How to Solve Sudoku: Core Techniques
Sudoku isn't about luck or arithmetic. It's about ruling out, one by one, the digits that can't go in a cell. This guide walks through the techniques you'll use most often, in the order you should learn them, so you can move from easy puzzles to medium and harder ones without getting stuck. Rows are counted from the top, columns from the left, and the nine 3×3 boxes are numbered 1 to 9 from top-left to bottom-right.
A standard sudoku, almost filled. Only the bottom-right cell is empty, and 9 is the only digit missing from its row, column, and 3×3 box. Difficulty on Playfulo is not an AI opponent. It is how many clues are filled in at the start.
Start
The last row already holds 1 through 8. The last column and the bottom-right box have no 9 either, so 9 is the only candidate. The marked cell is the one to fill.
Line 1 / 4
On the left, 9 is the only digit missing from the row, the column, and the box. On the right, 8 is written again and all three rules break.
Fits
The digit missing from the last row, the last column, and the box is the same digit.
Clash
The 8 in the row, the 8 in the column, and the 8 in the box are marked together. One overlap is enough to make the cell wrong.
Sudoku has no move-picking AI. The difficulty name is how many cells are cleared when the puzzle is built, and every puzzle has one solution.
A finished standard sudoku with the bottom-right cell cleared. Counting that row, column, and 3×3 leaves only 9. Difficulty as a clue count is how Playfulo builds its puzzles.
A sudoku grid has 81 cells arranged in nine rows, nine columns, and nine 3×3 boxes outlined in bold. There is exactly one rule: every row, every column, and every box must contain each digit from 1 to 9 exactly once. The digits printed at the start are clues and can't be changed.
To talk about techniques, you need a way to name cells. In this guide, 'row 3, column 7' means the third row from the top and the seventh cell from the left. Boxes 1, 2 and 3 run across the top band, 4, 5 and 6 across the middle, and 7, 8 and 9 across the bottom, so box 5 is the center of the grid.
Every cell belongs to one row, one column, and one box at the same time. Its possible digits are simply whatever is not already present in any of those three units. That idea sounds trivial, but every technique below, from the simplest to the most advanced, is built on it.
Cross-hatching means picking one digit and sweeping the whole grid for it. Say you choose 5. Imagine drawing a line through every row and column that already holds a 5. Inside any box that is still missing a 5, the cells not crossed by a line are the only places it can go. If just one such cell remains, that's your 5.
Here's a concrete case. Box 1 (rows 1–3, columns 1–3) has no 5 yet. There's a 5 at row 1, column 6, so row 1 is out. Another 5 sits at row 2, column 8, so row 2 is out too. That leaves row 3. A 5 at row 7, column 1 blocks column 1, and row 3, column 2 is already filled. The only spot left is row 3, column 3. When a digit has only one possible home inside a unit, it's called a hidden single.
Run cross-hatching for 1 through 9 in the same order every time. Digits that already appear many times on the board produce the most lines, so they resolve fastest. On easy puzzles, this technique alone usually carries you to the finish.
A hidden single is found by looking at a digit; a naked single is found by looking at a cell. Pick an empty cell, remove every digit that appears in its row, column, and box, and if only one digit survives, that's the answer.
Suppose row 3 already contains 1, 2, 4, 5, 6, 8 and 9, leaving empty cells at row 3, column 3 and row 3, column 7. The missing digits are 3 and 7. If column 7 already has a 3 somewhere, row 3, column 7 can't be 3, so it must be 7, and row 3, column 3 becomes 3. Units that are almost full are the fastest places to spot naked singles.
Whenever you feel stuck, go back to the rows, columns, and boxes with seven or eight digits filled. They have so few empty cells that checking them takes seconds, and they often hide a placement you skipped.
From medium difficulty up, it becomes hard to track candidates in your head. Pencil marks, also called notes, are small digits written in a cell to show which values are still possible. They're what make patterns like pairs and pointing visible, so every intermediate technique depends on them.
