Play hard sudoku online for free: a classic 9×9 sudoku with only 27 numbers given and 54 squares to solve, for confident solvers who want a real challenge. Every puzzle in this hard sudoku game is new and has exactly one answer, so it can always be solved by logic, but you will need techniques beyond the basics, such as naked pairs, pointing pairs and careful pencil marks. These free sudoku puzzles have hints, a mistakes counter and a timer, work on phones and tablets, and can be printed with an answer key.
Hard sudoku online: play and print
Play the hard sudoku above, then print the same level on paper: two hard 9×9 sudoku puzzles on a page and an answer key page with the solutions in red. The pictures also show a fresh hard puzzle and one that is half solved.
How hard is hard sudoku?
This game gives only 27 of the 81 squares at the start, compared with 40 on easy and 32 on medium. Fewer numbers mean fewer squares that have only one possible number, so the simple techniques run out of moves sooner and you must find the next step by looking for patterns. Every puzzle still has exactly one solution, and none ever needs a guess.
| Level | Numbers given | Squares to solve | Techniques you will use |
|---|---|---|---|
| Easy sudoku | 40 | 41 | Naked singles and hidden singles |
| Medium sudoku | 32 | 49 | Singles, pairs, pencil marks |
| Hard sudoku | 27 | 54 | Naked and hidden pairs, pointing pairs, boxes and lines |
Hard sudoku techniques you need
- Naked and hidden singles: always try these first. They are the engine of every solve, even on hard puzzles.
- Pencil marks: write the possible numbers small in each empty square. They turn a mess of possibilities into patterns you can see.
- Naked pairs: if two squares in the same row, column or box can only hold the same two numbers, those two numbers cannot go anywhere else in that unit.
- Pointing pairs and box-line reduction: if a number in a box can only go in one row, remove it from the rest of that row outside the box.
- X-wing (advanced): if a number can only go in the same two columns in two different rows, remove it from the rest of those columns.
Hard sudoku example: a naked pair
This 27-number puzzle needs a naked pair. After 20 naked and hidden singles, no square has only one choice left, but row 2 has a pair. The table lists what each square in row 2 can hold, with solved squares in bold:
| c1 | c2 | c3 | c4 | c5 | c6 | c7 | c8 | c9 | |
|---|---|---|---|---|---|---|---|---|---|
| row 2 | 5 | 1,6 | 1,6 | 6,7,9 | 3 | 4 | 8 | 2 | 6,7,9 |
The squares in columns 2 and 3 can each hold only 1 and 6. Two squares sharing the same two numbers is a naked pair: one of them must be 1 and the other 6, so neither number can go anywhere else in the row. That lets us remove 6 from column 4 and 6 from column 9, which opens up the rest of the puzzle. Note how a pair gives progress even though there is no square with a single choice.
Hard sudoku strategy: staying calm
- Do the easy squares first: before you hunt for pairs, make sure every single is filled in.
- Update your pencil marks: after each new number, cross it out of its row, column and box.
- Scan units one at a time: look at one row, column or box and ask what pairs and singles are in it.
- Take a break: sudoku rewards a fresh look, and many stuck puzzles solve in a minute after a rest.
- Use Hint wisely: it fills one square and counts as a mistake, so save it for when you have tried every technique.
The how to solve sudoku guide covers each technique, and Wikipedia’s sudoku solving algorithms article shows how computers solve the same puzzles.
Common mistakes in hard sudoku
- Forgetting to update pencil marks: after every new number, cross it out of its row, column and box, or old possibilities will mislead you.
- Seeing a pair that is not there: a naked pair needs two squares with exactly the same two possible numbers, not just one number in common.
- Rushing a guess: if you put in a number “to see what happens”, a wrong number can hide for many moves. This game turns wrong numbers red at once, which is a helpful safety net.
- Ignoring the boxes: many hard puzzles open up when you look at the 3 × 3 boxes and not just the rows and columns.
Hard sudoku is as much about patience as about cleverness, and it is a good test for children and adults who enjoy solving a puzzle from start to finish. If you can solve hard sudoku without hints, you are doing the same kind of reasoning that computer solvers use.
Printable hard sudoku puzzles
Use the printable sudoku maker to make hard 9×9 puzzles on paper, in black and white to save ink, with one, two or four to a page and an answer key page. Paper is a good way to practice pencil marks.
Hard sudoku: frequently asked questions
What makes a sudoku hard?
Fewer starting numbers and puzzles that need patterns such as naked pairs, not just squares with one possible number.
How many numbers does hard sudoku start with?
27 of the 81 squares, so 54 are left to solve.
Do you ever need to guess in hard sudoku?
No. Every puzzle in this game has exactly one solution that can be reached by logic. If you are stuck, look for a pair or use a hint.
Is hard sudoku online free?
Yes, it is free, and no account is needed. Best scores are saved only in the browser on this device.
What is the minimum number of clues in a sudoku?
17. In 2012 mathematicians proved that no 9×9 sudoku with a unique solution can have fewer than 17 given numbers.
Does it work on phones and tablets?
Yes. The game is built for touch first and also works with a mouse or keyboard on a computer.