If you have spent time on Sudoku forums or browsed collections of hard puzzles, you have probably seen grids labelled “SE 2.3” or “SE 7.1”. That number is the Sudoku Explainer rating, and it is the closest thing the Sudoku community has to a shared difficulty scale. Words like “easy”, “hard” or “fiendish” mean something different at every newspaper and app; an SE rating means the same thing everywhere. This guide explains where the rating comes from, how it is calculated, what the numbers really tell you, and where they fall short.
Where the SE rating comes from
SE stands for Sudoku Explainer, a free, open-source Java program written by Nicolas Juillerat in the mid-2000s. Its original goal was educational: you give it a puzzle, and it explains step by step how a human could solve it with pure logic instead of guessing. To decide which explanation to show first, the program needs to know which techniques are easier than others, so every technique received a numeric difficulty value. The rating of a whole puzzle grew naturally out of that system.
Because the program was free and its scale was consistent, enthusiasts adopted it as a common reference. Today hard-puzzle collections, forum challenges and many puzzle generators quote SE values. Several modified versions of the program exist and they can disagree slightly on the very hardest puzzles, but for the puzzles most people actually solve, the numbers are remarkably stable.
How the rating is calculated
The core idea fits in one sentence: a puzzle is as hard as the hardest step you are forced to take. Sudoku Explainer simulates a careful solver who always reaches for the simplest tool that still works:
- Look at the current grid and find every technique that can make progress, either by placing a digit or by removing a candidate.
- Choose the technique with the lowest difficulty value and apply it.
- Repeat until the grid is complete.
- The puzzle’s SE rating is the highest value among all the steps that were used.
Picture a puzzle that falls almost entirely to hidden singles, except for one moment where nothing simple is available and an X-Wing is the only way forward. After that, it collapses into singles again. The puzzle is rated 3.2, the value of the X-Wing, even though nearly all of the work was easy. The rating describes the highest wall you must climb, not the length of the road.
One detail surprises many players: a naked single (2.3) is rated higher than a hidden single in a box (1.2). In Sudoku Explainer’s model, a hidden single only requires scanning one digit across a box, while a naked single requires ruling out eight digits for one cell, which is harder to see without pencil marks.
Common technique ratings
The table below lists the values used by the original Sudoku Explainer for the most common techniques. Advanced techniques, chains in particular, are shown as ranges because their rating grows with the length and complexity of the chain, and different program versions treat them slightly differently.
| Technique | Approx. SE rating |
|---|---|
| Full House (last empty cell in a row, column or box) | 1.0 |
| Hidden Single inside a box | 1.2 |
| Hidden Single in a row or column | 1.5 |
| Naked Single | 2.3 |
| Locked candidates: Pointing / Claiming | 2.6 / 2.8 |
| Naked Pair | 3.0 |
| X-Wing | 3.2 |
| Hidden Pair | 3.4 |
| Naked Triple | 3.6 |
| Swordfish | 3.8 |
| Hidden Triple | 4.0 |
| XY-Wing | 4.2 |
| XYZ-Wing | 4.4 |
| Uniqueness techniques (Unique Rectangle, Unique Loop) | 4.5 – 5.0 |
| Naked Quad / Jellyfish / Hidden Quad | 5.0 / 5.2 / 5.4 |
| BUG (Bivalue Universal Grave) | ≈ 5.6+ |
| Aligned Pair Exclusion | 6.2 |
| Chains and cycles (X-Cycles, Y-Cycles, forcing chains) | ≈ 6.5 – 7.5 |
| Nishio, multiple and dynamic forcing chains | ≈ 7.5 – 9.0 |
| Nested / highly dynamic forcing chains | 9.0+ |
What the numbers mean in practice
- 1.0 – 1.5: Singles only. These are the gentle puzzles found in beginner books; you can solve them without pencil marks.
- 1.5 – 2.5: Still singles, but you need naked singles and row/column scanning. Comfortable for regular players.
- 2.6 – 4.4: Locked candidates, pairs, triples, X-Wing, Swordfish and the basic wings. This is where pencil marks become essential and where most “hard” newspaper puzzles live.
- 4.5 – 6.2: Uniqueness arguments, quads, Jellyfish, BUG and similar patterns. Expert territory.
- 6.5 – 9.0: Chains and forcing chains. Solving these by hand requires patience and a solid grasp of chain logic.
- 9.0 and above: Puzzles that need nested, highly dynamic chains. Enthusiasts often call them “diabolical”. Very few people solve them by hand; the hardest known puzzles sit around 11 to 12.
Limitations you should know about
SE is a useful scale, but it is not a perfect measure of how a puzzle feels. Keep these points in mind:
- It measures the peak, not the effort. A puzzle that needs one X-Wing and a puzzle that needs five X-Wings both rate 3.2. The second one is clearly more work.
- It ignores how hard a pattern is to spot. An X-Wing hidden among dozens of candidates can take far longer to find than one sitting in an almost empty grid, yet both get the same number.
- Humans do not follow the program’s order. You might use a different technique than the one Sudoku Explainer chose, or overlook a simpler move and solve the step a harder way. Your personal experience of the puzzle may differ from its rating in either direction.
- High ratings are less precise. In the chain range, the score depends on implementation details, so treat a 7.3 versus a 7.6 as “similar” rather than as a meaningful gap.
- The puzzle must be valid. Ratings only make sense for a puzzle with exactly one solution. A single mistyped digit can make a puzzle unsolvable or ambiguous.
This is why two puzzles with the same SE rating can feel very different: the rating tells you the toughest technique required, not how many times you need it or how well it is hidden.
How to use the Ozerlyn SE Rating calculator
You can check any puzzle with our free Sudoku SE Rating calculator. It takes less than a minute:
- Open the SE Rating page.
- Enter the puzzle’s givens into the grid, or import the puzzle as a line of 81 digits. Leave empty cells blank or write them as 0.
- If you just want to see how it works, load the built-in example first.
- Start the calculation. You will get the puzzle’s SE rating together with the solving techniques used along the way.
Very hard puzzles, especially those in the chain range and above, require a lot of computation and may hit the calculator’s time limit. If that happens, it is a strong hint that you are holding an extreme puzzle. Also double-check the givens: a typo is the most common reason a rating looks wrong.
Using SE ratings to become a better solver
The real value of the rating is that it tells you what to learn next. If you can comfortably solve puzzles around 2.3 but stall at 3.0, the technique list points you straight to naked pairs and X-Wings. A good habit is to pick puzzles slightly above your comfort zone, solve them, and then run them through the calculator to see which technique unlocked the hardest step. If you had to guess at some point, the list shows you the logical move you missed.
Ready to practise? Play a fresh grid in our Sudoku game at the difficulty that suits you, try today’s Daily Sudoku, and then paste any puzzle that felt tricky into the SE Rating calculator to find out exactly how hard it really was.