sudokUI › Sudoku difficulty rating

Sudoku difficulty rating

sudokUI rates a puzzle by solving it the way a person would. At every step it takes the easiest technique that makes progress and adds that technique’s score. The total is the rating.

Rate your own sudoku

The puzzle opens in sudokUI with its rating and band in the top bar. Nothing is uploaded: the rating is computed on your own device. No string at hand? Open sudokUI, choose New, then Custom, and type the puzzle onto the board. See also how this compares with HoDoKu.

Easiest technique first

Techniques are tried in a fixed order, from singles up to forcing nets, and the first one that works is applied. The rating therefore measures a path built from the easiest steps available, not the cleverest one.

Scores compatible with HoDoKu

Scores and search order are the defaults of HoDoKu, the reference sudoku analyser, so ratings can be compared with HoDoKu’s. A Naked Single costs 4, an X-Wing 140, a Forcing Net 700. Techniques HoDoKu does not know are scored next to their closest relatives.

Eight difficulty bands

The band is the higher of two readings: the band of the total score, and the band of the hardest technique needed. A puzzle needing a single fish, wing or kite is at least Tricky, and one needing several is at least Hard.

Rate any puzzle

Import an 81-character puzzle or type one in by hand. sudokUI first verifies that it has exactly one solution, then rates it before you play.

The eight difficulty bands

How fast is fast?

Rough benchmarks from the typical times online solvers report, spread the way solve times spread: most near the middle, a long tail behind. The world class mark is set by championship solvers. Writing your own marks takes longer than automatic candidates, and paper longer still, so each way of solving has its own table. A puzzle at the top of its band takes longer than one at the bottom.

Slow is where the slowest fifth begins. Typical is the median solver. Fast is faster than four solvers in five, Expert faster than 99 in 100, World class faster than 999 in 1,000. The app says where each of your solves lands.

Score of every technique

The cost added to a puzzle's rating each time the solver needs the technique, and how many of the puzzles sudokUI generates need it at least once (measured on 5,533,732 puzzles).