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45 Rule

45 RuleSTEP 1/3

Understand the 45 Rule

Before we look at the grid, remember one simple fact: the sum of all digits from 1 to 9 is exactly 45.

Every complete row contains the digits from 1 to 9
The same applies to every column and 3×3 box

That is why the digits in every complete Sudoku unit always add up to 45.

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45 RuleSTEP 2/3

Choose a unit to analyze

Now look at the row, column, or box that the game has just highlighted on the board. Pay attention to the digits that are already filled in and the empty cells that remain.

Check which digits from 1 to 9 are already present in the selected unit
Notice how many cells are still empty

In the next step, we will use the known digits to calculate the value of the missing cells.

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45 RuleSTEP 3/3

Calculate the missing value

Now add together all the digits already visible in the selected unit and subtract their total from 45.

Add all the filled digits, for example: 5 + 3 + 4 + 6 + 7 + 8 + 1 + 2 = 36
Calculate 45 − 36 = 9 and place the remaining digit in the empty cell

If only one empty cell remains in the unit, the result gives you the exact missing digit.

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