What kind of game is Shikaku?
Shikaku is a spatial logic game played on a rectangular grid scattered with numbers. Your job is to divide the entire grid into rectangles and squares so that each piece contains exactly one number, and that number equals the piece's area in cells. A 3 must live in a 1×3 strip; an 8 could be 1×8, 2×4, 4×2, or 8×1. When every cell belongs to exactly one rectangle, the board is done.
Unlike sudoku or kakuro there is no arithmetic to track — the whole game is geometry. Deduction comes from shape: prime numbers force thin strips, clues near walls have limited room to grow, and every placed rectangle becomes a wall that constrains its neighbors. A well-made Shikaku resolves as a chain of forced moves, never a guess.
A short history: from Nikoli to the world
Shikaku was invented in 1989 by Yoshinao Anpuku and published by Nikoli, the Japanese puzzle house that also popularized sudoku. Its full Japanese name is 四角に切れ (Shikaku ni kire), roughly "cut into squares." Like Nikoli's best designs, it needs no language or math background, which helped it travel: today it appears in international puzzle championships and newspaper puzzle pages alike.
Along the way it picked up other names — you'll see the same game called Rectangles, Divide by Box, Sikaku, or Cellblocks depending on the publisher. The rules are identical in every case.
Playing Shikaku on this site
This site publishes a shared daily Shikaku game every midnight US Eastern — the same board for every player, with a streak counter and a permanent archive so a missed day is never lost. Beyond the daily, there are more than 600 free puzzles across five board sizes (5×5 up to a giant 20×20) and three graded difficulty tiers.
Every board is machine-verified to have exactly one solution and graded by a solver that mimics human techniques, so an easy label genuinely means singles and forced moves, and a hard label genuinely demands deeper deduction. No account is needed; progress autosaves in your browser.