About
About crosssums.app
A free, fast, private way to play cross sums — the number crossword also known as Kakuro — with puzzles generated on your device and every one solvable by logic alone.
Play Cross SumsWhere it comes from
A puzzle with two names
Cross sums and Kakuro are the same puzzle: a crossword grid filled with digits 1 to 9, where each run of cells must add up to a clue and use no digit twice.
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Cross-Sums on the newsstand
Number crosswords built from sums are usually traced to the American magazine publisher Dell, which printed them as "Cross-Sums" from about 1950, with the design credited to a Canadian puzzle maker, Jacob E. Funk. They stayed a niche corner of puzzle books for decades.
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Kakuro in Japan
In 1980 the Japanese publisher Nikoli started running the puzzle under the name "Kasan Kurosu", shortened to Kakuro. It became a fixture of Japanese puzzle magazines, and in the mid-2000s newspapers in the UK and US brought it to a much wider audience under both names.
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This build
crosssums.app is an independent, modern implementation of the standard ruleset. The rules are unchanged; what is new is a board that works well with a thumb, puzzles generated fresh on your device every time, and tools such as pencil notes, hints, error checking, and undo that a printed page cannot offer.
How it works
Design and mechanics
Generated on your device
Each puzzle starts as a random layout of clue and white cells, repaired so that every run is between two and nine cells long and every white cell sits in both an across and a down run. Digits are then assigned so no run repeats one, and the clues are the sums.
Verified as unique
A constraint solver counts the solutions. If there is more than one, the generator pre-fills a cell where two solutions disagree and locks it, then checks again, until exactly one solution remains. You should never need to guess.
Instant and private
Puzzles, timers, and progress live entirely in your browser. Nothing is sent to a server to play, and your best times stay on your device. Sound is synthesized with the Web Audio API, so there are no audio files to download.
A calm board
Ink on ivory cells, amber clues that turn green when solved, red only when a rule is actually broken, and a number pad that is always in thumb reach. The grid stays above the fold on a phone, and starting a new puzzle never reloads the page.
Principles
What we hold to
Free, for real
The whole game is free. No energy meters, no paywall on hints, no account wall.
Honest difficulty
Three sizes, one solution each. Hints and checks are optional, and your no-hint record is tracked separately.
Accessible
Keyboard play, labelled controls and grid cells, visible focus, and contrast that meets WCAG AA.
Contact
Found a bug or have a suggestion? Email malam169@gmail.com. Rules questions are usually answered in the FAQ.
Play Cross Sums free