SUDOKU GUIDE
Solve a sudoku from a photo
A sudoku solver from photo reads the grid off your screen and fills in every square. Here's the hand method I use when I solve without one.
I used to stare at empty squares hoping the answer would show up. It doesn't. What works is counting what's missing, not staring at what's there.
Here's a real sudoku, solved from a photo:
- Take a decent photo.
Page flat, light even, no flash, fill the frame. Sudoku grids are all thin lines, so any blur turns 1s into 7s. Zoom in on the photo after and check you can read every digit. A tool solving from the photo only sees what the photo shows. - Find the fullest row, column, or box.
Don't start at the top left. Scan all nine rows, nine columns, and nine boxes, and start wherever the most squares are already filled. Fewer blanks means fewer options. - Use the only-one-place trick.
Pick a digit, say 4. Look at a row that's missing it. That 4 has to go in one of the empty squares, but the columns and boxes rule most of them out. When only one square is left, that's where it goes. This one move solves most easy and medium puzzles. - Pencil in candidates.
When you're stuck, write the small possibilities in each empty square. The moment a square has only one candidate left, ink it in. Then cross that digit out of every candidate list in its row, column, and box. - Look for pairs.
If two squares in the same row both only allow 2 and 7, nothing else in that row can be 2 or 7. That often cracks the next square open.
A small example.
A 4x4 grid, digits 1 to 4, same rules.
Row 1 has 1 and 3. It's missing 2 and 4. Look at column 2: it already has a 4 in row 2. So the 4 can't go in row 1, column 2. That square must be 2. The other blank in row 1 is 4.
Now column 4 reads 4, 2, blank, 1. It's missing 3, so row 3, column 4 is 3. Row 3 is now 2, blank, 4, 3 — missing 1, so row 3, column 2 is 1.
Keep going the same way and the grid fills itself.
When this doesn't work.
Really hard puzzles sometimes need trial and error: pick a square with two candidates, try one, and follow it until something breaks. If it breaks, the other candidate was right. Also, a bad photo kills everything. A crease through the grid or glare on one box hides digits from your eyes the same way it hides them from a camera. Fix the photo first.
I built PuzzleCrackerAI because I got tired of doing this by hand. It's a sudoku solver from photo: you take a picture of the grid and it solves it. First solve is free, no card needed.
Frequently Asked Questions
What is the best hand technique for solving a sudoku?
The only-one-place trick: pick a digit, find a row missing it, use columns and boxes to rule out squares until one remains.
How do I photograph a sudoku grid well?
Page flat, even light, no flash, fill the frame; zoom in and check every digit is readable.
What are pencil marks and pairs?
Small candidate digits per square — ink a square when one candidate remains, cross it out of its row, column and box; a pair (two squares, same two candidates) locks those digits out of the row.
Is it free?
Yes — first solve free, no card needed.