🧠 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.
Step 6: hidden pairs, the technique almost nobody uses
When the previous steps run dry, look for two numbers that only appear as candidates in the same two cells of a unit, even if those cells hold more pencil marks. Example: in a box, 3 and 8 only fit in two specific cells. Those cells belong to them: erase every other candidate from those two cells. You place no number right away, but that cleanup almost always reveals an immediate hidden single.
Step 7 (expert level): the X-Wing
If you play expert level, sooner or later you'll need it: find a number that in two different rows has exactly two candidates, sharing the same columns and forming a rectangle. The number must land on two opposite corners, so you can erase it from the rest of both columns. The extended explanation with more patterns lives in advanced sudoku techniques.
The stuck-board checklist
Save this sequence for when the board looks dead: first confirm every placed number cleaned up its neighbouring candidates; then hunt hidden singles unit by unit (not number by number); next chase naked pairs in the fullest units, where they are most likely to exist; finally check boxes where a number only fits on one line. If a full cycle brings no progress, the board is hiding a hidden pair or an X-Wing.
How long it takes and when to worry
A hard sudoku with 27 clues takes 20-40 minutes for a player with fresh technique; well-trained solvers go under 15. If you've been at it an hour, it isn't lack of talent: almost certainly a wrong pencil mark is poisoning the board. Reset the notes of the most tangled units and the knot usually unravels. More timing references in how long a sudoku takes.
Frequently asked questions
Can a hard sudoku be solved without notes?
Top players manage it, but it isn't efficient: from 27 clues onwards working memory can't hold every candidate. Using notes isn't cheating — it's the standard tool.
What if I find two possible solutions?
Recheck: every MundoSudoku board has a generator-guaranteed unique solution. If you see two valid paths, there's an earlier mistake on the board, usually a number placed on intuition.
How do I move from hard to expert?
Once you chain several hard boards without errors or long stalls, try expert (23 clues). The jump is real: on Sundays the daily sudoku is expert level and it's the best measuring stick.
