Permanently summing a column in an Org-mode table

Assign a field-name using the ^ mark:

|---+---|
|   | 1 |
|   | 2 |
|   | 3 |
|---+---|
|   | 6 |
| ^ | x |
|---+---|
#+TBLFM: $x=vsum(@1..@-1)

See The Org Manual, Section-3.5.9 Advanced Features.

The last row of a table is @> For example to get the sum for the third column in the last line, you can use the formula

@>$3=vsum(@2..@-1)

Maybe you have to adapt the @2, if you don't have a header line...

Related