About Sudoku Online
This is the most complete Sudoku implementation in our catalogue, and the one we would point a serious solver towards. The classic mode runs from Easy through Medium and Hard to Expert, and (crucially) every puzzle is guaranteed to have exactly one solution reachable through pure deduction. You never need to guess. If you are stuck, a logical step exists somewhere on the grid that you have not yet spotted.
Beyond classic play it offers four genuine variants, each adding a constraint rather than a cosmetic change. X-Sudoku requires both main diagonals to contain 1-9. Windoku adds extra hidden regions overlaid on the grid. Killer Sudoku replaces some clues with cages that must sum to a given total. Kropki marks adjacent cells whose values are consecutive or in a 2:1 ratio. Each rewires how you approach the grid, and together they represent a real progression path once classic Sudoku stops challenging you.
A proper notes mode lets you pencil in candidate digits and eliminate them as the grid fills, which is not optional equipment at Expert level: advanced techniques are effectively impossible to execute without it. There is no registration and nothing to install; the whole thing runs in the browser.
How to play Sudoku Online
- Choose a difficulty from Easy, Medium, Hard or Expert, or pick a variant mode.
- Fill the 9×9 grid so that every row, every column and every 3×3 box contains the digits 1 to 9 exactly once.
- Start with the cells that have the most filled neighbours: fewer remaining options means an easier deduction.
- Turn on notes mode to pencil multiple candidates into a cell, then eliminate them as neighbouring cells are resolved.
- Work in chains: each digit you place narrows the options for the cells around it, which opens the next step.
- Complete the grid to finish the puzzle. There is only ever one correct solution.
Controls
- Desktop: Click a cell to select it and type a digit, or click a number from the pad. Toggle notes mode for pencil marks.
- Mobile: Tap a cell to select it, then tap a digit from the on-screen pad. Use the notes toggle for candidates.
Tips and strategy
- Scan for naked singles first: any cell where only one digit is possible. On Easy and Medium boards this alone will carry you most of the way.
- Then look for hidden singles: a digit that can only go in one cell within a row, column or box, even though that cell has several candidates.
- Use notes properly from Hard upwards. Techniques like naked pairs, where two cells in a unit share the same two candidates and therefore lock those digits out of the rest, are invisible without pencil marks.
- When a digit is placed, immediately check its row, column and box for new singles. Momentum in Sudoku comes from following consequences, not from restarting your scan.
- Never guess. Every puzzle here is solvable by deduction, so a guess is just an admission you missed something, and it usually costs more time to unwind than the deduction would have taken to find.
Sudoku Online FAQ
Do I ever have to guess in this Sudoku?
No. Every puzzle is generated with exactly one solution that is reachable through pure logic. If you feel forced to guess, there is a deduction available elsewhere on the grid that has not been spotted yet.
What are X-Sudoku, Windoku, Killer and Kropki?
They are rule variants. X-Sudoku also requires both main diagonals to contain 1-9. Windoku adds extra overlaid regions. Killer Sudoku uses cages of cells that must add up to a stated total. Kropki places dots between adjacent cells that are consecutive or in a 2:1 ratio.
What is notes mode and do I need it?
Notes mode lets you pencil several candidate digits into a cell and remove them as the grid fills. It is optional on easy puzzles but effectively essential from Hard upwards, since most advanced techniques depend on seeing candidates.
Is this Sudoku free and does it need an account?
It is completely free and requires no registration. The game runs directly in your browser on Vzzle, on desktop and mobile alike.
