All puzzle guides

A little logic. A satisfying break.

How to play Shikaku

Turn the numbered clues into a complete tiling of rectangles. Each clue tells you an area, not a side length.

Play Shikaku

Already playing? Return to that tab to preserve its Undo history. These examples are separate from your board.

One clue, one rectangle

  1. Divide the grid into rectangles with horizontal and vertical sides.
  2. Each rectangle contains exactly one numbered clue. Its area in cells must equal that number.
  3. Cover every cell exactly once: no overlaps and no gaps.

A square is a rectangle too. The clue can be anywhere inside it, including a corner. Clue numbers stay visible while you play.

Start with the possible shapes

Before

After

Action: Add the rectangle from row 1, column 1 to row 2, column 2. Its four cells contain exactly one clue: 4.

A clue 6 can occupy 1 × 6, 2 × 3, 3 × 2 or 6 × 1. On a 5 × 5 grid the first and last shapes cannot fit. Any placement containing another clue is also impossible. A clue 1 always occupies its own single cell.

Eliminate overlaps

On this 2 × 2 example, the two clue-1 cells are forced. The clue 2 cannot extend down into either of them, so it must cover the top row.

2·
11

The resulting rectangles are row 1 columns 1–2, row 2 column 1, and row 2 column 2. Their areas are 2, 1 and 1, and all four cells are covered exactly once.

Ask who can cover a cell

If only one clue has any remaining rectangle that reaches an uncovered cell, that clue must cover it. Remove its candidate rectangles that miss the cell. This does not assume your guesses are correct.

Checked deductions

Forced Rectangle

A clue with one remaining rectangle excludes overlapping alternatives at other clues.

Sole Coverage

A cell covered by only one clue must belong to that clue’s rectangle.

Builds on: Forced Rectangle

Common Intersection

Cells shared by every remaining rectangle of a clue cannot belong to another clue.

Builds on: Forced Rectangle

Compatible Clue Pair

Two clues must choose rectangles that do not overlap each other. Discard another rectangle if it blocks every compatible pair.

Builds on: Common Intersection

A pair can rule out what one clue cannot

On a 10 × 10 board, call the clue 8 at row 6, column 2 A, and the clue 6 at row 8, column 1 B. After the simpler deductions, these are their complete remaining choices:

ClueRemaining rectangle corners
AA1: row 3, column 1 to row 6, column 2
A2: row 3, column 2 to row 10, column 2
BB1: row 3, column 1 to row 8, column 1
B2: row 4, column 1 to row 9, column 1
B3: row 5, column 1 to row 10, column 1
B4: row 7, column 1 to row 9, column 2

Before: the clue 8 at row 9, column 4 could appear to use row 9, columns 1–8.

If A uses A1, B1–B3 overlap it, leaving only B4. B4 overlaps that proposed row-9 rectangle. If A uses A2, A2 itself overlaps the proposal. These are all choices for A, so the proposal cannot leave space for both clues.

After: cross out row 9, columns 1–8 from that clue’s candidates. Do not draw either supporting clue yet: this deduction removes one possibility and does not select A1 or A2.

This example comes from public clues and complete checked domains. No stored answer or player guess is a premise. The same two-clue check supports v2 hints, including reopened codes whose Hard level remains closed for New.

The player explains independently checked eliminations until one rectangle remains for a clue. Apply adds that rectangle once. It never uses a stored answer as the deduction. Incorrect guesses receive recheck feedback first.

Choose corners, then commit

Ctrl+Z or Command+Z undoes the last edit or whole applied hint. Ctrl+Shift+Z or Command+Shift+Z redoes; Ctrl+Y also redoes. These work from the active board and its action buttons. Text fields keep their native editing shortcuts. Settings saves this game’s optional timer and Auto-check choice independently. Turn Auto-check off and use Check when you want to review your work.

Tap or click the first corner and then the opposite corner. A dashed preview and numbered corner markers show the proposed rectangle and its area. Choose Add rectangle to commit. With a mouse or pen, you can also drag corner to corner: the preview commits once when you release inside the board. Reverse drags work too. Escape, an interrupted drag or releasing outside the board cancels it. Touch scrolling never draws a rectangle; use two taps and Add rectangle. For a single-cell rectangle, choose the same cell twice.

On a keyboard, Shift+arrows anchors and extends or reduces a rectangle preview; releasing Shift commits once, and Escape cancels without an edit. Without Shift, press Enter or Space to anchor the first corner, move with arrows, then press Enter or Space a second time to commit. Held keys do not repeat commitment. Tab reaches Add rectangle and the other controls. Escape or Cancel preview abandons the proposal. Scrolling keeps the uncommitted preview so you can reach its controls. Leaving the editor cancels it. Neither action commits a rectangle.

To revise a rectangle, focus a cell inside it and choose Revise selected, choose new corners, then Replace rectangle. The original stays until commitment. Remove selected or Delete removes it. A new or revised rectangle replaces every existing rectangle wholly inside it. A partial overlap rejects the entire edit. One Undo restores the exact former set. Wrong areas or clue counts remain guesses you can Check, revise, remove or Undo.

Undo and Redo cover additions, revisions, removals, hints and restarts. Restart resets the timer and mistake count; Undo restores rectangles, not time or mistakes. Check marks wrong rectangles with a contrasting fill and an exclamation mark. A complete board needs legal rectangles and full coverage, not just plausible guesses.

Larger boards, checked levels

Choose 5 × 5, 7 × 7 or 10 × 10 at Easy or Medium. The default is 5 × 5 Easy. Easy uses forced-rectangle eliminations; Medium also needs sole coverage or common intersection. These are checked computational techniques, not human or publisher calibration. Hard remains unavailable: neither the original v1 engine nor the larger v2 construction met its admission threshold. V2 adds a stronger two-clue deduction, explained above; a stronger rule alone does not qualify a reliable New option. Original v1 codes, grades and saves keep their exact boards. New 10 × 10 boards use v2 codes.

Relaxed play has no mistake limit. An optional three-mistake challenge checks committed guesses and offers relaxed continuation. The optional timer pauses when the board is hidden or paused.

Progress saves in this browser. Your space lists all games together, latest played first. Share a code or link to the original board; download progress to carry rectangles and timer. Restore checks the canonical puzzle and asks before replacing a save. Other games stay untouched. If another tab changes a save, use that board’s recovery controls to download either copy before replacing it.

Shikaku offers New, Next and saved or shared boards. Today’s Shikaku and the published archive offer shared puzzles when available.