Sudoku

What it is

This is classic Sudoku on a 9×9 grid. Some cells are filled in at the start. You fill in the rest so that every row, every column and every 3×3 box holds each digit from 1 to 9 exactly once. Every puzzle can be solved by logic alone. No guessing needed.

There are five levels: Easy, Medium, Hard, Expert and Master. We do not grade a puzzle by how many numbers it gives you. The level depends on the hardest technique a puzzle needs. Easy puzzles need only singles. Medium adds pairs and pointing lines, Hard adds triples and X-Wing, Expert adds Swordfish and wing patterns, and Master asks for chains of reasoning. A new puzzle appears instantly.

There is also a daily puzzle. Everyone gets the same one on the same day (the day changes at midnight UTC), and its level changes through the week. Solve it on consecutive days to build a streak.

The grid is as large as your screen allows. The number pad sits under it on a phone and beside it on a wide screen. Each key shows how many of that digit you still have to place.

How to play

Tap a cell, then tap a number. If you prefer the other order, turn on Number first in Settings: pick a number, then tap as many cells as you like. On a keyboard, move with the arrow keys, type 1 to 9, press Backspace to erase, N for notes, Z to undo and H for a hint.

Notes are small pencil marks for the digits a cell could still take. Switch Notes on and the pad writes marks instead of digits. Auto notes fills them in for you. When you place a digit, it disappears from the notes in its row, column and box.

Selecting a cell highlights its row, column and box, and every cell with the same digit lights up as well. Highlight mistakes, on by default, marks a wrong digit as soon as you place it and keeps a mistake count. You can turn it off in Settings and play without it. Undo and Redo walk back and forward through your moves, and Erase clears a cell.

Hint is a short lesson. It lights up the cells that matter and names the technique. You can try to spot the pattern yourself, then ask for the explanation and apply the step with one tap. If a digit you placed is wrong, the hint points at it first. Reveal cell, in the menu, simply fills in the selected cell.

The timer pauses when you open a menu or leave the page. A finished puzzle shows your time, the level, your mistakes and your best time for that level. Best times do not count a puzzle in which you used a hint or Reveal cell. The game saves after every move, so you can close the page and come back.

Tips

• Scan for singles first: a digit that has only one place left in a row, column or box, or a cell that can take only one digit.
• Turn on Auto notes when you reach a harder puzzle, then look for cells with only two candidates.
• Pick a digit on the pad in Number first mode: every cell holding it lights up, so you can see which rows, columns and boxes already have it and where it still has room.
• When you are stuck, use Hint to learn the technique rather than to get the answer. The same pattern will come back in later puzzles.
• Step up one level at a time. Each level introduces one new family of techniques.

Questions people ask

Will I ever need to guess? No. Every puzzle can be solved by logic alone. No guessing needed.

What makes a Master puzzle harder than an Easy one? Not the number of given digits. A Master puzzle needs techniques such as chains, which take more steps of reasoning to see.

Do I need an account? No. Your stats, settings and the game in progress are saved on your device.

Can I play offline or with a keyboard? The game needs no network once it has loaded. The keyboard works as described above.

Does using a hint count against me? Hints are free. They only keep that puzzle from setting a new best time.

How does the daily streak work? Finish the daily puzzle on a day to count it. Finish it the next day too and the streak grows. Miss a day and it starts again.