Extreme Sudoku
Hard sudoku puzzles
Most "hard" sudoku in newspapers and apps fall to singles, pairs and the odd pointing pair. A really hard grid is different: it stalls again and again, and every time the next digit needs a pattern you have to go looking for.
What makes a sudoku hard
Clue count says very little. A grid with 24 givens can be gentle and one with 30 can be brutal. What matters is the logic the solve needs, and how often.
Our grade is set by the hardest technique a solver must use and by how many times the solve gets stuck at that level. A grid that is easy except for one clever step does not make the cut: it has to stay hard from the first stall to the last.
Every grid has exactly one solution and can be solved by logic alone. You never have to guess.
Our sudoku books use our top four levels only, Expert 5 to Expert 8. Expert 5 already needs wings and chains; Expert 7 and 8 need several long chains or almost-locked sets in one solve.
The techniques you will need
These are the advanced and expert techniques that come up most often in our sudoku walkthroughs. Each link explains the idea in plain words and shows it in real puzzles.
- ALS-XZ in 450 walkthroughs
- Two almost-locked sets (n cells with n+1 candidates) share a restricted digit X. Then a digit Z common to both must be in one of them, so cells seeing all of its copies cannot be Z.
- Nice Loops/AICs in 443 walkthroughs
- An alternating chain of strong and weak links. Whichever end is true, the conclusion holds, so the digit at the marked cell is ruled out.
- XY-Chain in 439 walkthroughs
- A chain of two-candidate cells, each linked to the next. Whichever way the start cell goes, one end of the chain holds the digit, so cells seeing both ends cannot.
- Finned Fish in 394 walkthroughs
- An X-Wing or Swordfish with one extra candidate (the fin). Cells that are in the fish pattern and also see the fin lose the digit.
- WXYZ-Wing in 376 walkthroughs
- Four cells holding four digits between them, where only one digit is not confined. That digit is removed from cells that see all its copies.
- Cell Forcing Chains in 337 walkthroughs
- Try each candidate of one cell in turn. Every choice leads to the same result, so that result must be true.
- 2-String Kite in 290 walkthroughs
- A row and a column each have a digit in exactly two places, linked through one box. One of the two far ends holds the digit, so a cell seeing both ends cannot.
- W-Wing in 247 walkthroughs
- Two cells with the same pair {X,Y} are joined by a strong link on X. One of them must be Y, so cells seeing both cannot be Y.
The full ladder, from naked singles up: sudoku techniques.
How to get better at hard sudoku
- Keep full pencil marks once the singles dry up. Every advanced pattern is a pattern in the candidates, and you cannot see it without them.
- Look for two-candidate cells first. XY-Wings, XY-Chains and Remote Pairs are all built from them.
- Check each digit on its own for X-Wings, Skyscrapers and 2-String Kites: scan one digit across the grid and look for rows or columns where it has only two places.
- When a chain is long, write it down in the margin. Holding seven links in your head is where most mistakes come from.
The toughest sudoku puzzles in our books
Ranked by level, then by how many times the solve needs an advanced or expert step. Each has a free step-by-step solution: try the puzzle from the book first, then check where you got stuck.
- Extreme Sudoku 1, puzzle 134 Expert 8 · 39 hard steps · key technique ALS-XZ
- Extreme Sudoku 1, puzzle 103 Expert 8 · 20 hard steps · key technique ALS-XZ
- Extreme Sudoku 3, puzzle 141 Expert 7 · 36 hard steps · key technique Nice Loops/AICs
- Extreme Sudoku 3, puzzle 27 Expert 7 · 35 hard steps · key technique ALS-XZ
- Extreme Sudoku 3, puzzle 70 Expert 7 · 31 hard steps · key technique Nice Loops/AICs
- Extreme Sudoku 3, puzzle 149 Expert 7 · 31 hard steps · key technique Nice Loops/AICs
- Extreme Sudoku 1, puzzle 128 Expert 7 · 29 hard steps · key technique Nice Loops/AICs
- Extreme Sudoku 3, puzzle 34 Expert 7 · 29 hard steps · key technique Nice Loops/AICs
- Extreme Sudoku 1, puzzle 133 Expert 7 · 28 hard steps · key technique ALS-XZ
- Extreme Sudoku 2, puzzle 83 Expert 7 · 28 hard steps · key technique Nice Loops/AICs
- Extreme Sudoku 3, puzzle 85 Expert 7 · 28 hard steps · key technique Nice Loops/AICs
- Extreme Sudoku 3, puzzle 71 Expert 7 · 27 hard steps · key technique Nice Loops/AICs
The books
Each book has 150 puzzles, one per page, ordered from the easiest to the hardest, with full solutions at the back and a QR code beside every solution that opens its step-by-step walkthrough. £9.99 / $11.99.
| Book | Expert 8 | Expert 7 | Expert 6 | Expert 5 | Solutions |
|---|---|---|---|---|---|
| Extreme Sudoku 1 | 2 | 26 | 50 | 72 | all 150 |
| Extreme Sudoku 2 | 0 | 50 | 50 | 50 | all 150 |
| Extreme Sudoku 3 | 0 | 50 | 50 | 50 | all 150 |
Those buttons open Extreme Sudoku 1; each book has its own page above.
Try one free
Every book has a free printable sample puzzle with its own step-by-step solution: see free puzzles. Prefer cages? Try hard killer sudoku. Or something with letters: the five-letter wordsearch.