Every Sudoku has exactly one solution, and every one can be solved by logic, with no guessing. These techniques take you from Easy puzzles to Hard ones, in the order you’ll need them.
Scan for easy placements
Pick a number and look across the grid. For each 3×3 box without it, check which rows and columns already contain it; often only one cell in the box is left. Start with numbers that appear most often.
Find cells with only one possibility
For an empty cell, rule out every number already in its row, column and box. If only one is left, that’s the answer. Easy puzzles can be solved with this step alone.
Hidden singles: one place for a number
Sometimes a cell has several possibilities, but a number fits in only one cell of its row, column or box. That number must go there. Normal puzzles need this technique.
Use notes on harder puzzles
Turn on Notes to pencil in the candidates for each cell. When you place a number, it’s removed from the notes in the same row, column and box automatically, so new singles appear as you go.
Pairs narrow things down
If two cells in a row, column or box can only be the same two numbers, those numbers belong in those two cells, so remove them from the other cells there.
If a number in a box can only go in one row (or column), it can’t go anywhere else in that row outside the box.
Work where the grid is fullest
Rows, columns and boxes with few empty cells give the quickest answers, and each answer makes the next easier. The number keys show how many of each number are still to place.
Avoid guessing
A wrong number turns red and counts as a mistake. If you’re stuck, a hint fills in one cell, which is better than a guess that throws the rest of the puzzle off.