Not quite everyone. I carry a GSI Cascadian plastic cup with the handle cut off. It is sturdy, easy on the lips with hot drinks, non-toxic and it weighs 1.6 oz. It has measurements from 1/2 to 1-1/2 cups cast into the inside of the cup. This is both my hot drink cup and my measuring cup.
Practice. With experience and practice, one can get pretty good at estimating quantities. The Scouts use to teach (do they still?) how to estimate measurements and also taught to use body parts to help, e.g., the width of a hand, the length of a knuckle, etc. Liquid volume measurements might be a little trickier but knowing the capacity of gear you carry or, e.g., how much is in a half-filled baggie, will let you get away from carrying a (single purpose) measuring cup.
That said, I carry a clear, Lexan-like insulated mug that has the volume levels clearly marked. Mine is from Campmor but I've seen the same thing at REI (just different logos). I really like the mug...it's fantastic at keeping things hot for a very long time. The one down side -- it weighs nearly 6oz. (Thermo Measuring Mug #82540-B $4.99)
If you or others choose to go with this mug, here's a tip: The plastic lid from a canister of Progresso bread crumbs fits the mug perfectly...providing a tight, secure seal. It adds a little bit more to the insulation and keeps the creepy crawlers and flyers out of your hot chocolate. The color even matches the markings on the mug.
"...inalienable rights...include the right to a clean and healthful environment..." Montana Constitution
In the 1970's, I carried genuine Sierra Club stainless wire handled cup. I scribed on the inside with a carbide marking tool the levels for 4, 8, 12 and 16 ounces of water. Use a measuring cup and a flat surface to fill the Sierra Cup to the levels and scratch away. I also scratched identifying marks on the bottoms of my Sierra Cups so that when I hosted a small party out backpacking, everyone could know their own cup.
Years later, I also used the plastic cup from a thermos bottle and it came with volume markings on the inside.
I don't know from where my current plastic cup with markings comes from. Makes a great coffee cup as the hot rim doesn't burn the mouth when it is plastic.
Like sarbar, I use the GSI Soloist cup, if I happen to be using that cook kit. If I'm using my Jetboil, it has a similar cup.
If I'm carrying only a pot (typically titanium, or lately the Optimus Terra Solo), it will be a pot that has measuring marks stamped into it - if the amount of water I need is between two marks, I interpolate (well, OK, it's really just a guess, but "interpolate" makes it sound like I actually know what I'm doing. )
In the past, when I used to use Nalgene bottles ("bless me, Platypus users, for I have sinned..."), I used the measuring marks on the side of bottle. Of course, that involved math: "Let's see, I've got 26 ounces of water in here, and I need to put 12 ounces in the pot...hmmm, 26 take away 12...if train A leaves the station..."
I get a kick out of how Sarbar says that - expecting someone to poke fun at her. LOL Now we get to continually remind you of the 5oz of savings that you are missing out on. Yes, nalgenes weigh more, but if you prefer them - then go for it. HYOH... and I mean it.
I always premeasure known liquids at home, then make marks on containers - like other have said. I generally use my SnowPeak mug that has stamped marks in the cup... easy!
Someone please use the term "meniscus" in their response.
I always forget and make it more complicated than it needs to be...it's just walking.
Well, not really, but it was the only way I could think of to work it into the response. I didn't figure anyone would be interested in how you should adjust measurement point because of the surface tension causing the water to have a concave surface.
Loc: S. Chicago, IL
I'm really surprised, I though there would be more ppl that just put some in till it looks right like I do. A little here a little there, dulp to much! But it still seems to come out ok for me that way
Loc: Gateway to Columbia Gorge
Two possibilities: (1) Get a plastic mess kit cup (0.8 oz.); they have measurement marks. (2) Put scratch marks on the inside of your pot at the one cup and two cup levels (put in a cup of water and have the pan on a level surface, make the scratches on two sides). For between one and two cups, you can approximate. Believe it or not, the only time you have to measure ingredients exactly when you're cooking is if you are baking cakes from scratch. Approximate works fine for nearly everything else.
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey
Loc: The Third Maine
As others have posted, a guess is usually good enough for my measuring needs. However, I still have a 30 year old Palco green plastic measuring cup from a mess kit from my earliest backpacking days And I take it with me everywhere. It's got ounce graduations, and it makes a nice extra drinking cup.