Calcudoku vs KenKen: Same Puzzle, One Trademark, and Why It Matters

I was seven when I found the book. Ten days later, every puzzle in it was finished.

An Old Book and a Seven-Year-Old

I was seven years old when I found an old puzzle book — I don't remember where it came from, might have been a relative's, might have been lying around the house. It had grids in it. Not Sudoku (I didn't know Sudoku yet). These grids had dotted cages with numbers and math symbols inside them. No instructions I could follow. I figured out the rules by staring at the solved example on the first page.

I finished the entire book in ten days. After school, every day, nothing else. My parents probably thought I was studying.

That puzzle has a complicated name situation. In 2004, a math teacher named Tetsuya Miyamoto created it for students at a private classroom in Yokohama. His whole philosophy was built around not teaching — hand students a puzzle and let them figure out the math themselves. He called it 'Kashikoku Naru Puzzle.' Roughly: 'the puzzle that makes you smarter.' It sold over 1.5 million copies in Japan.

The Trademark That Split a Name

And then the naming got complicated. An American toy inventor named Robert Fuhrer found the puzzle at the Bologna Book Fair in 2007 and trademarked it as 'KenKen.' The Times of London picked it up in 2008. The New York Times followed in 2009.

But you can trademark a name, not a mathematical concept. So anyone who wants to publish the same puzzle without paying licensing fees uses a different name — Calcudoku, Mathdoku, whatever. Same grid, same cages, same rules. Different five letters on the cover.

The rules take thirty seconds. You get a grid (4×4 up to 9×9) divided into cages outlined with dotted lines. Each cage has a target number and an operation — say '12×' in a two-cell cage, meaning the two digits multiply to 12. Could be 2 and 6, could be 3 and 4. Fill the grid so no digit repeats in any row or column.

Why It Still Breaks My Brain

What seven-year-old me didn't understand (and what still gets me now) is that the arithmetic and the logic aren't separate steps. You can't just factor the cages and place digits — the row constraints change which factorizations are even possible. A cage marked '6+' in a row that already has a 1 and a 5 is a completely different problem than the same cage in an empty row. I was a math kid in South India where this stuff was encouraged, and Calcudoku still broke my brain. In the best way.

Miyamoto's students in Yokohama still play it. The 30,000 teachers using the KenKen brand in American classrooms assign it. And every midnight, a new Calcudoku daily challenge shows up on Cliko Games — seven difficulty levels, from a gentle 4×4 warm-up to a 9×9 Cyborg grid that will cost you a lunch break.

If you're coming from Sudoku and want something that asks more of your brain, this is it. And if you want pure arithmetic chaos, try Mad Add next.