When you assign a formula to a simple column reference like ‘`$3=`’, the
same formula is used in all fields of that column, with the following
very convenient exceptions: (i) If the table contains horizontal
separator hlines with rows above and below, everything before the
first such hline is considered part of the table *header* and is not
modified by column formulas. Therefore a header is mandatory when you
use column formulas and want to add hlines to group rows, like for
example to separate a total row at the bottom from the summand rows
above. (ii) Fields that already get a value from a field/range
formula are left alone by column formulas. These conditions make
column formulas very easy to use.

To assign a formula to a column, type it directly into any field in
the column, preceded by an equal sign, like ‘`=$1+$2`’. When you press
` TAB` or

Instead of typing an equation into the field, you may also use the following command:

`C-c =`(`org-table-eval-formula`

) ¶-
Install a new formula for the current column and replace current field with the result of the formula. The command prompts for a formula, with default taken from the ‘

`TBLFM`’ keyword, applies it to the current field and stores it. With a numeric prefix argument, e.g.,`C-5 C-c =`, the command applies it to that many consecutive fields in the current column.