Question 1

How do you play Hard Sudoku?

Accepted Answer

Fill the 9×9 grid so that every row, every column and every 3×3 box contains the digits 1 to 9 exactly once. You are given some digits to start; the rest you work out. No arithmetic is involved at any point — the digits are just symbols, and the game would play identically with nine colours.

Question 2

Is this free? Do I need an account?

Accepted Answer

It is free and there is no account. Nothing is installed, nothing is bought, and your puzzle, settings and statistics are kept in this browser rather than on a server.

Question 3

Does every puzzle have exactly one solution?

Accepted Answer

Yes, and it is checked rather than assumed. Every time a clue is removed while a puzzle is being built, the generator counts the solutions of what is left and puts the clue back if the answer is no longer one. A sudoku with two solutions is not a hard sudoku, it is a broken one — you reach a point where logic runs out and only guessing separates the branches, with no way to tell from the inside. It is most of the cost of making a puzzle and it is not optional here.

Question 4

How is the difficulty decided?

Accepted Answer

By the hardest technique the puzzle actually forces you to use — not by how many clues it starts with. This one is hard, meaning it can be finished with box-line reasoning and hidden pairs. Clue count is the usual measure elsewhere and it is close to meaningless: a 24-clue puzzle solvable by hidden singles is easier than a 30-clue puzzle that needs an X-Wing, and anyone who has done a few hundred can feel that.

Question 5

How many clues does a puzzle start with?

Accepted Answer

It varies, and deliberately so — the generator removes clues until removing another would either break the single solution or push the puzzle past its grade, so the count is a result rather than a target. A 9×9 usually lands in the low-to-mid twenties. The theoretical minimum for a 9×9 with one solution is seventeen.

Question 6

Can I use pencil marks?

Accepted Answer

Yes. Turn Notes on and the digits you press are written small into the cell instead of filled in. Placing a real digit clears that digit from every cell that shares a row, column or box with it, which is the bookkeeping that makes notes tedious on paper. "Fill pencil marks" writes every candidate at once if you would rather start from there.

Question 7

What does the hint button do?

Accepted Answer

It finds the next step a solver would take and tells you what kind of step it is and which cells it rests on — not the digit. Press it again and it fills that cell in. Hints come from the solver rather than from the answer key, which means a hint is always a move you could have found yourself, and it can be explained. A hint that just reveals a digit teaches nothing.

Question 8

Will it stop me entering a wrong digit?

Accepted Answer

No, and that is on purpose. Sudoku is a game about being allowed to be wrong and finding out — blocking wrong digits turns it into a game of poking cells until one accepts. A digit that clashes with another in its row, column or box is highlighted immediately, and the mistake counter records it, but the digit goes in.

Question 9

Can I undo?

Accepted Answer

As many times as you like, and redo to put them back. Notes are restored along with digits, so undoing a placement brings back the pencil marks that placement cleared.

Question 10

Can I replay the same puzzle?

Accepted Answer

Yes. Every puzzle has a number, and the same number always produces the same grid — the whole thing is generated from that one value. Share the link and the other person gets your exact puzzle.

Question 11

What is a naked single?

Accepted Answer

A cell with only one digit left that fits, once you have ruled out everything already present in its row, its column and its box. It is the simplest step in sudoku and the one every puzzle starts with.

Question 12

What is a hidden single?

Accepted Answer

A digit that can only go in one cell of a row, column or box — even though that cell might still look like it has several options. Hidden singles are what most of an easy puzzle is made of, and learning to look for them is the single biggest jump in speed a beginner makes.

Question 13

What is a naked pair?

Accepted Answer

Two cells in the same group that both have exactly the same two candidates. Between them they will use both of those digits, so no other cell in that group can — even though you do not yet know which is which. It is the first technique that eliminates rather than places.

Question 14

What is pointing, or box-line reduction?

Accepted Answer

If a digit can only go in one row within a box, then it must be somewhere on that row, so it can be struck from the rest of that row outside the box. The same argument runs the other way, from line to box. It is reasoning across two groups at once, and it is where most people say sudoku started feeling hard.

Question 15

What is an X-Wing?

Accepted Answer

A digit that in two different rows can only go in the same two columns. Those four cells form a rectangle, and whichever way it resolves, both of those columns are used up — so the digit can be struck from the rest of both columns. It is the first technique that reasons about a shape rather than a group.

Question 16

Do I need to guess?

Accepted Answer

Never on a puzzle graded below expert — every step has a reason you can find. On an expert puzzle you may reach a position where no single technique applies, and the usual method there is to follow one branch and see whether it contradicts itself. That is still deduction, just longer. This puzzle is hard.

Question 17

Is there any maths in sudoku?

Accepted Answer

None. Nothing is ever added, and the digits could be nine letters or nine colours without changing a single move. What sudoku actually exercises is constraint reasoning — repeatedly asking what a group still needs and where it can still go.

Question 18

What is the timer for?

Accepted Answer

Nothing is lost by ignoring it. It is there because a lot of people race their own previous time, and for nobody else. There is no penalty, no limit, and no leaderboard.

Question 19

Can I play on a phone?

Accepted Answer

Yes, and the input is built for it rather than adapted to it. A 9×9 grid on a phone gives cells around 38 pixels across, which is below what a thumb can reliably hit — so you tap a cell to select it and then press a number on the pad below, where the keys are sized properly. On a desktop the keyboard does the same job: arrow keys move, number keys enter, and backspace erases.

Question 20

What do the numbers on the keypad mean?

Accepted Answer

The small figure on each key is how many of that digit are still to be placed. When it reaches zero the key dims. Counting how many 9s are already down is bookkeeping rather than deduction, and everybody does it by hand otherwise.

Question 21

What other sudoku sizes and kinds are there?

Accepted Answer

This site runs one engine across all of them: Easy Sudoku, Medium Sudoku, Hard Sudoku, Expert Sudoku, Mini Sudoku (4×4), 6×6 Sudoku, Sudoku X, Windoku. The grid size, the box shape and any extra regions are configuration, which is why a 4×4 and a Sudoku X behave identically in every other respect.

Question 22

Is this puzzle solvable without any special tricks?

Accepted Answer

Not entirely. You will need notes and at least one technique that eliminates candidates rather than placing digits. That is what the grade means.