# How can 128 ounces = 133.5 ounces? **URL:** \ **Category:** Factual Questions\ **Created:** [February 1, 2016, 7:53pm UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876 "2016-02-01T19:53:46Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [February 1, 2016, 11:03pm UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/21 "2016-02-01T23:03:47Z")
Simple way of thinking about this. Stop talking about water. You are comparing volume and mass. Water has nothing to do with this. You cannot compare volume and mass by themselves - you need a reference density. It is a historical curiosity that the systems used the density of water. Convenient, but arbitrary. As noted, when something changed - like the manner in which length was standardised for one system, the relationship was broken. Even then, the manner in which the mass measure was created required an exact temperature, and won’t work otherwise. So, the US defined the volume - the fluid ounce - defined with dimensions in inches. They then worked out a mass from this. They go on to create a system with reference masses, crafted ever so carefully in metal. These are the actual definitions of mass measure. Then someone comes along and creates a more carefully definition of the inch, which is accepted world-wide, and - oops - the fluid once is ever so sightly different in size. So, what to do? The answer is nothing. Accept that the inch has changed, and thus the volume it defines has changed too. So the fluid once is no longer exactly tied to the mass once via the density of water at a specific temperature. No big deal. The metric system didn’t see such large scale changes in the definition of the fundamental units that anything mattered. It is worth noting that the metric system no longer ties the definition of mass to a volume of water, and in principle, one litre of water probably does not exactly equal one kilogram anymore. But the disparity is infinitesimal. ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [February 1, 2016, 11:14pm UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/22 "2016-02-01T23:14:51Z")
> [@TroutMan](#): > > _A pint’s a pound, the world 'round_ is close enough for most things, but it’ll bite you sometimes. A pint’s only a pound in the US. Imperial pints are 20oz rather than 16oz, and hence ~1.25lbs. ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [February 1, 2016, 11:19pm UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/23 "2016-02-01T23:19:27Z")
> [@Gorsnak](#): > > A pint’s only a pound in the US. Imperial pints are 20oz rather than 16oz, and hence ~1.25lbs. Yes, the Imperial pint is bigger, but the Imperial ounce is also **smaller** (29.57 ml vs. 28.41 ml). So it would be about 1.20 lbs. ---
**Author:** ![TroutMan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/troutman/32/6721_2.png) [@TroutMan](https://boards.straightdope.com/u/TroutMan)\ **Post date:** [February 1, 2016, 11:27pm UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/24 "2016-02-01T23:27:19Z")
> [@Gorsnak](#): > > A pint’s only a pound in the US. Imperial pints are 20oz rather than 16oz, and hence ~1.25lbs. According to the U.S. Mnemonic Standards Board, there is no world beyond the borders of the U.S. 😉 ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [February 1, 2016, 11:40pm UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/25 "2016-02-01T23:40:26Z")
Also, something which is completely irrelevant to your question, is that grams measure mass, whereas ounces measure weight. A subtle difference that makes no difference if you always do your measurements in a gravity field of 9.8 m/s^2. So if you mass 70 kilos you’ll still mass 70 kilos on the Moon. But if you weigh 150 pounds on Earth you’ll only weigh 25 pounds on the Moon. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [February 2, 2016, 12:03am UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/26 "2016-02-02T00:03:17Z")
> [@Lemur866](#): > > Also, something which is completely irrelevant to your question, is that grams measure mass, whereas ounces measure weight. Given that NIST provides conversions from grams to ounces without reference to location, I don’t know that this is always the case. [Look here:](http://physics.nist.gov/Pubs/SP811/appenB9.html) > [@](#): > > ounce (avoirdupois) (oz) kilogram (kg) 2.834 952 E-02 > ounce (troy or apothecary) (oz) kilogram (kg) 3.110 348 E-02 NIST cannot be wrong in this, by definition. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [February 2, 2016, 12:08am UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/27 "2016-02-02T00:08:43Z")
> [@TroutMan](#): > > _A pint’s a pound, the world 'round_ is close enough for most things, but it’ll bite you sometimes. I don’t think I’ve ever needed a more accurate weight but I never picked up on that difference. I don’t even remember where I learned that. Oh well, life would be pretty dull if there was nothing left to learn. But on that basis my life should be much more exciting than it is. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 2, 2016, 2:13am UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/28 "2016-02-02T02:13:27Z")
The American system of units is a royal mess. Sometimes, the pound is treated as a unit of force, with the corresponding unit of mass being the slug. Sometimes, the pound is treated as a unit of mass, with the corresponding unit of force being the poundal. And sometimes, it’s treated as being a unit of both force _and_ mass, which can’t possibly be consistent but which engineers insist upon doing anyway, which does terrible violence to basically all of the formulas in physics. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [February 2, 2016, 2:46am UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/29 "2016-02-02T02:46:14Z")
> [@Johnegee](#): > > Now my question is What difference does it make how many mL are in a US fluid ounce? I mean, how does that resolve the contradiction? It means a fluid ounce does not WEIGHT an ounce !. Google knows that if you convert an ounce to volume, you must be referring to “fluid ounces” , and it shows “fluid ounce” in its answer… If you convert an ounce to grams, you must be talking about weight ounces, so it drops the “fluid” and says “ounce to grams” in the answer. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [February 2, 2016, 2:51am UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/30 "2016-02-02T02:51:25Z")
