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To play Sudoku, fill the empty cells with digits 1–9 while making sure every row, every column, and every 3×3 box contains each digit exactly once.
The easiest way to begin is not to stare at all 81 cells at once. Find an area with very little left to solve, rule out what cannot fit, make a placement only when the grid forces it, and then use that new digit to look for the next move.
For a first puzzle, remember this loop:
look → rule out → place → re-scan.
Before your first move: remember the three rules
Every empty cell belongs to one row, one column, and one 3×3 box.
A digit can be placed only if it does not duplicate the same digit in any of those three units. But a digit being legal does not automatically mean it is proven. If several digits still fit, you need more information before you place one.
For beginners, that single distinction prevents a lot of trial and error: do not ask only “could this digit go here?” Ask “can I prove that this digit has to go here?”
Step 1: Read the Sudoku grid
A classic Sudoku has:
- 9 rows;
- 9 columns;
- 9 boxes, each measuring 3×3 cells;
- 81 cells in total.
Some cells already contain fixed clues. The rest are yours to solve.
Every time you inspect an empty cell, think about its row, column, and box together. Every time you place a digit, those same three units are the first places to inspect again.
- Target cell
- Row
- Column
- Box
Step 2: Find a last missing digit
One of the easiest ways to make your first move is to find a row, column, or box with only one empty cell.
Suppose a row contains:
1 2 3 4 5 6 7 _ 9
The only missing digit is 8, so the empty cell must contain 8.
This pattern is often called a Full House. You may also hear Last Digit used for the same basic idea, especially when discussing the final missing value of a unit or grid.
The name matters less than the reasoning: the unit must contain 1–9 exactly once, eight digits are already present, and only one digit is missing.
Step 3: Scan rows, columns, and boxes
Most moves are not as obvious as an eight-filled row.
A useful next method is to choose one digit and look at where it already appears. Suppose you are searching for 5 inside a particular box. Existing 5s in intersecting rows and columns rule out cells that cannot contain another 5.
If those restrictions leave only one valid position for 5 in the box, you can place it there.
This general process is called scanning. A common beginner form is crosshatching, where row and column restrictions help isolate the only position for one digit inside a box.
You can scan box by box, or follow the same digit across the whole grid. Both views help you stop thinking about all nine digits at once.
Step 4: Eliminate digits that cannot fit
For any unsolved cell, begin with the idea that the digits 1–9 are possible, then eliminate what the grid forbids.
Remove any digit that already appears in:
- the cell's row;
- the cell's column;
- the cell's 3×3 box.
Whatever remains is still a candidate for the cell.
If only one candidate remains, you have found a Naked Single and can place it.
There is another kind of forced move called a Hidden Single. Instead of asking how many candidates one cell has, ask where one digit can go inside a row, column, or box. If that digit has only one possible position in the unit, it belongs there even if the cell itself still shows several candidates.
You do not need to memorize the names before playing. The important idea is that a placement should be forced from the current grid.
Step 5: Place only digits you can justify
A common beginner mistake is to find a digit that is allowed in a cell and assume it must be correct.
Imagine a cell can legally contain either 2 or 7. Neither digit conflicts with the row, column, or box. That means both are still candidates — not that one can be chosen at random.
A safe placement has an explanation such as:
- this is the only digit missing from the row;
- this cell has only one candidate left;
- this is the only position for 6 in the box.
As puzzles become harder, the explanation may involve a larger pattern, but the principle stays the same.
Step 6: Add pencil marks when you need them
On easy puzzles, scanning may reveal enough moves without writing anything inside the empty cells.
When possibilities become harder to remember, use pencil marks or notes to record candidates.
For example:
{2, 7}
inside a cell means that, based on the current grid, you are tracking 2 and 7 as the remaining possibilities for that cell.
Candidates are information, not instructions to guess. Their purpose is to make restrictions visible so that later deductions can remove possibilities or force placements.
You also do not have to fill every candidate immediately. Some players begin with selective notes and move to fuller candidate notation only when the puzzle requires it.
