Where I live and in other countries, pretty much all food products have a label with calories and nutrients per 100g or 100ml. If you want to e.g. compare the calory density of two different brands of ketchup, you simply look at the calories per 100ml and immediately know which one has more. If a packaging contains e.g. 300g and you know the calories per 100g, it's easy to figure out how many calories you consume by eating all of it or half of it. It's also easy to track your macros. If you eat 160g of pasta and the pasta has 72g of carbs per 100g, then you simply do 72 x 1,6 to know you ate 115,2g of carbs.
How do you do this in America with serving sizes, metric units for nutrients and imperial units to measure weight?