How to Play Kakuro
Select a Puzzle
Click 'New Game' to open the puzzle selector. Choose from Easy (5×5 or 6×6), Medium (7×7), or Hard (9×9) puzzles. The difficulty controls the number of cells and how many forced combinations appear.
Understand the Grid
Black cells with a diagonal slash are clue cells. The number in the upper-right is the sum for the cells to the right (across run). The number in the lower-left is the sum for the cells below (down run). Click a clue cell at any time to see all valid digit combinations for that run.
Fill White Cells
Click a white cell to select it, then press a digit key (1-9) or tap the on-screen number pad. Each digit must be unique within its horizontal and vertical run and must help the run total equal its clue. Use Pencil mode (P key) to enter candidate notes. Use Check to highlight errors in red.
Complete the Puzzle
When all white cells are filled correctly, the puzzle auto-completes and shows your solve time and hint count. Use Undo (Ctrl+Z) to backtrack, and Hint (when a cell is selected) to reveal a single correct digit if you get stuck. Arrow keys move between cells; the timer pauses if needed.
Frequently Asked Questions
What is the difference between Kakuro and Sudoku?
Sudoku and Kakuro share a grid format and the constraint that digits must not repeat within a group, but they differ fundamentally in how groups are defined and what the constraint is. In Sudoku, digits 1-9 must each appear exactly once in every row, column, and 3×3 box — the constraint is completeness. In Kakuro, each run (any length from 2 to 9 cells) must contain unique digits that sum to a specific target — the constraint is arithmetic. Kakuro runs vary in length, which creates far more varied combinatorial challenges. Kakuro also has no equivalent to Sudoku's box constraint, relying instead entirely on the sum targets and the no-repetition rule.
Are there combinations I should memorize?
Yes — experienced solvers memorize the 'forced' combinations where only one digit set is possible. The most important are: 2-cell runs: sum 3 = {1,2}, sum 4 = {1,3}, sum 16 = {7,9}, sum 17 = {8,9}. Three-cell runs: sum 6 = {1,2,3}, sum 7 = {1,2,4}, sum 23 = {6,8,9}, sum 24 = {7,8,9}. Four-cell runs: sum 10 = {1,2,3,4}, sum 30 = {6,7,8,9}. Five-cell runs: sum 15 = {1,2,3,4,5}, sum 35 = {5,6,7,8,9}. These give you immediate certainty about which digits go in those cells, which then constrains intersecting runs and often unlocks much of the puzzle.
What does the combination helper show?
When you click a clue cell in the grid, the combination helper popup appears and lists every valid digit set for that run — meaning every subset of unique digits from 1-9 that has the correct length and sums to the clue value. For example, a 3-cell run summing to 16 has six valid combinations: {1,6,9}, {2,5,9}, {2,6,8}, {3,4,9}, {3,5,8}, and {4,5,7}. By comparing the valid sets for the across run and the down run through a cell, you can intersect them to determine which digits are possible for that cell. The helper is your best tool for quickly seeing what is constrained.
How do pencil marks work?
Pencil marks (also called candidates) are small digit notes you can write inside a cell when you are not yet certain of the final answer. Toggle pencil mode with the P key or the Pencil button in the toolbar. In pencil mode, pressing a digit toggles it on or off as a candidate — it appears as a small number in the cell's 3×3 micro-grid. When you have narrowed down a cell to a single candidate, switch back to pen mode and enter the final digit. Entering a digit in pen mode automatically clears all pencil marks in that cell. Pencil marks are essential for solving medium and hard puzzles efficiently.
Is there always a unique solution?
All well-formed Kakuro puzzles have exactly one solution. This is a defining property of properly constructed Kakuro puzzles — the combination of all run sum constraints and the no-repetition rule is designed to uniquely determine every cell. If a puzzle has multiple solutions, it is considered malformed. All six built-in puzzles in this game have been manually verified to have exactly one solution. You should never need to guess: if you appear to have two equally valid options, re-examine your combination analysis — one path will be eliminated by considering the intersecting run constraints more carefully.
What do the keyboard shortcuts do?
This game is fully keyboard-accessible. After clicking a white cell to select it, use the digit keys 1-9 to enter values, and Backspace, Delete, or 0 to clear. Arrow keys move the selection to the nearest white cell in that direction, skipping over black and clue cells. P toggles between pen and pencil mode. Ctrl+Z undoes the last move; Ctrl+Y or Ctrl+Shift+Z redoes. The timer pauses when you use the pause button. On mobile, the on-screen number pad at the bottom of the right panel provides tap access to all digits and the erase button.