A flour sack or flour bag is a bag or sack for flour. Large bulk bags as well as smaller consumer sizes are available.
A wide variety of wheat flours are available. Flour can also be made from other grains, roots, nuts, etc. Packaging engineers and food scientists need to understand the properties of the particular flour, intended handling and logistics systems, and desired shelf life.  Package forms and materials can be matched to these needs.
Flour is often shipped from the miller to bakeries, institutions and other bulk uses. Sizes range from 10 kg to 100 kg. One traditional construction was cheap cotton bags. These printed cotton bags were sometimes viewed as Collectables; other times the flour sack fabric was repurposed into a variety of household items.
Current practices are to use multi-wall paper sacks. Some include a layer of plastic film for barrier properties and insect control.
Woven polypropylene bags are also used for high strength. One variety, Purdue Improved Crop Storage bags, also includes inner plastic bags.
Consumer packages are often bags or sacks constructed of paper. Plastic films are also used, sometimes with reclosable features. Stand-up pouches of flour have recently been introduced. 
Insects can be a probem, When available. a suitable insecticide can be used; Care must be used to ensure product safety. Hermetic plastic bags also help.
When insect infestation is noted, one method of stopping further growth is to freeze the sacks of flour for several days.