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 RET or C-c C-c with point still in the field, the formula is stored as the formula for the current column, evaluated and the current field replaced with the result. If the field contains only ‘=’, the previously stored formula for this column is used. For each column, Org only remembers the most recently used formula. In the ‘TBLFM’ keyword, column formulas look like ‘$4=$1+$2’. The left-hand side of a column formula can not be the name of column, it must be the numeric column reference or ‘$>’.
Instead of typing an equation into the field, you may also use the following command:
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.