Hard sudoku online

What makes this puzzle hard

The number of digits alone does not decide the difficulty here. The generator checks every puzzle and only rates it Hard when it truly needs pairs and pointing cells. Spotting missing digits will not get you to the end. So a hard puzzle will not turn out to be an easy one in disguise, and yet every one can be solved by logic, without a single guess.

The techniques you need

Start with notes: pencil in the candidates in every empty cell, because you will not see pairs without them. A pair is two cells in one row, column or box holding the same two digits, for example 3 and 8. You can then remove those digits from the other cells of that group. Pointing cells work in a similar way: when a digit can only sit in one row within a box, it goes from the rest of that row. After every removal, check single cells again, because new ones often appear.

The rules of this game

You have three hints, and three mistakes end the game. A mistake costs 120 points and the score pool shrinks every second, so pace counts too. An interrupted game waits under Continue, and this link will not erase a game you have already started. A hint does not fill in a digit right away, it shows the simplest logical step. When Hard stops being a challenge, Expert, Master and Extreme are waiting above it.

How to start a hard puzzle

Do not fill in notes straight away. Go through the digits first, starting with the most frequent one, and check where it can stand in each box. That fills the first cells quickly and cheaply. When those moves run out, turn on notes and add candidates across the whole board. Look for pairs in boxes first, because they are easiest to see, then in rows and columns. If you get stuck, do not guess: a mistake costs points, and a hint shows you the technique you are missing.