Filling your carboy (Also the Imp vs US Gallon problem)

So I'm 6 brews in (4 all grain) now and have figured out a bunch of my mistakes, but the US vs UK gallon thing still confuses me no end.
Carboys are 6 US gallon, 5 Imperial, and I know that most recipes list in US gallons, but why is it that a lot of recipes don't use the full volume of the carboy/primary? I thought it was bad to leave a lot of airspace over the beer, or is that a misconception too?

Part of your confusion may be that a US gallon IS an imperial gallon. The rest of the world uses UK or metric measurements.

It is. You want plenty of head space for primary fermentation or you will have a mess on your hands when that krausen shhoots all over your ceiling.
I do my primary in 6-1/2 gallon and 8 gallon buckets.
"IF" you do a secondary, then you want limited head space

menerdari said:

It is. You want plenty of head space for primary fermentation or you will have a mess on your hands when that krausen shhoots all over your ceiling.
I do my primary in 6-1/2 gallon and 8 gallon buckets.
"IF" you do a secondary, then you want limited head space

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This. And when you have active fermentation happening, the headspace will soon fill with co2, pushing the air out.

rhoop said:

Part of your confusion may be that a US gallon IS an imperial gallon. The rest of the world uses UK or metric measurements.

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Sorry, wrong. 1 Imperial gallon = 1.2 US gallons

menerdari said:

Sorry, wrong. 1 Imperial gallon = 1.2 US gallons

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Yeah, I have a 10 imperial gallon keg that holds 12.2 gallons of water, so I can confirm this in practice.

Wow, I've never caught until now that an imperial gallon is not part of the imperial measurement system, but is in fact a metric measurement. So the US uses imperial measurements, but not imperial gallons. Everybody else uses metric measurements, and imperial gallons. Who thought of that.

Didn't mean to thread jack, definitely +1 on the extra head space comments.

There are no gallons in the metric system. Only liters are used to measure liquid. I think the imperial gallon is the way they used to measure liquid, but not since I've been educated. From a 32 year old Canadian.

Overall it's ridiculous and I hate it either which way, but I force myself to just concede and use all US standards, it's just easier. Except Liters. How Liters somehow remains so highly used in a dominantly US forum, when a quart is practically the same thing....but again, it's all nuts.

Would that we could all just have a single way of measuring, but then what would all the calculators do, right?

But the real question is when you go to a brew pub in the US and order a pint do you really get a pint.?

It confuses me how it came to this. Probably some colonial taxation thing...

Imp. gallon = 4 Imp. quart
US gallon = 4 US quart

Imp. quart = 2 Imp pint
US quart = 2 US pint

so far so good...
now it starts to break down.

Imp pint = 20 Imp. oz.
US pint = 16 US oz.

1US oz. = 1.041 Imp. oz.

BTW. US and Imperial measurements are both "metric" (literally measurement) systems. What you mean to say when you say "metric" is "S.I. (System International) metric".