Nonogram Solver, Step by Step
How to use the nonogram solver
Type the clues one line at a time: the row clues from top to bottom, and the column clues from left to right, with spaces between the numbers, like 3 1 2. An empty row is 0. The size of the grid comes from how many lines you type, up to 40 by 40. If the rows and columns do not ask for the same number of filled squares, the page says so before anything else, because that means a typo.
Next step fills in whatever follows by logic, one row or column at a time, highlights that line, and says why. Keep pressing to walk through the whole puzzle, or step back to look again. Show the answer fills the grid at once and says whether it is the only solution.
How to solve nonograms, step by step
Overlap. Start with the long clues. In a row of 15, a block of 10 must cover the middle 5 squares wherever it slides, because it can only move 5 squares either way. Add up all the blocks in a line plus one gap between each: whatever is left over is how far each block can slide, and any block longer than that has squares that are certain.
Edges and known squares. Once some squares are filled or crossed out, each block has fewer places it can go. Work out the first and last place each block could start, given what is already known; squares every position covers are filled, and squares no block can reach are crossed out. This is what the solver means by where the blocks can go, and it does most of the work in most puzzles.
Finished lines. When a line has all its filled squares, cross out the rest. And when no line gives anything more, try a square both ways: if filling it would leave some row or column with no way to fit its clue, it must be empty. The solver does that too, and says so.
Make your own nonogram
In Draw your own, tap squares to draw a picture and the clues appear around it. The page checks as you go whether the clues have exactly one solution, which is what makes a nonogram fair: many pictures, especially ones with lots of loose single squares, can be read more than one way. Solve these clues copies them across so you can watch the solver work through your own puzzle.
How it was checked
The heart of the solver works out, for one row or column, what every possible placement of its blocks agrees on. Before it went up it was compared against brute force, every placement listed one by one, on 3,000 random partly-filled lines, and agreed on every one. Then it solved all 42 puzzles from the daily nonogram to their stored pictures, found each to have exactly one solution, and explained every one from the first square to the last without a single wrong square.
More logic puzzles
There is a new daily nonogram to play every day, and the sudoku solver explains its moves the same way. If you like puzzles where every step is forced and nothing is luck, our brain teasers are the same feeling in wood and metal. There are more solvers on the puzzle tools page.
Frequently Asked Questions
How do you solve a nonogram step by step?
Start with the long clues: add up a line's blocks plus one gap between each, and any block longer than the room left over covers some squares wherever it goes. Then, as squares get filled and crossed out, work out where each block can still lie in each line. Squares every position covers are filled, squares no block can reach are crossed out. Repeat until done.
What do the numbers in a nonogram mean?
Each number is a block of that many filled squares in a row or column, in order from left to right or top to bottom, with at least one empty square between blocks. A 0 means the line is empty.
Do nonograms have only one solution?
A fair one does, but some sets of clues fit several pictures. The solver tells you when that happens, and the draw-your-own mode checks your picture has exactly one solution.
What is the difference between a nonogram, picross and a griddler?
None: they are names for the same puzzle. Nonogram comes from Non Ishida, one of its inventors in 1987; Picross is Nintendo's name for it, and griddler is used on several puzzle sites.
Can every nonogram be solved without guessing?
Most published ones can, line by line. Harder ones need a step that tries a square both ways and rules one out because it makes some line impossible. The solver uses that too, and says when even that is not enough.