How to Play Sudoku

New to Sudoku? Follow these quick steps to build momentum. Each puzzle starts with “givens” (fixed digits). Your job is to fill the remaining squares without repeating numbers in any row, column, or 3×3 box.

  1. Step 1: Understand the goal

    Every row, column, and 3×3 box must contain the digits 1 through 9 exactly once. Givens are fixed, everything else is yours to solve.

  2. Step 2: Scan for forced numbers

    Look across each row, column, and box for cells that already have eight digits placed. The final missing digit is a guaranteed placement.

  3. Step 3: Use pencil marks

    Toggle candidates in Ruedoku to track which numbers are still possible in a cell. This keeps complex grids organized and prevents guesswork.

  4. Step 4: Cross-hatch boxes

    When a digit can only appear in one row or column within a box, note that restriction. It will often unlock placements in intersecting units.

  5. Step 5: Repeat and reassess

    After every placement, rescan the puzzle. Fresh singles and new candidate eliminations appear quickly when you keep the board tidy.

Sudoku Solving Techniques

Improve your solve rate with proven strategies. Ruedoku hints explain each move and Auto Play demonstrates how these techniques unfold on real puzzles.

Naked Singles

A cell with only one candidate must take that value. These often appear after you pencil mark possibilities or place a neighboring digit.

Example: Row 4 is missing just a 5 in column 7, so place 5 there immediately.

Hidden Singles

When a digit is the only candidate within a unit (row, column, or box), it is hidden among other marks. Clear the noise and place it with confidence.

Example: In box 6, only one cell allows a 9, so the 9 belongs to that square.

Locked Candidates

If a candidate is restricted to one row or column inside a box, eliminate that digit from the rest of that row or column outside the box. Ruedoku hints call this out as “pointing pairs”.

  • Locked candidates come in two flavors: pointing (box to line) and claiming (line to box).
  • Use them early to reduce clutter before searching for advanced patterns.

Example: If 3s only appear in the top row of a box, remove 3 from other cells in that row.

Naked Pairs & Triples

Two cells that share the same two candidates (or three cells sharing three candidates) lock those digits to those cells. Remove the digits from every other cell in that unit.

  • Pairs and triples work in rows, columns, and boxes—check all three after you spot one.
  • If a pair spans a box edge, revisit the intersecting unit for hidden singles.

Example: Two cells in column 5 can only be 2 or 8; eliminate 2 and 8 from the rest of column 5.

Box-Line Interactions

Also called pointing pairs or claiming. If a digit is confined to one line within a box, remove it from the rest of that line. Conversely, if a digit is restricted to one box within a line, clear it from the remainder of the box.

  • Trace the candidate across both the box and the line to ensure the restriction really exists.
  • Combining this with naked pairs frequently opens up new singles.

Example: All the 7s in row 2 fall inside the left-middle box, so remove 7 from the other cells in that box.

X-Wing

Spot rectangles where a digit appears in exactly two cells of two different rows (or columns). Those four corners form an X-Wing and let you eliminate the digit from the intersecting columns or rows.

  • X-Wings come in both row-based and column-based forms—scan candidates in both directions.
  • Candidate highlighting tools make it easier to confirm the four-corner rectangle before you eliminate digits.

Example: If the digit 4 appears in exactly two cells on rows 1 and 5 sharing columns 3 and 8, remove 4 from all other cells in columns 3 and 8.

Difficulty Guide

Easy: Solvable with singles and straightforward scanning.

Medium: Expect locked candidates and the occasional naked pair.

Hard: Requires layered candidate work plus techniques like box-line interactions.

Ultra Hard: Minimal clues with advanced patterns such as X-Wings. Mastering pencil marks keeps these beasts manageable.