How to Solve Sudoku: The Only Four Techniques Most Puzzles Need
You do not need twenty named techniques. Four will solve almost every Easy, Medium and Hard Sudoku, and they build on each other in order.
Search for Sudoku techniques and you will find swordfish, XY-wings, unique rectangles and a dozen other intimidating names. Almost none of them are needed. Four techniques, applied in order, will finish nearly every puzzle you meet outside the Expert tier.
The rules, stated precisely
A 9x9 grid, divided into nine 3x3 boxes. Every row, every column and every box must contain each digit from 1 to 9 exactly once. That is the entire rule set, and every technique below is a consequence of it.
Technique 1: naked singles
Look at an empty cell. Cross off every digit already present in its row, its column and its box. If exactly one digit survives, that is the answer.
This is the technique beginners use exclusively, and on Easy puzzles it is often enough on its own. Work through the grid cell by cell; each solved cell creates new naked singles elsewhere, so a single pass rarely finds them all.
Technique 2: hidden singles
Now flip the question. Instead of asking "what can go in this cell", ask "where can this digit go in this row?"
Take a row with several empty cells. Consider the digit 7. If 7 can only legally go in one of those cells — because every other empty cell in the row already has a 7 in its column or box — then that cell is a 7, even if it also permits other digits.
Hidden singles are the technique that unlocks Medium puzzles, and most people find them only after being told to look for them. Scan by digit, not by cell.
Technique 3: pencil marks
On Hard puzzles you will run out of singles. This is where you stop holding candidates in your head and write them down.
In every empty cell, note the small digits that are still legal there. This is slow — the first full pass takes several minutes — but it converts the puzzle from a memory exercise into a visible one. Every technique after this operates on the pencil marks rather than the grid.
Keep them updated. A stale pencil mark is worse than none, because you will trust it.
Technique 4: pointing pairs and box-line reduction
Look at one 3x3 box and one digit. If every possible position for that digit inside the box happens to sit in the same row, then that digit must be somewhere in that row inside this box — which means it cannot be anywhere else in that row outside the box.
That single deduction eliminates candidates in two other boxes, and it usually cascades into a hidden single somewhere. The mirrored version — where the candidates in a row all fall inside one box, eliminating them elsewhere in the box — is called box-line reduction and works identically.
The order that actually works
- Sweep for naked singles until you find none.
- Scan digit by digit for hidden singles.
- If stuck, fill in complete pencil marks.
- Hunt for pointing pairs, then go back to step 1 — eliminations almost always create new singles.
Repeat. On Easy through Hard, this loop terminates in a solved grid.
Never guess
A properly constructed Sudoku has exactly one solution reachable by logic. If you are guessing, you have missed something — and a wrong guess propagates silently for twenty cells before it contradicts itself, at which point unwinding it is worse than starting over.
Every puzzle generated on our Sudoku page is verified to have a unique solution before it is served, so there is always something findable.
Speed comes later
Beginners often try to go fast before they go correct. Solve five puzzles slowly and completely, writing pencil marks even when you think you do not need them. Speed is what happens when the scanning becomes automatic, and that takes repetition rather than effort.