Yeah is fl.oz on consumer containers _really_ seen as a volume measurement, i.e. thought of in terms of conversion "How many of these to a gallon", "what does a 6-pack weigh" and so on?

Or are they more like t-shirt sizes "I know I want a 16oz can to drink and I know how big they are"?

When I go to buy liquor it seems to already be mostly in metric, sizes are 500ml, 750ml, and liter.

If I understand correctly, the FPLA ( https://en.wikipedia.org/wiki/Fair_Packaging_and_Labeling_Ac... ) requires grocery items to be labeled in both American units and metric units. Metric-only is illegal in America but legal and common in Canada. I have shopped in American and Canadian supermarkets and speak from personal observations.

It seems the exception is that wine imported into the US can keep metric-only labeling, so I have indeed seen bottles labeled only "750 mL", sold in the US.

Mexico too. Anyone immigrated from basically anywhere is more used to buying in metric. And some products have already made the conversion, e.g. soda bottles.

An interesting case on grocery items is that solid things that you pour (breakfast cereal, flour, sugar, salt, for example) the nutrition label gives the serving size in a US customary volume measure and in a metric mass measure.

For example my breakfast cereal label says a serving is 1 cup or 45g.

Liquids seem to give a US customary volume and a metric volume. For example the milk I put on that cereal says a serving is 1 cup or 240mL.

Checking some sauces I have on hand thick liquids (ketchup and sriracha) are US volume and metric mass like pourable solids and thin liquids (like soy sauce) are both US and metric volume like milk.

Butter is also US volume and metric mass.

A little bit of research says that this is actually regulatory. For things where how much you can fit in a given volume varies quite a bit depending on how you pack it the FDA requires that the metric units on the label be mass units, and that nutritional information is for that amount of mass of the thing.

The actual amount of flour in a cup of flour for example can vary quite a bit depending on how much air got in when you poured it. If the label says it is 110 calories for 1/4 cup (30g) and you want to actually use 440 calories of flour in something you should measure out 120g rather than 1 cup.

For things like milk 1 cup is going to have the same amount of milk no matter how you pour it, so they use volume measurement for both customary and metric.

Force and mass is what always drove me crazy in engineering school. I assume that courses in the US have largely given up trying to pay some lip service to common older units but sorting out pounds-mass, pounds-force, and slugs? Pretty sure I couldn't do it today.

The rocket company I worked at designed their orbital rocket in inches and lbm. Engine flow rates in lbm/s, temperatures in deg Rankine, thrust in lbf. Btu/hour/inch^2/degR heat transfer coefficients.