Levels 4 and 5 of five

Hard sudoku online: the hard and expert levels

A hard sudoku asks for more than crossing out. On levels 4 and 5 you need pairs, pointing pairs and the heavier techniques. You never have to guess: every sudoku here can be solved with logic and has exactly one solution.

What makes a sudoku hard?

Not the number of digits that are already there, but the hardest technique you need along the way. On levels 1 and 2, crossing out gets you a long way. On hard (level 4) you need naked and hidden pairs, plus the intersections of a row and a box. On expert (level 5) the heavier techniques join in, and one good move can take ten minutes of looking.

Every sudoku was solved beforehand with the same techniques you use. If the solver could not get through without guessing, the puzzle does not make it in. That is the promise of every level: there is always a logical next step.

The five levels

Very easy. Crossing out square by square is enough. A nice way to get to know the game.
Easy. You need the hidden single: which digit fits in only one place in this box?
Medium. Pointing pairs, pairs and triples join in, and you start using notes.
Hard. The X-wing and the XY-wing are the key here. Without notes it gets tough.
Expert. The hardest techniques, and long chains of small conclusions.

On the sudoku page you pick the level with one tap, and that choice stays until you change it. What the techniques are is explained on how to solve sudoku.

Stuck on a hard sudoku

Getting stuck is part of it, and there are ways to move on. Notes make the grid visible: put the digits that can still go in every empty square, and the pairs jump out by themselves. Hint tells you which step comes next and why, without filling in the digit.

If there is a wrong digit somewhere, every deduction runs into a wall. Check then marks the squares that are wrong, so you can move on instead of thinking against an impossible grid.

A fresh sudoku, made for you

Besides the regular collection, you can have one made. The site then digs out a new sudoku on the spot and measures it, until it lands on the level you picked. That takes half a second and gives you a puzzle nobody else has had.

From the collection you never get the same one twice: the site keeps track of which ones you already had. Once you have played through a whole level, a new one is made for you right there.

Play a hard sudoku Pick your own level All the techniques in a row Solve a sudoku together Take the tour