Sudoku 
Easy Sudoku
Forty givens: counting, cross-hatching and naked singles finish nearly every board.
Easy SudokuWritten by Brian Hamilton and Karan Hamilton.
Brian and Karan are the puzzle lovers and game makers behind Sudoku Online Puzzles. You can learn more on the About page.
Guides are reviewed for clarity, accuracy, and usefulness for solvers. See our editorial standards, puzzle testing process, or send a correction.
The hard part of a first sudoku is not a clever pattern. It is knowing where to look next. Two moves, both called singles, finish every Easy and Medium puzzle we deal, and this guide shows each of them on a real Easy board from our generator, then puts them into a routine you can follow on any puzzle.
Singles are enough for your first hundred puzzles
We dealt 200 Easy and 200 Medium boards and solved them taking the simplest move each time. Naked and hidden singles finished all 400. Naked singles alone finished 192 of the Easy boards but only 94 of the Medium ones: the extra difficulty of Medium is hidden singles, not a new technique.
What are the rules of sudoku?
Fill the grid so that every row, every column and every 3×3 box holds the digits 1 to 9 once each. The digits already printed are the givens: 40 on our Easy boards, 32 on Medium. A proper puzzle has exactly one answer, so every digit can be worked out; you never have to guess. Rows, columns and boxes all follow the same rule, so they share one name, houses.
How to read R4C5
Our guides name a square by its row and column, counting rows from the top and columns from the left: R4C5 is row 4, column 5. Boxes are numbered 1 to 9 in reading order, so box 2 is the top middle one. The sudoku terms page has the rest of the vocabulary.
Where should a beginner start?
With the fullest house you can find. A row, column or box with only two or three gaps is a small question: which of the few missing digits goes where? Count what it already has, list what is missing, then check each gap’s column and box for one of those digits.
That is the whole trick of the opening: turn the 81-square grid into small questions about one house. On this board row 4 answers two squares at once, and each digit you place makes its row, column and box a little fuller for the next question.
How do you cross-hatch a box?
Cross-hatching asks the question the other way round: not “what goes in this square?” but “where does this digit go in this box?” Pick a digit the box is missing. Every copy of it in a row or column that runs through the box rules out that line inside the box. If only one empty square is left, the digit goes there.
A digit with one place left in a house is a hidden single, and on a fresh Easy board there are plenty: a median of 17 before anything is written in, against 8 squares down to one digit. Work through the digits 1 to 9 against each box in turn; it is slower to describe than to do.
What is a naked single?
A square that can hold only one digit, because its row, column and box already contain the other eight. Where cross-hatching follows one digit around, this check stays on one square. It is the first thing our Sudoku Helper looks for, and it is called a naked single because nothing hides it once you look.
The best squares to check are the ones in crowded rows and columns, since their houses have the most digits to rule things out.
What should you do after placing a number?
Look at its row, column and box again straight away. A new digit is one more thing ruling out squares around it, and the next single is often right next to it.
That gives a routine for any puzzle at these levels:
- Start with the fullest houses and count what they are missing.
- Cross-hatch: take each digit in turn and check every box that still needs it.
- Check crowded squares for naked singles.
- After each placement, rescan the row, column and box it sits in.
- When the steps above find nothing, do the same for rows and columns instead of boxes, then write pencil marks.
Say the reason before you write the digit
“The only place for a 5 in this box”, or “the only digit this square can take”. If you can’t say it, don’t write it. A guessed digit that happens to be wrong can sit there for a long time before anything shows it, and then everything placed after it is in doubt.
What if there is no obvious move?
Then the next digit is usually a hidden single in a row or column, not a box. Medium boards often start that way: of our 200, only 94 can be finished by naked singles alone, and 6 have no square down to one digit at all when they are dealt.
Here the digit-by-digit question finds what the square-by-square one cannot. If you still find nothing, write pencil marks, the digits each empty square can still hold. Our games’ Notes button lets you write them yourself, and Start Notes fills in every empty square at once. Once the marks are in, a naked single is a square with one mark left, and a hidden single is a digit that appears only once in a house. Keep them up to date after every placement: a stale mark is how a false move gets made, and sudoku mistakes shows what that looks like.
What should you learn after singles?
Nothing, until you play Hard. Our Easy and Medium puzzles never need more than singles. On Hard, 23 boards in 200 need a locked candidate as well, a digit confined to one line of a box; on Expert, 17 need a naked pair. Our sudoku techniques page lists every technique in the order to learn it, and sudoku difficulty levels explains what each of our levels asks for. To drill singles on their own, the Sudoku Trainer has ten positions for each kind.
