Capacity is the standard way to sell a lunch bag and it is close to useless, because it describes volume and the problem is shape.

Boxes are wider than people think

A standard rectangular meal-prep box has a bigger footprint than most bags allow.

Two centimetres short and it will not lie flat, whatever the litre figure says.

An angled box ruins the bag

Forced in at a slant, it stops the bag closing and tips the contents.

People stop using the bag rather than the box, because the box was theirs first.

So quote the base

Internal base length and width, against a container the buyer names.

It costs nothing to state and it is the number that decides the outcome.

Mixed groups: size to the largest

An average-sized bag fails everybody at the top of the range.

The largest common footprint works for everybody and wastes very little.