Step 7: Re-scan after every placement
Sudoku is iterative.
A new solved digit can:
- complete a row, column, or box;
- remove a candidate from several peer cells;
- reveal a Naked Single;
- reveal a Hidden Single;
- create a locked candidate, pair, or another pattern elsewhere.
After placing a digit, first inspect the affected row, column, and box. Then return to your broader scan.
This prevents a common inefficiency: finding a useful move and immediately searching for something complicated while an easy consequence has just appeared nearby.
Your first Sudoku solving loop
Use this loop whenever you are unsure what to do next:
- Look for a row, column, or box with one missing digit.
- Scan one digit across the grid.
- Look for cells with only one candidate.
- Look for digits with only one possible position in a unit.
- Update your candidates if you are using notes.
- Re-check the areas affected by the last move.
- Repeat from the top.
As your skill improves, intermediate and advanced techniques extend this loop rather than replacing it.
From your first move to your first few deductions
A beginner solve usually develops as a chain of small consequences rather than one spectacular technique.
You might begin with a last missing digit in a nearly complete row. That new digit removes a candidate from cells in the same column and box. One of those cells may now have only one candidate left, creating a Naked Single. That placement can then restrict a digit to one remaining position in a neighboring box, creating a Hidden Single.
The important pattern is the feedback loop:
forced placement → new restrictions → simpler grid → next forced placement.
When a puzzle stops producing that kind of direct progress, candidates and intermediate techniques give you ways to create new eliminations and restart the same loop.
How do you know when a Sudoku is complete?
A Sudoku is complete when all 81 cells are filled and every row, column, and 3×3 box contains the digits 1–9 exactly once.
In a digital game, the interface may confirm completion automatically. On paper, you can check each unit for duplicates or missing digits.
A grid being full is not enough if an earlier mistake created a duplicate. Correct completion means the entire grid satisfies all three classic constraints.
What should you do if you get stuck?
First, do not assume you need to guess.
Re-check:
- rows, columns, and boxes with many solved cells;
- digits that have very few available positions inside a box;
- candidates that changed after your last placement;
- Naked Singles and Hidden Singles you may have missed;
- the area around your most recent useful deduction.
If no basic move remains, the puzzle may require an intermediate technique such as Locked Candidates or a subset like a Naked Pair. That is a signal to learn the next level of logic, not to abandon the rules.
Common beginner mistakes
Ignoring the box
A digit can be absent from the row and column but still be blocked by the 3×3 box.
Treating a possible digit as a solved digit
If a cell can contain 2 or 7, neither one is proven simply because both are legal.
Forgetting to update notes
Old candidates can make later patterns look valid when they are not. If a placed digit removes a candidate from its peers, your notes need to reflect that change.
Looking at only one part of the grid for too long
A Sudoku is interconnected. If one area stops producing progress, another area may contain an easier move that unlocks it later.
Searching for advanced techniques too early
If an easy deduction is available, use it. Advanced patterns are valuable when the puzzle requires them, not because complicated moves are inherently better.
FAQ
Where should I start a Sudoku?
Start with rows, columns, or boxes that already contain many digits, then scan individual digits for units where only one position remains.
Should I fill in all candidates immediately?
Not necessarily. Many players begin with scanning and add candidates when obvious placements stop appearing. Harder puzzles benefit more from systematic candidate notation.
What is the easiest Sudoku technique?
Finding the last missing digit in a row, column, or box is one of the simplest deductions. Naked Singles and Hidden Singles are the next essential beginner techniques.
What if I make a mistake?
Undo the entry if you can, then identify why it was not logically justified. Checking the failed assumption teaches you more than simply replacing the digit.
Is there a best order for filling a Sudoku?
No fixed cell order works for every puzzle. A useful beginner strategy is to take the simplest forced move available, update the grid, and scan again.
What to learn next
Continue with How to Solve Sudoku for a more systematic process from beginner deductions into harder candidate patterns. Then study Scanning, Sudoku Candidates, Naked Singles, and Hidden Singles as separate concepts.