Honestly, it's not that ridiculous because everybody has a different idea of portions. 100g is a nice round number to calculate carb content with (a opposed to say 76 or 43 or something) and quite a few foods like rice the recommended single portion is 100g per person anyway.
When they say per portion on the cereal box for instance it is 100% WAY off what you'll pour into your bowl anyway. The only thing that seems reliable per serving is the 250ml average glass of soft drink unless a particular food is individually packaged.
Sorry it's frustrating you to have to work all this stuff out xxx
Agree with you that 100g is easier to work out. If they made it random numbers I don't think my brain could cope.
I thought the ridiculousness referred to things that come in a portion serving eg a pre packaged sandwich, and then giving the info per 100g. It's rather unlikely that a sandwich will be 100g or that someone would only eat an exact weight of something like that. You tend to eat either a half or the whole sandwich so giving info for 100g rather than the 'portion' doesn't seem to make sense in that situation.
For everything else especially when I have my scales I don't mind a bit of maths
I'm afraid I use mental arithmetic! When Iwas 10 and 11 my class teacher used to say that a monkey could calculate if it had a pencil and made us use only our brains. In fac he called written sums "Monkey maths". Idon't know what he would have made of calculators. He didn't live to see them. Remember there's no need to calculate to decimals, because the carb count in the food, on the panel isn't that accurate.
Hana
I agree with the OP. Okay we can just look on the back and work it out ourselves which is what I've grown accustomed to doing but how wonderful it would be to just look on the front and see carbs (and protein while we're at it ) on the front along with the other numbers. Some items have 'per portion' on the back next to the 'per 100g' which is great.
I've noticed over that last year that carb info in supermarket food is disappearing in favour of calories (which is, of course, next to useless for type-1s).