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A quick guide to solving Sudoku, and everything Sudoku Player UK can do.

Instructions — How to Solve a Sudoku Puzzle

The rules

A Sudoku grid is 9×9 cells, divided into nine 3×3 boxes. Some cells start filled in (the "clues") and the rest are empty. To solve the puzzle, fill every empty cell with a digit from 1-9 so that:

  • Each row contains every digit 1-9 exactly once
  • Each column contains every digit 1-9 exactly once
  • Each 3×3 box contains every digit 1-9 exactly once

That's it — no maths or arithmetic involved, just logic. A properly set puzzle always has exactly one solution, so you never need to guess: every move can be worked out for certain.

Start with the obvious

Scan each row, column, and box for digits that are almost complete — if a row already has eight of its nine digits filled in, the last empty cell can only be the missing one. The same logic applies to boxes: if a box is missing only one or two digits, check which cells in that box could still take them, given what's already in their row and column.

Pencil marks (candidates)

Once the obvious placements are done, most puzzles need a more systematic approach: for each empty cell, work out which digits could possibly go there — i.e. which digits aren't already used in that cell's row, column, or box. These possibilities are called candidates, and jotting them down in the corner of the cell is called pencilling in. (SPUK can do this automatically — see Pencil Mode below.)

Naked singles & hidden singles

Once you have candidates marked in, two patterns solve most cells:

  • Naked single — a cell has only ONE candidate left. That digit must go there.
  • Hidden single — a digit appears as a candidate in only ONE cell within a row, column, or box, even if that cell also has other candidates. That digit must go there, since nowhere else in that row/column/box can take it.

Every time you place a digit, remove it as a candidate from every other cell in the same row, column, and box — that single step usually reveals several more singles, and solving becomes a chain reaction.

When you get stuck

Harder puzzles eventually run out of singles and need more advanced techniques (pairs, pointing candidates, X-Wings, and so on) — worth searching up once you've got the basics down. In the meantime, a hint reveals one cell's correct value so you're never stuck for good — see Hints below.

Features — What Sudoku Player UK Can Do

Playing a puzzle

  • Click New Game and pick a difficulty — Easy, Mild, Medium, or Hard.
  • Click a number on the number pad to select it (it highlights, and matching values light up on the board), then click a cell to place it.
  • Or skip the number pad entirely: click a cell, then press 1-9 on your keyboard.
  • Your in-progress game is saved automatically, so navigating to another page and back — or closing the browser — picks up right where you left off.

Pencil mode

Toggle the ✎ button (in the header, or press P) to have SPUK automatically calculate and display every valid candidate for each empty cell. Selecting a number then highlights that digit's candidates in red across the whole board — handy for spotting hidden singles.

Hints, erase & undo

  • Hint (?) — click it, then click an empty cell to reveal its solution. The number of hints available per game is set in Settings.
  • Erase (✕) — click it, then click a cell you entered to clear it. Given clues can't be erased.
  • Undo (↶) — instantly reverts your last move.

Keyboard shortcuts

  • 1-9 — enter that digit into the selected cell
  • P — toggle pencil mode
  • Arrow keys — move the selected cell
  • Backspace / Delete / 0 — erase the selected cell
  • H — arm the hint action
  • Ctrl+Z (or Cmd+Z) — undo
  • Escape — clear the current selection, or close an open modal

Timer & scoring

The timer only counts active play-time — it pauses while you're on another page or the browser's closed, so stepping away never inflates your time. Completed games score points based on difficulty (Easy 4 · Mild 6 · Medium 8 · Hard 10), minus one point per hint used.

Accounts & the leaderboard

You can play as a guest with no account, but guest games aren't saved or scored. Sign up to appear on the Leaderboard (filterable by today/week/month/year/all-time), and make sure your Profile — name, country, and city — is filled in, since incomplete profiles are excluded from the leaderboard.

Settings

Signed-in players can customize their defaults from the Settings page:

  • Whether pencil mode is on by default for new games
  • Your default difficulty
  • How many hints you get per game (0-3)
  • Whether the Total Missing / Hints Remaining counts show in the header
  • Which side(s) the number pad docks on — any combination of top, bottom, left, and right (left/right need a wide enough screen, and fall back to bottom on phones)
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Docs — Sudoku Player UK