Sudoku from easy to expert, with pencil notes, highlighting of repeated digits, undo and a timer. Every puzzle has exactly one solution.
What is Sudoku?
Sudoku is a logic puzzle played on a 9x9 grid, where you fill every empty cell with a digit from 1 to 9 so that each row, each column and each of the nine 3x3 boxes contains every digit exactly once. Some cells are filled in at the start; these are the givens, and they cannot be changed.
No arithmetic is involved. The digits could just as well be letters or colours. A well-made Sudoku has exactly one solution, and you can reach it by reasoning alone, without guessing.
How to play Sudoku
The grid has 81 cells grouped into 9 rows, 9 columns and 9 boxes. Every cell belongs to one row, one column and one box, and those 20 other cells are its peers. A digit may not appear twice among a cell and its peers.
Select an empty cell and enter a digit. If you are not sure yet, switch on Notes and write small candidate digits in the cell, as many as you like. When you place a real digit, our game removes that digit from the notes of all its peers for you.
The game marks conflicts: if a digit appears twice in a row, column or box, both copies are marked in the conflict colour. A conflict check only tells you that two digits clash; a wrong digit that does not clash with anything yet will not be marked, so careful logic still matters.
The puzzle is solved when all 81 cells are filled with no conflicts. Because every puzzle here has a single solution, a full grid without clashes is always the correct one.
Sudoku strategy: techniques from easy to expert
Scanning: pick a digit and look at where it already sits. Each copy rules out its row, column and box. If only one cell in a box is left for that digit, it goes there. Doing this for all nine digits solves much of an easy puzzle.
Naked single: a cell where eight digits are already ruled out by its peers can only hold the ninth. Hidden single: within one row, column or box, a digit that has only one possible cell must go there, even if that cell has other candidates.
Naked pair: if two cells in the same row, column or box have exactly the same two candidates, say 3 and 7, those two digits belong to those two cells, and you can remove 3 and 7 from every other cell in that unit. The same works with three cells sharing three candidates. Hidden pair: if two digits can only go in the same two cells of a unit, those cells hold only those digits, and their other candidates can be erased.
Locked candidates: if, inside a box, every possible cell for a digit lies in one row, that digit cannot appear anywhere else in that row outside the box. The reverse also holds: if in a row all candidates for a digit fall inside one box, remove the digit from the rest of that box.
X-Wing: if a digit has exactly two possible cells in each of two rows, and those cells line up in the same two columns, the digit must occupy one diagonal pair of those four corners. Either way, it fills both columns, so you can remove it from every other cell in those two columns. The same pattern works with columns and rows swapped, and Swordfish extends it to three rows.
Good habits: fill in notes only when scanning stops giving you digits, keep them complete, and update them after every placement. When you seem stuck, look for the unit with the fewest empty cells first.
Sudoku difficulty levels and variants
Our Sudoku has four levels, based on how many givens the puzzle starts with: Easy has 44, Medium 36, Hard 30 and Expert about 25. Every puzzle is created fresh when you start, and each one is checked to have exactly one solution. Fewer givens usually means you need the stronger techniques above, though the count alone does not decide everything; where the givens sit matters too. For reference, it has been proved that a Sudoku with a unique solution needs at least 17 givens.
Other variants exist, such as Killer Sudoku with sum cages, diagonal Sudoku, overlapping Samurai grids and small 4x4 or 6x6 grids for beginners. Our game plays the classic 9x9 form.
History of Sudoku
The modern puzzle first appeared in the United States in 1979 under the name Number Place. A Japanese puzzle publisher took it up in the 1980s and gave it the name Sudoku, a short form of a phrase meaning that each digit must appear only once. Around 2005 it spread across the world through daily newspapers, and it has been a fixture of puzzle pages ever since. The idea of filling a grid so that no symbol repeats in a row or column is much older and goes back to Latin squares studied by mathematicians in the eighteenth century.
Playing Sudoku on Hevagames
Sudoku on Hevagames plays in the browser on phones and computers, with nothing to install. Tap a cell and use the number pad; each digit button shows how many of that digit are still missing and fades once all nine are placed. Selecting a cell highlights its row, column and box, and if the cell holds a digit, every other cell with that digit. Tapping the same digit again clears the cell.
On a keyboard, type 1 to 9, hold Shift with a digit to add a note, press N to switch notes mode, use the arrows to move, Backspace, Delete or 0 to erase, and U or Ctrl+Z to undo. Undo keeps up to 300 steps, and your puzzle, notes and timer are saved if you leave.
Scoring: each level has a base of 1000, 2000, 3000 or 4000 points, minus one point per second you take, never less than a tenth of the base. Your best time and best score are kept for each level on your device. The board scales to the screen and follows the light or dark theme.
How to play
- 1Fill the grid so every row, column and 3×3 box contains the digits 1 to 9 once.
- 2Select a cell, then tap a number on the pad or type it.
- 3Turn on Notes to write small candidate digits in a cell instead.
- 4A digit that repeats in its row, column or box is shown in red.
Controls
Tips
- Placing a digit removes that digit from the notes in the same row, column and box.
- The small number under each pad button shows how many of that digit are still to place.
- Selecting a filled cell highlights every cell with the same digit.