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Type or paste your sudoku, and this solver works through it one step at a time. Each step names the technique it used, shows the cells involved and says what changed, whether a digit was placed or candidates were removed. Ask for one hint when you are stuck, step through the whole solve, or let it run to the end.
The first time you press Hint, Next step or Solve all, the solver checks the givens. It stops and tells you if two equal digits share a row, column or box, naming both cells, if the puzzle has no solution, or if it has more than one. It solves only a puzzle with exactly one solution. With two or more, the grid has no single answer for a step to lead to.
Before its first step, the solver fills every empty cell with its candidates, the digits it can still take, unless you start from your own pencil marks. That fill is the first entry on the timeline.
If you already have candidates marked on paper, tick Start from my own pencil marks before you begin. After the check, add your marks cell by cell, then choose a step, and the solver works from your marks instead of filling in its own. Cells you leave empty get every digit they can still take, and a mark that clashes with a digit already placed in the same row, column or box is dropped.
If a cell's marks leave out the digit the solution needs there, the solver will not start. It names the cell and the digit, for example "This note removes the solution's 6 at R1C1", and offers to put the missing digits back or to use its own candidates instead.
Each step is titled with the technique's name, the same name the techniques guide uses, and links to the lesson that teaches it once that lesson is written. Every step goes on a timeline. Click any earlier step and the board goes back to how it looked at that moment, candidates included. The left and right arrow keys move between steps.
For most techniques, the How tab walks through a step in short phases, such as Phase 2 / 4, with the cells highlighted on the board. The up and down arrow keys move between phases. The Why tab explains the logic and ends with What changed. When the timeline holds more than one family, chips let you hide a family from it. The Summary view counts the techniques used, the values placed and the candidates eliminated, and Export copies the puzzle and the numbered list of steps to your clipboard as plain text.
Before each step the solver tries its techniques in a fixed order that starts with the simplest, and takes the first one that works. These are the families, in the order the Solve up to menu lists them:
A hidden single, for example, is a digit with only one possible cell left in its row, column or box. A pointing pair is a digit whose only two cells in a box share a row or column. One of them must hold the digit, so any other cell in that line, which sees both, loses it.
We ran the solver on 41 medium puzzles the site served. It finished all 41 with its techniques alone, in 58 to 68 steps after filling in the candidates. Singles did most of the work, and in 28 of the puzzles they were all it needed. Pointing pairs came up in 11 puzzles, claiming groups in 4, naked pairs in 3 and a W-wing in 2. Fish, chains and ALS moves never appeared, so the later chapters of the techniques guide are for harder grids than these.
The solver knows the techniques listed above and no others. On a few famously hard puzzles, such as Easter Monster, none of them applies even at the first step. When that happens, the solver says so: this puzzle needs a technique the solver does not have yet. It does not place a digit without telling you. It offers Solve all instead, which fills the remaining cells from the solution and says that those cells have no technique behind them.
Each technique linked above has its own lesson with worked boards, and the sudoku techniques guide puts them all in solving order. When a step confuses you, open its lesson, then come back and step through it again.