Sudoku has exactly one rule, and every technique here is just a different way of applying it. Fill the 9x9 grid so that each of the nine rows, nine columns, and nine 3x3 boxes contains the digits 1 through 9 exactly once. No arithmetic, no diagonals โ the difficulty comes entirely from figuring out, cell by cell, which digits are still possible once you account for everything that touches it. This guide walks through the techniques in the order you should reach for them. Play along at Sudoku, where every new game is freshly generated in your browser by a backtracking solver, so no two boards repeat.
Controls are simple: tap a cell, then tap a number on the on-screen keypad to fill it in. New Game starts a fresh puzzle, and Reveal shows the complete solution instantly if you want to check your work or bail out of a puzzle that's beaten you. Everything below assumes you're solving with your eyes and, where useful, a scrap of paper โ this build has no on-screen pencil-mark layer, which is actually an advantage for learning the techniques properly instead of leaning on software to track candidates for you.
The One Rule and the Three Constraint Groups
Every cell belongs to exactly three groups at once: its row, its column, and its 3x3 box. A digit can only go in a cell if it doesn't already appear anywhere in any of those three groups. This overlapping structure is the entire engine of the puzzle โ a single placed digit can eliminate candidates from up to 20 other cells simultaneously, which is why placing one correct digit often cascades into several more becoming obvious.
Scanning and Cross-Hatching
The most basic technique is scanning: pick a digit, say 6, and check each of the nine boxes. If a box already has a 6, skip it. If not, look at the rows and columns that already have a 6 elsewhere and cross out every cell in the box that lies in one of those rows or columns โ this is cross-hatching. If only one empty cell in the box survives the cross-out, place the 6 there. Repeat for each digit, then repeat the whole pass again once you've placed a few, since each placement changes what the next scan finds.
Naked Singles and Hidden Singles
Naked Singles
Look at a single empty cell. List every digit already placed in its row, column, and box, and cross each off. If only one digit โ say 7 โ remains, that's a naked single: 7 is the only digit that could legally go there, because 1 through 6, 8, and 9 are all already used somewhere that cell can see. This is the easiest deduction in the game and the first thing to check on any empty cell.
Hidden Singles
Hidden singles work from the other direction. Pick a box and a digit, say 4, and ask which empty cells could still legally hold it. If four of the box's cells are filled, and among the remaining five, four sit in a row or column where a 4 already appears elsewhere, only one empty cell in that box remains a legal home for the 4 โ even if that cell still shows other candidates. Fill it in. It's "hidden" because the 4 was buried among other candidates rather than standing alone.
Pencil Marks and Keeping Them Honest
Once scanning and singles stop finding anything, track candidates properly: for each empty cell, write down every digit not yet eliminated by its row, column, or box. Since this build has no notes tool, keep them on scratch paper, working a handful of cells at a time. What matters most is keeping the marks honest โ the instant you place a digit, strike it from the pencil marks of every cell in its row, column, and box. Stale pencil marks are the single biggest source of solving errors, since every technique from here depends on accurate candidate lists.
Naked Pairs and Triples
If two cells in the same row, column, or box both have pencil marks reduced to exactly the same two candidates โ say both can only be 3 or 8 โ one cell is a 3 and the other an 8, even though you don't know which is which. Either way, no other cell in that shared group can be a 3 or an 8, so strike both from every other cell's marks in that group. The same logic extends to naked triples: three cells whose combined pencil marks use only three digits total lock those digits out of the rest of the group.
Hidden Pairs
Hidden pairs mirror naked pairs. If two candidates โ say 2 and 6 โ each appear in the pencil marks of only the same two cells within a shared group, those two digits must occupy those two cells, regardless of what other candidates those cells still list. Strip every other candidate from those two cells; this often unlocks a naked single elsewhere once the clutter is gone.
Pointing Pairs and Box-Line Reduction
Pointing pairs work between a box and a line. If, within one box, every remaining cell that could hold a digit lies in the same row or column, that digit must eventually land in that row or column inside this box โ so you can eliminate it from every other cell in that row or column outside the box. Box-line reduction runs the reverse: if every cell in a row or column that could hold a digit falls inside one box, eliminate it from the rest of that box. Both often break a puzzle that otherwise looks stuck.
Building a Repeatable Solving Order
Random hunting wastes time and misses easy deductions. Use a fixed order: sweep the grid for naked and hidden singles and clear every one you find; once that stalls, fill in full pencil marks for the remaining cells; scan for naked pairs and triples; check for hidden pairs; then look for pointing pairs and box-line reductions. After any elimination, go back to step one โ a single removed candidate can turn a crowded cell into a fresh naked single, and re-checking the easy techniques first is always faster than pushing straight to advanced ones.
Guessing, Contradictions, and Recovery
A well-formed sudoku puzzle never requires a guess. If you're genuinely stuck, either a technique further down this list hasn't been applied yet, or a digit was placed incorrectly earlier and the pencil marks downstream of it are wrong. Don't restart the whole grid. Work backward through your recent placements and re-verify each against all three of its constraint groups โ a contradiction almost always traces to a single wrong digit a few moves back, not a systemic problem. Correct it, redo pencil marks for any affected cell, and continue forward. Reveal will show the finished solution if you want to confirm your own answer, but treat it as a check, not a shortcut.
Common Mistakes
- Forgetting to update pencil marks after every placement. One stale candidate can produce a false naked single or hide a real one.
- Jumping to pairs and triples before exhausting singles. Advanced techniques are slower to spot โ clear every naked and hidden single first.
- Confusing a naked pair with a coincidence. Both cells need the exact same two candidates for the elimination to hold.
- Guessing instead of admitting a missed technique. Work through the solving order again from the top.
- Restarting the whole grid after a contradiction. Trace the error back through your last few placements instead.
Frequently Asked Questions
Do I ever have to guess in Sudoku?
No, not in a properly generated puzzle. If you're guessing, you've either missed an available deduction or made an earlier placement error โ both are fixable without luck.
What's the difference between a naked single and a hidden single?
A naked single is a cell with only one possible candidate left. A hidden single is a cell where one candidate is the only cell in its row, column, or box that can hold that digit, even if the cell shows other candidates too.
Do I need to track pencil marks for the whole grid at once?
No. Work on a handful of cells at a time and rely on scanning and singles for as long as they keep working before committing to full pencil marks everywhere.
I think I made a mistake โ do I have to start over?
No. Trace back through your last several placements and check each against its row, column, and box. The error is almost always recent and local.
What should I do after placing a digit?
Immediately remove that digit from the pencil marks of every cell in its row, column, and box, then check whether that creates any new singles nearby.
Is using the Reveal button cheating?
It's a tool for checking your answer or seeing the solution when truly stuck, not a technique. Relying on it instead of working through the deductions above won't make you faster next time.
Related Guides
- How to Play Minesweeper โ another pure logic-deduction game built on the same "prove it, don't guess" foundation.
- How to Play Nonogram โ a grid-logic puzzle where row and column constraints do similar work to Sudoku's three groups.
- How to Play Klondike Solitaire โ a logic puzzle with cards instead of numbers.
- A Beginner's Guide to Puzzle Games โ a good starting point if logic puzzles are new to you.
- Best Puzzle Games โ more logic and deduction games to try once Sudoku clicks.