Here’s a two-page thread by the same OP from about one year ago on essentially the same topic. [What formula switches from pints to milliliters? - Factual Questions - Straight Dope Message Board](http://boards.straightdope.com/sdmb/showthread.php?t=750297) He proved impervious to our educational efforts then. I predict a similar result here. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [February 2, 2016, 2:54am UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/31 "2016-02-02T02:54:36Z")
> [@Lemur866](#): > > So if you mass 70 kilos you’ll still mass 70 kilos on the Moon. But if you weigh 150 pounds on Earth you’ll only weigh 25 pounds on the Moon. Well force is best measured in Newtons, but because we use scales to measure mass on earth, we might use them to measure force due to gravity on the moon… ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [February 2, 2016, 3:01am UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/32 "2016-02-02T03:01:16Z")
> [@TroutMan](#): > > According to the U.S. Mnemonic Standards Board, there is no world beyond the borders of the U.S. 😉 Isn’t the British mnemonic something like “a pint of water is a pound and a quarter” or something to that effect? There’s an equivalent saying, but I’m not going to bother to look it up. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [February 2, 2016, 3:08am UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/33 "2016-02-02T03:08:05Z")
> [@LSLGuy](#): > > Here’s a two-page thread by the same OP from about one year ago on essentially the same topic. [What formula switches from pints to milliliters? - Factual Questions - Straight Dope Message Board](http://boards.straightdope.com/sdmb/showthread.php?t=750297) > > He proved impervious to our educational efforts then. I predict a similar result here. Ah, I see. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [February 2, 2016, 3:10am UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/34 "2016-02-02T03:10:10Z")
I think the answer to the OP is simply: 128 ounces = 133.5 ounces because 128 fluid ounces = 133.5 ounces. Going any further in to it just seems to confuse the issue. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [February 2, 2016, 2:46pm UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/35 "2016-02-02T14:46:12Z")
> [@Johnegee](#): > > First, thanks for the link. I read the article three times and paid special attention to your quote therefrom. Now my question is What difference does it make how many mL are in a US fluid ounce? I mean, how does that resolve the contradiction? In one conversion, you’re using **Imperial** fluid ounces; in the other you’re using **US** fluid ounces. The two are not the same (they’re about 4% different). So your two conversions give you answers that are 4% different. It has nothing to do with volume vs. mass; it’s simply that you’re using two units (Imperial Fluid Ounces and US Fluid Ounces) that sound similar and are close to the same size, but actually slightly different. So you’re of course getting two slightly different answers. Now, volume vs. mass _could_ be an issue, depending on what you’re measuring, the temperature, and how accurate you need to be, etc. For instance, a cubic centimeter of water at room temperature is going to be about 0.998 grams. But that’s not what’s going on in your calc; you’re just using two different units. ---
**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\ **Post date:** [February 2, 2016, 7:03pm UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/36 "2016-02-02T19:03:57Z")
Some of you have brushed by the fact that the old rhyme is nearly correct about the weight of water, but different for other things measured in fluid ounces. Until I checked, I didn’t know just _how_ different. While a pint of water weighs a little more than a pound, and a US gallon of water a little more than **8 pounds** , a US gallon of gasoline (petrol) weighs only a little more than **6 pounds**. ---
**Author:** ![Ornery\_Bob](https://avatars.discourse-cdn.com/v4/letter/o/da6949/32.png) [@Ornery\_Bob](https://boards.straightdope.com/u/Ornery_Bob)\ **Post date:** [February 2, 2016, 8:16pm UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/37 "2016-02-02T20:16:00Z")
[How Much Does Water Weigh?](http://bowlofplenty.blogspot.com/2009/01/how-much-does-water-weigh.html) A fluid ounce of water at standard temperature and pressure weighs 1.04 oz. There’s your 4% difference. ---
**Author:** ![Ranger\_Jeff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ranger_jeff/32/2065_2.png) [@Ranger\_Jeff](https://boards.straightdope.com/u/Ranger_Jeff)\ **Post date:** [February 3, 2016, 3:34am UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/38 "2016-02-03T03:34:32Z")
Fluid ounces are a measure of volume. They’re in the same family as cups, pints, quarts, and gallons. Ounces are a measure of weight. They’re in the family of pounds and tons. Think of it this way: comparing fluid ounces to weight ounces is generally like comparing oranges to suspension bridges. In a sane system, there would be different names for them. Like in the metric system. ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [February 3, 2016, 3:51am UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/39 "2016-02-03T03:51:20Z")
> [@Ranger\_Jeff](#): > > In a sane system, there would be different names for them. Like in the metric system. You mean like when they called a unit the ton(ne) and said “screw it, 2000 is close enough to 2200”? Or when they said a calorie is the same thing as 1000 calories? ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [February 3, 2016, 6:18am UTC](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876/40 "2016-02-03T06:18:54Z")
IIRC, the sensible system (the real system) was imperial measure. An imperial gallon of water was 10 pounds. (in the rough and ready days, they did not worry about thermal expansion, but obviously at or around room temperature…) 4 quarts to a gallon, two pints to the quart, 2.5 cups to the pint, 8 (fluid) ounces to the cup… (IIRC a trivia I never verified - a cubic foot of water is 6 gallons, weighs 60 imperial pounds. if it was Perrier, it cost 60 pounds. ) 8 x 2.5 x 2 x 4 = 160 - tada! An ounce of water (imperial) was an ounce weight. Two and a half cups to the pint… why do you think the Hobbitses were excited that beer came in pints? [Previous page](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876.md?page=1) [Next page](https://boards.straightdope.com/t/how-can-128-ounces-133-5-ounces/744876.md?page=3)