Notes are only useful if they're accurate. Check all three units before writing a cell's candidates, and every time you place a digit, erase it from the notes in that row, column, and box. In Playfulo's notes mode, placing a final digit clears that cell's notes and automatically removes the digit from notes in the same row, column, and box, which takes care of most of that bookkeeping.
Don't fill all 81 cells with notes from the start. Solve everything you can with cross-hatching and singles first, then add notes to the cells that remain. Notes written on a half-solved grid are shorter and much easier to read.
A naked pair appears when two cells in the same unit have exactly the same two candidates and nothing else. If row 5, column 1 and row 5, column 4 both show only {2, 7}, those two digits must split between those two cells. You can erase 2 and 7 from every other cell in row 5, and if the two cells also share a box, from the rest of that box as well.
A naked triple is three cells in one unit whose candidates, combined, use only three digits. They don't all need the same three: {1, 4}, {4, 8} and {1, 8} together only use 1, 4 and 8, so it still works. Remove 1, 4 and 8 from the other cells of that unit.
A hidden pair works in reverse. If, inside a box, the digits 4 and 9 can each go in only the same two cells, those cells must hold 4 and 9. Even if the cells show other candidates, you can wipe everything except 4 and 9 from them. Hidden pairs are easy to miss when notes are crowded, so try counting how many cells each digit can occupy within a unit.
Pointing pairs use the overlap between a box and a line. Say that inside box 5 (rows 4–6, columns 4–6), candidate 6 only appears in row 4. Box 5's 6 must then land in row 4, so you can remove 6 from the rest of row 4 outside the box: columns 1–3 and 7–9. Two such cells form a pointing pair; three form a pointing triple.
Box-line reduction is the mirror image. If, in column 2, candidate 8 only appears in the cells that belong to box 7 (rows 7–9), then column 2's 8 must sit inside box 7. That means you can remove 8 from the other cells of box 7, namely columns 1 and 3 in rows 7 through 9.
Neither technique places a digit directly, but both trim candidates and create new singles. A large share of the sticking points in medium puzzles are broken by one of these two moves.
Knowing the techniques is one thing; applying them in a sensible order is another. Treat the list below as one loop, and whenever you place a new digit, go back to step 1. Repeating the cheap techniques first keeps your notes lean and cuts down on mistakes.
The most frequent mistake is checking only one unit before placing a digit. A number might be missing from the row but already present in the column or box, so always check all three. The second is forgetting to clean up notes after placing a digit. One wrong candidate quietly corrupts every deduction that follows.
The third is guessing when stuck. Well-made sudoku puzzles are designed to be solved by logic, so running the routine once more is almost always faster than a guess. Playfulo highlights conflicting digits in red and offers undo and a single-cell hint. When you use a hint while practicing, try to explain to yourself why that digit had to go there; that's how the hint turns into a skill.
A good progression is to build cross-hatching speed on easy, learn notes and pairs on medium, then move up to hard. Playfulo's daily sudoku gives everyone the same puzzle each day on Korea time and keeps your streak in this browser, which makes it a handy way to build a one-puzzle-a-day habit.
Start with cross-hatching: pick a digit, rule out every row and column that already contains it, and place it wherever a box has only one possible cell left (a hidden single). Then look at nearly full rows, columns, and boxes for cells with only one candidate (naked singles). Those two techniques solve most easy puzzles.
Add notes once cross-hatching and singles stop producing placements. With fewer empty cells, your notes stay short and accurate. Check the row, column, and box before writing candidates, and erase a digit from the notes in all three units every time you place it.
A naked pair is two cells in the same row, column, or box that both contain exactly the same two candidates. Those two digits must go in those two cells, so you can remove them from every other cell in that unit. For example, if row 5, column 1 and row 5, column 4 are both {2, 7}, erase 2 and 7 from the rest of row 5.
It's better not to. Properly constructed sudoku puzzles can be solved with logic alone, and guessing is hard to unwind when it's wrong. If you're stuck, double-check your notes and reapply pairs, pointing pairs, and box-line reduction in order.