It is also a quick way to find a typing mistake. A single wrong number in a clue is the most common reason a hand-copied puzzle refuses to come out, and the checks below usually point straight at it.
Entering the clues
- Set the number of rows and columns to match your grid.
- Type each row's clue from left to right and each column's clue from top to bottom, with the numbers in the order they are printed.
- For a row or column with no filled cells, enter 0.
- Solve. If the clues fail a basic check, the solver says which line is wrong before it tries anything else.
What the clue check catches
Two mistakes are easy to spot without solving anything. First, a clue that can't fit its line: blocks need at least one empty cell between them, so a clue of 3 1 2 needs 3 + 1 + 2 plus two gaps, eight cells in all. On a 7-wide row that is impossible, and the solver names the row.
Second, the totals must agree. Every filled cell is counted once by its row and once by its column, so all the row clues added together must equal all the column clues added together. If the rows come to 112 and the columns to 109, at least one number was copied wrong, and you know roughly how far out it is.
How it solves
The solver works the way a careful person does. It goes through the lines one at a time, fills the cells every possible arrangement of the clue agrees on, crosses out the cells none of them can reach, and repeats until nothing changes. On a 15-wide row with a clue of 11, for example, the middle seven cells are filled before anything else is known.
If line-by-line logic stalls, it tries a cell both ways and follows the consequences across the grid; a choice that leads to a contradiction must be wrong. Only after that does it fall back on a full search, which is how it can tell one solution from several.
To learn those same moves yourself, read how to solve nonograms, which walks through each one on short example rows.
When the answer is several solutions, or none
Several solutions means the clues allow more than one picture, so no amount of logic could finish the puzzle without a guess. When the solver finds two different grids that both fit, the cells where they differ are where the ambiguity lives. In printed puzzles this usually points to a typo; in your own puzzles it means the drawing needs a change.
No solution means the clues contradict each other somewhere even though each line fits on its own. Recheck the clues one line at a time against the source. Swapped numbers, such as 2 4 typed as 4 2, are a frequent culprit because they pass the totals check.
Very large grids with many possible solutions can take longer to settle. If the solver reaches its time limit, it says so instead of guessing an answer.
Private by design
Everything runs in your browser. The clues you type are never sent to us or stored anywhere, so you can check an unpublished puzzle or a competition entry without it leaving your computer. If you would rather draw a picture and have the clues worked out for you, use the nonogram maker instead, and for puzzles already checked for a single solution, try our nonograms.