When using Excel for your daily work, you will often need to insert rows and columns.

Remember, when you insert rows and columns, Excel changes the references so that your formulae do not appear to change materially, however, there are some things to watch out for.

Example with 2 columns, and an addition formula:
=B3+A3

When you insert a column in between A and B, the formula adjusts like so:

Note: nothing is materially affected - you are still adding 10 + 2

Now, suppose unstead that you use the SUM formula:

... When you insert a column, the formula adjusts as you'd expect, but if you look carefully there is a difference. Now, cell B3 is included in the summation:

SO... if you want the addition of new columns to be included in your summation formula, use the SUM function... If you don't, you have to use simple addition.

Practise sheet

.. same works for rows.

Video training on Inserting Rows and Columns: