Scanning the grid efficiently
On an easy or medium grid, scanning often carries you from start to finish. It still pays to do it methodically.
1. Scan by bands
A band is three boxes in a line: rows 1 to 3, 4 to 6 or 7 to 9, and likewise for columns. Pick a digit and look at one band. If it already appears in two of the three boxes, it occupies two of the band's three rows: in the third box it can only sit on the remaining row. The columns often narrow that row down to a single cell.
Then move on to the next band, and to the vertical bands. A full pass over the grid for one digit takes a few seconds.
2. Start with frequent digits
The keypad shows how many copies of each digit are left to place. Digits with only two or three left are the most constrained, so start with them. A check mark means a digit is finished and you can forget about it.
3. Lock a digit to chain placements
A long press on a keypad digit (or the L key) locks it: every copy is highlighted and scanning becomes visual. Each empty cell you tap receives that digit, so you can place several in a row without going back to the keypad. The lock lifts once all nine copies are placed.
Notes, at the right time
4. Note one group, not the whole grid
When scanning stops placing anything, pick an almost full row, column or box and note the candidates of its empty cells. The game refuses an impossible note and flashes the cells that block it, which is a free sanity check.
To note a digit in several cells at once, drag your finger across the empty cells (or Shift + drag on a computer), then tap the digit. It is only noted where the rules allow it.
5. Look for pairs
Two cells in the same group that share only the same two candidates, say 3 and 8, form a pair: those two digits are reserved for them. Strike them from the other cells of the group. This often reveals a hidden single elsewhere. This technique and more advanced ones are covered in our guide to sudoku solving techniques.
6. Let the game clean up your notes
When you place a correct digit, it disappears from the notes in its row, column and box. After each placement, check the neighbouring cells: a cell down to a single candidate is a digit to place right away.
Protecting your score
7. Weigh mistakes, jokers and time
Every digit is checked instantly. A wrong digit turns red and locks the grid until you remove it with the undo button (Annuler); it still counts as a mistake. On a 9 × 9 grid, a mistake costs about 10 points.
The Joker costs about 60 points. It reveals the digit of the selected cell if it is empty, so select the cell that is blocking you before using it. Revealing the same cell twice is only charged once.
Time matters too. The reference is 5 minutes on easy, 8 on medium and 12 on hard. Finishing twice as fast earns up to a 20% bonus; past the reference the penalty grows slowly and is capped at 25%. Staying stuck for many minutes can therefore cost more than a joker.
To review the rules and a sample box, read how to play Sudoko. For a quicker grid, try Sudolito, SIWA's 6 × 6 sudoku.