How to play Helpful Sudoku

Start playing

Choose Play today for the day's Medium puzzle, or choose a difficulty on Today. Select an empty cell and use the number buttons or type 1–9. Original clues cannot be changed during play. Difficulty describes the puzzle; the Helpfulness settings control how much assistance you receive.

Sudoku rules

Fill each row, column, and 3×3 box with the digits 1–9, once each. In X Sudoku, each of the two long diagonals must also contain 1–9 once each. A legal candidate does not conflict with digits already placed; it is not necessarily the final answer.

Choose your assistance

Basic Sudoku switches all Helpfulness options off. Auto-check conflicts highlights conflicting entries and empty cells with no legal candidates. Smart keypad and keyboard restrict number and note entry to legal candidates. With smart input off, you can enter any digit or pencil mark, including a wrong one. Show all candidates fills empty cells with calculated possibilities and enables conflict checking and smart input. Changing assistance returns you to normal number entry.

Pencil marks (Notes)

When Show all candidates is off, press Notes or N to enter or remove pencil marks. Press the same digit again to remove its mark. Notes do not fill the cell. Press Notes again, or G, to return to number entry. Turning on Show all candidates hides Notes and exits pencil mode. Existing manual notes are retained; smart input can hide marks that are no longer legal.

Naked and hidden singles

A naked single is an empty cell with just one legal candidate. A hidden single is a digit that has only one possible cell in a row, column, box, or X diagonal, even when that cell has several candidates. One-click naked singles lets you fill a single-candidate cell by selecting it. Highlight naked and hidden singles marks forced moves without playing them.

Hints

Hint selects and explains an available naked or hidden single. If none is available, it may suggest a cell with few candidates; that is a place to investigate, not a proven answer. Read the Hint panel, then enter the digit yourself. Asking for a hint does not place a digit.

Undo, redo, rollback and restart

Undo reverses your last entry, pencil mark, or machine move; Redo restores it. Rollback steps back to the most recent branching choice. Restart clears your progress on the same puzzle. Erase or Backspace/Delete clears the selected editable cell. The activity box records moves and important messages. The end summary separates user and machine placements and both single techniques.

Watch the solver

Start fills forced moves and, when needed, tries a choice and rolls back from contradictions. Pause stops the animation. Choose Very slow (3 seconds per move), Slow (1 second), Normal (0.2 seconds), Fast (0.04 seconds), or Very fast (0.005 seconds). Instant Solve clears guesses and notes, then solves from the original clues. Solver placements count as machine moves.

Import and edit a puzzle

Enter clues in the Import grid, load text or a CSV, or review a photo import when image recognition is configured. Download sample.csv for an example: nine rows of nine values, with 0 for blanks. Choose the correct Sudoku type. Finish editing & check verifies that there is exactly one solution. Play puzzle is available only after a successful check. Clicking a cell or choosing Edit puzzle returns to entry mode and requires another check. Imported puzzles can also be edited from their Play session controls.

Appearance and keyboard

Settings offers System, Light, and Dark themes and Sage, Soft blue, Ochre, and Plum accents. Use arrow keys to move the selection, 1–9 to enter digits, N to toggle Notes, G for number entry, U for Undo, and R for Redo. Keyboard entry is paused while the solver is animating.

Keeping your progress

Appearance and assistance preferences are saved on this device where storage is available. Automatic saving and resuming of the current game is not implemented yet. Keep the original puzzle if you want to return to it after closing the app.

How we describe difficulty

These are Helpful Sudoku generator rules, not universal Sudoku standards. Branching choices count guesses on the successful solution path, excluding abandoned branches. Singles counts come from the generator solver run. A human may use advanced logic instead. Previously generated collections keep their existing labels until reclassified.

Classic difficulty: Trivial

No branching choices or hidden singles, with at least 40 clues. Solved using naked singles alone; a good starting point with assistance switched off.

Classic difficulty: Easy

No branching choices or hidden singles, with fewer than 40 clues. Solved using naked singles alone, with more empty cells to work through.

Classic difficulty: Medium

No branching choices. Hidden singles are used, but do not outnumber naked singles.

Classic difficulty: Hard

One branching choice, or no branching choices with more hidden singles than naked singles. Advanced human techniques may avoid the solver’s branching choice.

Classic difficulty: Extreme

Also called Expert. Two to five branching choices on the successful solution path. Singles still help, but more difficult decisions remain.

Classic difficulty: Nightmare

Six or more branching choices on the successful solution path. Our most demanding classic puzzles; advanced logic or extensive searching may be needed.

X Sudoku difficulty: Trivial

X Sudoku with no branching choices or hidden singles and at least 40 clues. Naked singles alone solve the puzzle, including deductions on both diagonals.

X Sudoku difficulty: Easy

X Sudoku with no branching choices or hidden singles and 35 to 39 clues. Solved using naked singles alone.

X Sudoku difficulty: Medium

X Sudoku with no branching choices and no more hidden than naked singles. Includes naked-singles-only puzzles with fewer than 35 clues.

X Sudoku difficulty: Hard

X Sudoku with one to three branching choices, or no branching choices with more hidden singles than naked singles.

X Sudoku difficulty: Extreme

Also called Expert. X Sudoku with four to nine branching choices on the successful solution path. Both diagonals must also contain each digit once.

X Sudoku difficulty: Nightmare

X Sudoku with ten or more branching choices on the successful solution path. Our most demanding X puzzles; substantial searching may be needed.

Free puzzle packs

Open Packs to create ten printable PDFs in a ZIP. Mixed difficulty includes Easy, Medium and Hard only. Choose clues and/or candidates, with optional solutions, or select All three. Choose large or compact A4, create the pack, then Download ZIP.

Basic Sudoku with starting notes

In Settings, choose Basic Sudoku with starting notes beside Basic Sudoku (no help). Both presets switch off all automatic Helpfulness options. Starting notes also fills legal possibilities once for each fresh or restarted puzzle, respecting X Sudoku diagonals, and enters Notes mode. Manage the marks yourself: they are not automatically removed as other cells change. Selecting this preset after moves have been made leaves your current board and notes intact and applies the fill to your next fresh or restarted puzzle. Turning on another Helpfulness option leaves this preset; existing notes remain. Show all candidates hides manual notes without deleting them. Choosing plain Basic Sudoku stops automatic prefilling for future games. Filling a cell clears its own notes; Undo restores them. Use Notes or N to switch between pencil marks and normal digits. This starting aid is recorded in the activity log and assistance summary.

Playing on a phone

The board fits the available screen width. Board size sets your preferred size on larger displays; small screens cap it so every cell stays within reach. Menus and solver speeds wrap onto another line where needed. Reduce Motion can make play feel smoother on a phone.

You may be able to install the website from your browser. This opens the same game in an app-style window; installation does not guarantee offline play. An existing installation may open the homepage until its launch settings update: select Play Sudoku to continue.