🧠 How to solve a hard sudoku step by step
By Miguel Arechaga
We have all been there: the sudoku was going fine, and suddenly... nothing. No obvious cell, everything blocked. The difference between an easy and a hard sudoku is that the hard one forces you to use techniques, not just look. Here is the full method, in the order worth applying it.
Step 1: exhaust the obvious
Go through every number 1 to 9 and look for rows, columns and boxes where it can only fit in one cell. This basic scan yields few cells in a hard puzzle, but they are free. Do not move to step 2 until it is exhausted.
Step 2: write the candidates
This is where real sudoku begins. In every empty cell, note down all the numbers that could go there (the pencil marks). It is tedious the first time, but it is the foundation of every advanced technique. In our online game you can toggle notes with one button.
Step 3: hidden singles
Look at the candidates of a row, column or box. If a number appears as a candidate in only one cell of that unit, it must go there, even if the cell has other marks. This technique solves the most cells at medium levels.
Step 4: naked pairs and triples
If two cells in the same unit hold exactly the same two candidates (say 4 and 7), those numbers will occupy those two cells. Consequence: you can erase 4 and 7 from every other cell of that unit. Triples (three cells, three candidates) work the same. This cleanup usually unlocks new singles: go back to step 3.
Step 5: pointing candidates
If, inside a box, a number can only go in cells that sit on the same row (or column), then that number will occupy that row inside the box... so you can erase it from the rest of the row outside the box. Subtle but extremely powerful in hard puzzles.
If you get completely stuck
- Recheck your pencil marks: 90% of blocks are a forgotten or wrongly erased note.
- Switch numbers: drop the one you are chasing and scan another; the mind gets tunnel vision.
- Rest for 2 minutes: seriously, it works. You will come back and see the naked pair that was right there.
Above all: practise. Start with our daily sudoku and level up gradually.
