Custom cell format

To define a custom format, follow these steps:

   1.    Enter sample text for the format in a cell, and then select that cell (Excel then displays the sample text in the new format, which helps see the effects of your changes).

   2.    Do one of the following:

• Right-click on the selection and choose Format Cells... in the popup menu:
  
• On the Home tab, in the Number group, click the dialog box launcher:
  
• Press Ctrl+1.

   3.    In the Format Cells dialog box:

• On the Number tab, in the Category box, select the Custom item:
  
• In the Type list box, select the custom format you want to base your new custom format. Excel displays the details for the type in the Type text box.
• If the details for the type extend beyond the Type text box, double-click in the Type text box to select all its contents, issue a Copy command (for example, press Ctrl+C), and then paste the copied text into a text editor, such as Notepad. (For a shorter type, you can work effectively in the Type text box. For a longer type, it's easier to have enough space to see the whole type at once.)
• Enter the details for the four parts of the type, separating the parts from each other with a semicolon:
  • If you write formats with conditions such as [< 1000] and [≥ 1000], the format for the calculated condition is applied,
  • Otherwise:
    • If you write two formats, the first applies to positive and zero values and the second applies to negative values
    • If you write three formats, the first applies to positive values, the second to negative values, and the third to zero
    • If you write four formats, they apply to positive, negative, zero, and text values, respectively:

    <positive>; <negative>; [zero]; [text]

• If you're working in a text editor, copy what you typed and paste it into the Type text box. Check the sample text to make sure it seems to be correct.
• Click the OK button.

Codes for custom number formats

Code	Description
General	Displays the number in general format.
#	Displays only significant digits and does not display insignificant zeros.
0 (zero)	Displays insignificant zeros if a number has fewer digits than the zeros in the format.
?	Displays a space for insignificant zeros on either side of the decimal point so that the decimal points align when formatted in a fixed-width font. It can also be used for fractions with different numbers of digits.
.	Decimal point.
%	Percent symbol.
,	Thousand separator.
E-, E+, e-, e+	Scientific notation.
$, -, +, /, (, ), :, space	Displays this character.
*	Repeats the next character to fill the full width of the column:
_ (underscore)	Leaves a space equal to the width of the next character.
“text”	Displays text enclosed in double quotes.
@	Text placeholder.
[color]	Displays characters in the specified color – black, blue, cyan, green, magenta, red,  white , or  yellow .
[condition value]	Custom criteria for each number format section.

See also this tip in French: Format de cellule personnalisé.