Sudoku Strategies: The 4 Techniques That Solve Most Grids

We tested 4 Sudoku techniques on 4,000 puzzles

We ran the puzzle generator from our Sudoku game 1,000 times for each level and gave every grid to a simple solver that only knows the techniques in this guide. It never guesses. The table shows how often each set of techniques finished the whole grid.

Two things stand out. First, an Easy grid starts with about 11 cells you can fill right away, while an Expert grid has less than one, so on harder levels you have to create your first openings with notes. Second, the hidden single is the big jump: on Hard it lifts the solve rate from 19% to 79%.

Naked single: the trick from the video

A naked single is a cell that can only hold one digit, because the other eight already appear in its row, column or box. The video shows a clean example in the center cell of the grid:

1. Row: 2, 6 and 9 are already placed.
2. Column: 3, 7 and 8 are placed.
3. Box: 1 and 4 are placed.
4. What is left: 2, 6, 9, 3, 7, 8, 1 and 4 are used. Only 5 remains, so the cell is a 5.

The video grid was set up so that this cell is easy to see. In a real game, look for cells in crowded rows and boxes first. On NitroPlayer, selecting a cell lights up its row, column and box, which makes this count quick.

Hidden single: one place left for a digit

A hidden single works the other way around. Instead of asking "which digit fits this cell?", you ask "where can this digit go in this box?".

Pick a digit, say 7, and a box that does not have a 7 yet. Trace every 7 already on the board through its row and column. Each of those lines rules out cells in your box. If only one empty cell survives, the 7 goes there, even if that cell could hold other digits too. This scan is often called cross-hatching.

Do the same for rows and columns: if a row has only one cell where a 4 can go, place it. In our test this one technique is what turns most Medium and Hard grids from stuck to solved.

Naked pairs: two cells, two digits

When singles run out, turn on notes (the N key or the Notes button) and write the candidates in each empty cell. Then look for two cells in the same row, column or box whose notes are exactly the same two digits, for example 3 and 7.

Those two cells must hold the 3 and the 7 between them, in some order. So no other cell in that unit can be a 3 or a 7. Erase both digits from every other note in the unit. That often leaves a cell with a single candidate, which is a new naked single.

The same logic works with three cells that share three digits (a naked triple), but pairs are far more common.

Pointing: when a box points along a line

Look inside one box at a single digit's candidates. If every possible spot for that digit in the box lies on the same row (or the same column), the digit must end up on that line inside this box. So it cannot appear on that line anywhere outside the box. Remove it from those cells.

The mirror version is called box line reduction: if a row's only spots for a digit are all inside one box, remove that digit from the other cells of that box. Both moves only remove notes, but they regularly unlock a hidden single a few cells away.

A solving routine that works on every level

1. Scan for naked singles in the most crowded rows, columns and boxes.
2. Cross-hatch each digit from 1 to 9 through every box to find hidden singles.
3. Repeat steps 1 and 2 after every placement. Each new digit removes options from three units at once.
4. Write notes only when singles dry up. NitroPlayer removes a placed digit from the notes in its row, column and box automatically, so your candidates stay clean.
5. Look for pairs and pointing in the notes, then go back to step 1.

A hint fills one correct cell but costs 500 points, the same as 50 seconds on the clock, so one more full scan is usually worth it. Hints are off in duels, and a wrong digit there locks your board for 10 seconds.

When the four techniques are not enough

In our test, 41% of Expert grids still had empty cells after singles, pairs and pointing. Those grids need at least one more advanced pattern, such as an X-Wing (a digit that appears exactly twice in each of two rows, in the same two columns) or hidden pairs.

You never need to guess. Every NitroPlayer grid is checked to have exactly one solution, so there is always a logical next step. If you are stuck, recheck your notes first: a single missing candidate is the most common reason a grid seems impossible.