Rules
Sudoku is played on a square grid split into boxes by thick lines. The classic grid is 9×9, with nine 3×3 boxes. Some cells start with a digit. These givens are bold and can’t be changed. Your job is to fill every other cell.
Rows, columns and boxes are called houses. A digit can appear only once in each house, so every digit on the board rules itself out of the rest of its row, its column and its box. There’s no arithmetic. The digits are just symbols, and letters would work the same way.
Every Sudoku on PuzzleKite has exactly one solution, and you can reach it by logic alone. You never need to guess.
Basic moves
On PuzzleKite, difficulty depends on the techniques a grid needs, not on how many digits it gives you. Easy grids need only the first two moves below. Medium grids add hidden singles. Hard grids need notes, plus the patterns in section 4.
Full house Easy
When a row, column or box has one empty cell left, that cell takes the missing digit. It’s the simplest move in Sudoku, and it shows up more and more as the grid fills.
Naked single Easy
Pick an empty cell and look along its row, down its column and around its box. Rule out every digit you see. If only one digit is left, that’s the answer.
Row 5, column 5 and the middle box already hold these eight digits between them:
Only 6 is left for the centre cell.
Hidden single Medium
Now flip it around. Pick a digit and a box, then find every cell in that box where the digit could still go. When only one cell is left, the digit goes there, even if that cell could hold other digits too.
- The circled 2 in row 4 rules out the box’s top row.
- The 2s in columns 1 and 2 rule out the rest of those two columns.
- One cell is left. 2 can only go here in the middle-left box.
Notes Hard
When singles dry up, pencil in the digits each open cell could still be. Turn on Notes with the button or the N key, then tap digits to switch marks on and off. When you place a digit, it disappears from the notes in its row, column and box.
Common mistakes
Almost everyone makes these at first. Each one has a simple fix.
Guessing to get unstuck
A guess can look fine for twenty moves and then fall apart. If you’re not sure, write a note, not a digit.
Forgetting the box
It’s easy to check the row and column and skip the 3×3 box. Every digit has to fit all three houses.
Noting everything too early
Easy grids fall to singles. Filling every cell with notes first is slow and hides the obvious moves.
Leaving a red digit
A digit that repeats in a house turns red as soon as you place it. Fix it right away, before ten more moves bury it.
This 2 repeats the circled 2 further down column 1, so it shows red straight away.
Advanced moves
Singles and notes carry a hard grid a long way, but somewhere it stalls. These four patterns are what the Hard grade is made of. Each one uses the notes in one house to rule digits out of another, or out of the rest of the same house.
Pointing Hard
When every cell in a box that can still take a digit sits in one row or column, the box points along that line: the digit can’t go anywhere else in the line, so strike it from the rest of the row or column.
In the middle-left box, every 8 note sits in column 1. So 8 can’t go anywhere else in column 1, and it comes off the two open cells in the top-left box. The upper one is left with a single note, 1, so it is solved.
Claiming Hard
Claiming is pointing in reverse. Read along a row or column: when every cell that can still take a digit sits inside one box, the line claims the digit for that box, so it comes off the box’s other cells.
The same board, read the other way. In column 1, 5 can only go in the two yellow cells, and both sit inside the middle-left box. The column claims 5 for that box, so 5 comes off the box’s other two open cells.
Naked pair Hard
Two cells in one house, the same two notes, nothing else. Those digits must fill those two cells, in some order, so the pair owns them and they come off every other cell in the house.
Later in the same grid, in the right-middle box: two cells hold only 7 and 9. The pair owns both digits, so 7 and 9 come off the box’s top cell, and 6 is all that’s left there.
A hidden pair is the same find from the other side: two digits that can only go in the same two cells of a house. Whatever else is noted in those cells can go. Here, 1 and 4 fit only the two cells on the right, so the 6 noted in the upper one has to go too. Hidden pairs hide behind crowded notes, so hunt for digits with few places left, as you would for a hidden single.
Naked triple Hard
Three cells, three digits. The cells don’t each need all three notes; between them they hold only those three digits. The triple owns them, so strike the three from the rest of the house.
Hidden triples work like hidden pairs, one size up: three digits that fit only the same three cells of a house, shedding every other note in those cells. They are the rarest pattern here, but when a hard grid stalls they are worth one slow look.
Next steps
Once singles feel natural, move up a level. Each one adds a new idea.
- EasySingles onlyFull houses and naked singles.Play Easy →
- MediumAdd hidden singlesOne place left for a digit in a house.Play Medium →
- HardNotes and patternsPointing, claiming, pairs and triples.Play Hard →
A hard grid here usually needs one or two of the patterns above, and singles do the rest. The best way to learn them is to meet them: play on Hard, keep notes, and let Hint show you the step you missed.