+X+Y=14, so X+Y=7, but they can't be (3,4) because 3 is already used...". This is formal reasoning practice.
Working memoryHolding the state of 36–81 cells plus 9–16 node constraints in your head is a serious memory workout.
PatienceUnlike action games, Nodoku rewards deliberate pacing. Each puzzle takes 5–40 minutes and requires sustained attention.

Unlike passive entertainment, Nodoku's session is bounded — one puzzle, one clear outcome. It fits naturally into a daily routine alongside Wordle, Connections and other daily puzzle games.

History of Sudoku and Its Variants

  • Number Place (1979). Howard Garns, a retired American architect, published the first Sudoku-style puzzle in Dell Pencil Puzzles.
  • Sudoku (1984). Nikoli, a Japanese puzzle publisher, named the puzzle "Sudoku" (数独) and popularized it in Japan.
  • Sudoku boom (2004–2005). The Times of London published its first Sudoku in 2004, sparking a worldwide explosion in popularity.
  • Killer Sudoku (2000s). Combined Sudoku with arithmetic cages — a direct ancestor of Nodoku's node mechanic.
  • Nodoku (now). A browser-native variant with sum nodes at 2×2 corners — free, no ads inside the game, with daily puzzles and archive.

For more background, see the Sudoku Wikipedia entry and the Killer Sudoku article.

Is Nodoku Safe and Private?

Yes. Nodoku is 100% browser-based. All puzzle generation, solving, verification, and rendering run in JavaScript inside your browser tab. Your current board state, timer, hints used, and completion status are saved in localStorage on your device. Nothing is transmitted to a server.

Frequently Asked Questions About Nodoku

What is Nodoku?
Nodoku is a free online Sudoku variant. In addition to standard Sudoku rules (fill every row, column and box with each digit exactly once), each gold node on the grid shows the sum of the four cells touching it. That sum must match once those four cells are filled.
Is Nodoku free?
Yes, completely free. No download, no sign-up, no in-app purchases.
Does Nodoku work on mobile?
Yes. The board scales to fit any screen, and the on-screen number pad works with touch. Keyboard input is also supported on desktop.
What are the gold nodes?
Each gold node sits on the corner between four cells and shows the sum of those four cells. Nodes turn green when the sum is correct, and red when the current values make the sum impossible.
Does every puzzle have a unique solution?
Yes. Every generated puzzle is verified to have exactly one solution before it is shown. You never need to guess.
What is Notes mode?
Notes mode lets you pencil in possible digits in a cell, just like real Sudoku. Press N or tap the Notes button to toggle. When you place a digit, matching notes in the row, column and box are automatically cleared.
What's the difference between Daily and Unlimited?
Daily puzzles are seeded by the current date, so every player worldwide solves the exact same puzzle. Unlimited creates a fresh random puzzle each time you press Play.
Is my progress saved?
Yes. Daily puzzle progress, timers, hint counts and completion status are saved in your browser's localStorage. Nothing is uploaded to a server.
Can I play offline?
Once the page loads, Nodoku runs entirely in your browser. You can keep solving puzzles without internet until you close the tab.
Why is it called Nodoku?
It is a portmanteau of "node" (the gold sum nodes) and "Sudoku" (the underlying puzzle). The nodes are the unique feature that separates it from standard Sudoku.
How does the timer work?
The timer starts when you begin a puzzle and pauses when you open the menu or switch browser tabs. It saves periodically so your progress is not lost.
Does Nodoku require any plugins?
No. It runs on HTML5 + JavaScript in any modern browser. No Flash, no Java, no WebAssembly required.

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External Resources