The problem with measuring dimensions is bags are floppy, bulky things that shift shape. They can fold over, squish down, flatten, stand up straight....
It's not best to store feed in the open bag anyway. You should buy an appropriate sized plastic storage container or sealed lid garbage can for your feed. It keeps it fresher and the nutrients from breaking down. If you get large amounts that will last a long time buy 2 containers. A big one for most of it and a small for say 5-7 days worth. That way you aren't letting air in the big container every day or 2 and only opening it about once a week. The nutrient levels in your feed will remain much better. It's even a usda requirement for licensing to have your feed in a sealed container with a label on it.