Doodle CubSudoku has one rule: fill the 9x9 grid so that every row, every column and every 3x3 box contains the digits 1 to 9, each exactly once. Some digits are given at the start. You use them to work out the rest by logic; a well-made puzzle never needs a guess and has exactly one solution. Here is how to start, step by step.
The grid
- 81 cells in 9 rows and 9 columns.
- Nine 3x3 boxes, outlined with thicker lines.
- Givens: the digits already printed. You can't change them.
No maths is involved. The digits could just as well be letters or colours; you only need to keep track of which ones are used.
Finding your first digit
Pick a digit that appears often among the givens, say 5. Look at one box that doesn't have a 5 yet. Check the rows and columns that run through that box: every row or column that already contains a 5 rules out its cells in that box. If only one cell is left, the 5 goes there. This is called a hidden single, and it solves most of an easy puzzle.
Repeat for every digit, then for every box. Each digit you place gives you new information for the next.
The other basic move
Look at a single empty cell. Check its row, column and box. If eight different digits already appear among them, the cell must be the ninth. This is a naked single. It works best in crowded parts of the grid.
Pencil marks
When singles run out, write the possible digits for each cell in small numbers. These pencil marks make patterns visible. Keep them up to date: when you place a digit, remove it from the marks in its row, column and box.
In Doodle Sudoku you can write digits by hand: write big for an answer and small for a pencil mark. You can also type them, or hold Shift for pencil marks.
A first walkthrough
- Scan digit by digit. Start with the digit that appears most often. Check every box for hidden singles.
- Scan the busiest rows and columns for naked singles.
- Go round again. Each new digit can unlock others.
- Fill in pencil marks when you stop finding singles.
- Look for pairs: two cells in a unit that share the same two candidates. Those digits can be removed from the rest of the unit.
- Check your work. If a row ends up with two of the same digit, something went wrong a few steps back.
Difficulty levels
Puzzles get harder not because they have fewer givens, but because they need more advanced techniques. Doodle Sudoku grades puzzles by the hardest technique a human solver needs:
| Tier | Techniques needed |
|---|---|
| Warm-up | Singles only |
| Solid | Pairs |
| Sharp | Pointing pairs, box-line reduction |
| Expert | X-wings, triples |
| Fiend | XY-wings, simple colouring, swordfish |
A new Daily puzzle arrives each day: easy on Monday and hardest on Saturday.
How long does a puzzle take?
An easy puzzle takes most beginners 10 to 20 minutes and drops to under five with practice. Hard puzzles can take an hour the first time. Doodle Sudoku keeps your best time and your average of the last ten solves for each tier, so you can watch yourself improve. If you like an extra challenge, Strict mode checks every digit as you enter it: three mistakes end the puzzle, but a Strict solve scores 25% more.
Beginner mistakes
- Guessing. It feels faster, but a wrong guess can survive for many moves before it breaks.
- Forgetting to update pencil marks. Stale marks hide the next move.
- Only scanning rows. Boxes hide the most hidden singles.
Stuck on a harder puzzle? Our guide to sudoku tips when stuck covers seven techniques in the order to try them.