Not sure if you really mean brackets or you actually mean parentheses
parentheses ( ) ; brackets [ ] ; braces { }
If you really mean parentheses, here's a simple way. First highlight
the cell(s) you want formatted this way, then:
1.)Do the menu sequence Format>Cells
2.)Choose the Number tab
3.)In the Category field, choose Number
4.)In the Negative Numbers field, pick either of the 2 numbers (black
or red) enclosed in parentheses. (Optionally choosing Decimal Places
and 1000 Separator while you're in that screen.)
5.)Now, any cell containing a negative number within the range you
highlighted will be enclosed in parentheses.
However, if you really do mean brackets and not parentheses, it's just
a little more complicated. Again, highlight the cell(s) you want
formatted this way, then.
1.)Do the menu sequence Format>Cells
2.)Choose the Number tab
3.)In the Category field, choose Custom
4.)In the Type field, enter #,##0;"["#,##0"]" for no decimal places or
#,##0.00;"["#,##0.00"]" for 2 decimal places. (Be careful to include
those quotation marks as Excel doesn't allow entering brackets in a
custom format without them!)
5.)Now, any cell containing a negative number within the range you
highlighted will be enclosed in brackets.