How to play Sudoku: four difficulty levels and solving tips

Sudoku modes

• Play solo (ひとりで遊ぶ): pick one of four difficulty levels (if you close the page partway, you can pick up where you left off)
• Today's Puzzle (今日の問題): one new puzzle every day, with the difficulty changing by day of the week
• Random Match (ランダムマッチ): race to solve online
• Room Match (ルーム対戦): race a friend using a room ID

Basic rules of Sudoku

• Fill each empty square with a number from 1 to 9.
• Each row (horizontal line) contains each number from 1 to 9 exactly once.
• Each column (vertical line) contains each number from 1 to 9 exactly once.
• Each 3×3 box marked off by thick lines also contains each number from 1 to 9 exactly once.
• Every puzzle has exactly one solution and is built to be solvable by logic alone, without guessing.

How to play on Asobi Lounge

• Select a square and enter a number with the number buttons (or the 1–9 keys on a computer keyboard).
• Turn notes on to pencil in small candidate numbers. When you enter a number, it is automatically removed from the notes in the same row, column and box.
• You can undo and redo, use hints (with an explanation of the reasoning) and pause. In the settings you can switch "Mistake check" (ミスチェック), which shows numbers that differ from the answer in red, and "Highlight same numbers" (同じ数字を強調) on or off.
• In versus play, your opponent's progress (the share of squares filled) is shown on the screen.

Techniques needed at each difficulty

Puzzles on this site are graded by how hard the techniques (lines of reasoning) needed to solve them are. Counting over 300 puzzles at each level, Easy puzzles start with 38–39 given numbers and Normal puzzles with 30–32.

• Easy: you can solve it just by looking for squares in a row, column or box where "this number can only go here."
• Normal: you also use squares where "only one candidate is left," because just one number can still go in that square.
• Hard: you need techniques that cut down candidates, such as pointing, where the way candidates line up in a box removes candidates from the same row or column, and pairs, where two squares share two numbers between them.
• Expert: you need techniques that look at candidate patterns across the whole grid, such as triples, X-Wing, XY-Wing and Swordfish.

Today's Puzzle and versus puzzles

Today's Puzzle is one puzzle per date: Easy on Mondays and Tuesdays, Normal on Wednesdays and Thursdays, Hard on Fridays and Saturdays, and Expert on Sundays. Your time is recorded for each day you solve it.

In versus play, both players get the same puzzle, disguised by swapping the numbers and rearranging rows and columns. The grids look different but are exactly as hard, so you cannot copy your opponent's grid, and you compete purely on solving speed.

Keyboard controls on a computer

• 1–9: enter a number (a note when notes are on)
• Shift + 1–9: write or erase a note
• Arrow keys: move between squares
• Backspace, Delete, 0: erase
• N: notes on/off, H: hint, P: pause/resume
• Ctrl + Z / Ctrl + Y: undo / redo

Tips

• Start with the rows, columns and boxes that already have the most numbers filled in; it is easier to find a number that fits there.
• Scanning by number, where you focus on one number and check every place it could go across the whole grid, works especially well on Easy and Normal puzzles.
• If two squares in the same row each have only two candidates, and they are the same two numbers (a naked pair), you can remove those two numbers from the other squares in that row.
• When you get stuck, write out each square's candidates again in note mode. A hint shows the name of the next technique you can use and why it works.

Frequently asked questions

Does Sudoku involve luck?

No. Every puzzle has exactly one solution and is built to be solvable by logic alone.

How do the difficulty levels differ?

They are graded by the techniques needed. Easy needs only finding squares where a number "can only go here," Normal also uses squares with "only one candidate left," Hard needs narrowing down candidates with pointing and pairs, and Expert needs advanced techniques such as X-Wing and XY-Wing.

Can I stop and continue later?

In solo mode, your grid in progress is saved on this device, and you can pick up where you left off the next time you open it.

Is Sudoku free to play?

Yes, it costs nothing. Games against the CPU and solo modes work right away without signing in. Online play (random matches and private rooms) and saving stats or leaderboard scores require signing in with a Google account. To play with a friend, your friend also needs to create an account and register as your friend.

More puzzles and deduction

• Hit & Blow
• Blind Mission
• Dots & Boxes
• Yacht
• Block Territory
• Minesweeper