Sudoku SE Rating: the complete guide
Written by: Ozerlyn Editors · Last updated:
The SE rating (Sudoku Explainer rating) is a number, usually between 1.0 and about 12, that measures how hard a Sudoku is to solve with pure logic. It is set by the single hardest technique the puzzle forces you to use, not by how many clues it has. Ozerlyn's free calculator runs the Sudoku Explainer engine itself, so the number you get is a real SE rating.
Key facts
- SE stands for Sudoku Explainer, a free Java solver written by Nicolas Juillerat.
- The rating equals the hardest step needed to finish the puzzle; easy steps elsewhere do not lower it.
- Ratings are given to one decimal place: 1.2 is a hidden single in a box, 3.2 an X-Wing, 4.2 an XY-Wing.
- A puzzle that can be solved with singles alone rates 2.3 or lower; anything above about 8.8 needs dynamic forcing chains.
- The serate batch tool reports three numbers, ER/EP/ED; when people say "the SE rating" they mean ER.
- Ozerlyn's calculator runs the Sudoku Explainer engine on our server, so you do not need to install Java.
What the SE rating measures
Sudoku Explainer solves a puzzle the way a careful human would. At every step it looks for the easiest technique that makes progress, applies it, and repeats. Each technique has a fixed difficulty value, and the puzzle's rating is the highest value used on the whole solving path. If a grid can be finished with singles except for one X-Wing, it is rated 3.2, because that one X-Wing is the step that stops a beginner.
This "bottleneck" approach is why the SE rating is more honest than counting clues. A 17-clue puzzle (17 is the proven minimum for a unique solution) can be easy, while a puzzle with 30 clues can require chains. Clue count tells you how much is filled in; the SE rating tells you what you need to know.
ER, EP and ED: the three serate numbers
The batch rater "serate", added to Sudoku Explainer by the programmer known as gsf, prints three ratings. ER (explainer rating) is the hardest step needed to solve the whole puzzle. EP (pearl rating) is the hardest step needed before the first digit can be placed. ED (diamond rating) is the hardest step needed for the first candidate elimination, so ED ≤ EP ≤ ER.
Puzzle collectors call a puzzle a "diamond" when all three numbers are equal, and a "pearl" when ER equals EP but ED is lower. For everyday use only ER matters, and it is the number our calculator shows.
How to rate a puzzle on Ozerlyn
Type the givens into the grid above (or paste a puzzle with Import), then press "Calculate SE Rating". The puzzle must be a standard 9×9 Sudoku with exactly one solution; grids with duplicate digits, fewer than 17 clues or several solutions are rejected with an explanation. The result shows the SE rating, a difficulty label and every technique Sudoku Explainer needed, with how often each was used.
Calculating a new puzzle can take a few seconds, because the engine really solves it step by step. Puzzles that someone has already rated come back instantly. To keep the service fast for everyone, new calculations may ask you to sign in with a free account and are limited to a few per minute.
SE rating of each technique (Sudoku Explainer 1.2.1)
These are the values Sudoku Explainer assigns. Chains and loops get a range because longer chains score a little higher.
| SE rating | Technique |
|---|---|
| 1.0 | Last value in a block, row or column |
| 1.2 | Hidden single in a block |
| 1.5 | Hidden single in a row or column |
| 1.7 | Direct pointing |
| 1.9 | Direct claiming |
| 2.0 | Direct hidden pair |
| 2.3 | Naked single |
| 2.5 | Direct hidden triplet |
| 2.6 | Pointing (locked candidates) |
| 2.8 | Claiming (locked candidates) |
| 3.0 / 3.2 / 3.4 | Naked pair / X-Wing / hidden pair |
| 3.6 / 3.8 / 4.0 | Naked triplet / Swordfish / hidden triplet |
| 4.2 / 4.4 | XY-Wing / XYZ-Wing |
| 4.5 – 5.0 | Unique rectangles and unique loops |
| 5.0 / 5.2 / 5.4 | Naked quad / Jellyfish / hidden quad |
| 5.6 – 6.0 | Bivalue universal grave (BUG) |
| 6.2 | Aligned pair exclusion |
| 6.5 – 7.5 | X-cycles, Y-cycles, bidirectional cycles and forcing chains |
| 7.5 | Aligned triplet exclusion |
| 7.6 – 8.1 | Nishio forcing chains |
| 8.2 – 8.7 | Cell and region forcing chains |
| 8.8 – 9.6 | Dynamic forcing chains |
| 9.1 – 11.9 | Dynamic forcing chains with nested chains |
What the difficulty labels mean
Sites draw the lines between levels differently. These are the labels our calculator shows next to the rating.
| SE rating | Label | What you will need |
|---|---|---|
| below 1.5 | Beginner | Hidden singles only; no notes needed. |
| 1.5 – 2.9 | Easy | Singles and locked candidates (pointing, claiming). |
| 3.0 – 4.9 | Medium | Pairs, triples, X-Wing, Swordfish, XY-Wing and XYZ-Wing; full candidate notes. |
| 5.0 – 6.9 | Hard | Quads, Jellyfish, unique rectangles, BUG and short chains. |
| 7.0 – 8.9 | Very hard | Forcing chains, Nishio, cell and region forcing chains. |
| 9.0 and above | Expert | Dynamic and nested forcing chains; rare even among hard puzzles. |
SE rating: frequently asked questions
What is a good SE rating for a beginner?
Start with puzzles rated below 2.0, which need only hidden singles and simple direct eliminations. Once those feel easy, move to 2.3–2.8 (naked singles and locked candidates), then to 3.0–4.4, where pairs, X-Wing and XY-Wing appear.
What is the highest possible SE rating?
In Sudoku Explainer 1.2.1, ratings for normal Sudoku go up to about 11.9 (dynamic forcing chains with nested dynamic chains). Puzzles rated above 10 are very rare and are mostly found in collections of the hardest known Sudokus.
Does the number of clues decide the SE rating?
No. The SE rating depends only on the hardest logical step needed. A puzzle with the minimum of 17 clues can be easy, and a puzzle with many clues can still need advanced techniques.
Why can two puzzles with the same SE rating feel different?
The rating records only the hardest step. One 3.2 puzzle may need a single X-Wing, another may need several, and long stretches of singles also take time. The SE rating measures how much you need to know, not how long it takes.
What do ER, EP and ED mean?
They are the three numbers from the serate tool: ER is the hardest step to solve the whole puzzle, EP the hardest step before the first digit is placed, and ED the hardest step before the first candidate is removed. The usual "SE rating" is ER.
Is Ozerlyn's SE rating the same as Sudoku Explainer's?
Yes. Our calculator sends your grid to a server that runs the Sudoku Explainer Java engine, so you get the same rating you would get by running the program on your own computer, without installing anything.
Why was my puzzle rejected?
The calculator needs a valid 9×9 Sudoku with exactly one solution. It rejects grids with a repeated digit in a row, column or box, grids with fewer than 17 clues, and grids that have no solution or more than one.
Are Ozerlyn's own puzzles rated with SE?
Yes. Every puzzle in our Sudoku levels is rated with Sudoku Explainer before it is published, so a level's puzzles stay within a consistent SE range.