Sudoku guide

A guide to the daily Sudoku on OverTime: choosing a difficulty, using notes well, finding the next digit, and how your time is scored.

How to play Sudoku

  • 9x9 grid
  • 3 puzzles a day
  • Fastest time wins

Goal

Fill the nine-by-nine grid so every row, column and three-by-three box contains each digit from 1 to 9 exactly once.

  • The square you selected
  • Same digit as the selected square
  • Clashes with its row, column or box

The rules

  • No digit may repeat within a row.
  • No digit may repeat within a column.
  • No digit may repeat within one of the nine three-by-three boxes.
  • The clues you start with are fixed and cannot be changed; every puzzle has exactly one solution reachable by logic alone.

Notes and technique

Use pencil marks to record the candidates for a square rather than guessing. The two workhorse techniques are the naked single - a square with only one candidate left - and the hidden single, where a digit can only go in one square of a row, column or box even though that square has several candidates.

Difficulty and scoring

  • Each difficulty is a separate puzzle and is tracked separately on the same day.
  • Your time is the score: finish faster to rank higher.
  • Puzzles reset at midnight Melbourne time and are the same for every player.

Before your first grid

Sudoku opens on a menu rather than a board. Choose Easy, Medium or Hard, check your best and average times for that level, then press Start Game.

The three levels are separate puzzles. Easy begins with 46 given digits, Medium with 36 and Hard with 26, and each has exactly one solution.

Beside the grid on a wide screen, or under it on a phone, sit the controls. You get a Normal and Notes switch, a number pad, Clear, Undo and an Auto-fill notes toggle.

On a keyboard, type 1 to 9 to place a digit and use the arrow keys to move. Backspace, Delete or 0 clears a square, Space swaps between Normal and Notes, and Ctrl+Z or Cmd+Z undoes.

A sound way to start

Start with the digit that already appears most often. Each copy rules out a row, a column and a box, so the remaining copies have few places to go.

The number pad helps here. Each button shows how many of that digit are placed out of nine, and a digit with all nine placed greys out.

Sweep each box for hidden singles before you write any notes. They are the quickest placements on the board, and each one opens up its neighbours.

Reading the grid as it fills

Once the easy placements dry up, switch to Notes and mark the candidates for the busiest box. With Auto-fill notes off, a square shows its notes only while it is selected or under the pointer.

Auto-fill notes writes every legal candidate into every empty square. It also clears a digit from the notes around a square when you place it, which saves a lot of tidying.

Look for pairs. When two squares in a row can only hold the same two digits, clear both from the other notes in that row.

Selecting a filled square highlights every matching digit on the board. Use it to see at a glance which rows and columns still lack that digit.

Mistakes that cost time

A slow solve is rarely about a hard step. It usually comes from one of these.

  • Trusting the board to catch errors. A square turns red only when its digit clashes with its row, column or box, so a wrong digit with no clash stays quiet.
  • Guessing to break a stall. Every puzzle can be solved by logic, so a guess only hides the step you missed.
  • Pressing Start before you are ready. The on-screen clock waits for your first digit, but your ranked time runs from the moment the board appears.
  • Ignoring Undo. When a chain of placements goes wrong, stepping back to the last sure move is faster than hunting for the bad digit.

Scores and leaderboards on OverTime

Your score is 1,000 minus your solving time in seconds. A solve that takes 1,000 seconds or longer, which is 16 minutes and 40 seconds, scores zero.

Each difficulty has its own leaderboard, so a Hard time is only ever compared with other Hard times. The boards cover today, this week, this month, this season and all time.

Your first solve of a level each day is the one that counts. You can replay a level you have finished, but the replay is not recorded.

After a solve, the results screen shows your time and difficulty alongside your streak and games played.

Where to go next

Sudoku has no Unlimited mode, but three levels a day give you room to practise. Work up from Easy to Hard as your times drop.

To test your speed against a person, Battle Sudoku has two players race to solve the same grid. Mine Sweep offers a different kind of logic, where each number tells you how many mines touch its square.