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My wife and I have noticed that a lot of the drinks or liquids we buy have a measurement of "fl oz".

I was surprised to ascertain that fl oz. is indeed a measure of volume. I might be a little dense here but why the hell do I need the "fl"? Does this provide us with any more information?

Also for "heavier" liquids couldn't fl oz be different from oz? So if a recipe asked for 10 oz of gravy and I have a jar that is 10 fl oz, what if they gravy is 11 oz? Serenity now!

blankip
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Because a fluid ounce and a dry measure ounce (both volume measurements) are about 20% different, though "dry measures" (other than the measuring scoops and spoons in a USA kitchen) have become far less common as most product is now marketed by weight, not volume.

https://en.wikipedia.org/wiki/Dry_measure

A US dry quart is 1101 cc, while a US liquid quart is 946.4 cc (and an imperial quart is 1137 cc either way, since they evidently didn't have the bifurcation.)

A fluid ounce (volume) of water weighs about an ounce (weight) - a jar that is 10 fl ounces is a volumetric measure and can weigh whatever it likes - more for mercury (or syrup, for a less toxic and more food example) and less for ethanol. The continued use of fl mostly helps to distinguish volume from weight in current labeling, practically speaking.

Ecnerwal
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    Things would be a lot simpler if we could just use the metric system everywhere. Unfortunately, switching to the metric system is surprisingly complicated. – mrog Nov 05 '15 at 16:38
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    No duh. Many decades ago we were "prepared for that great coming change" in school, and yet it proceeds (or not) at a snail's pace. – Ecnerwal Nov 05 '15 at 16:43
  • But why fl oz opposed to other units of volume that we already have? – blankip Nov 05 '15 at 17:53
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    The non-fluid ounce (called an "ounce") is **not** a volume measurement, it is a weight measurement. There are 16 ounces to a pound, and a pound is clearly a weight measurement. The fluid ounce is a volume measurement and is confusingly named. @blankip Outside of the metric system, there are not really good volume measurements to replace the fluid ounce. The metric system is not generally used in the United States, and even if that changed today many recipes would still have fl. oz. written in them. – Todd Wilcox Nov 05 '15 at 18:35
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    @ToddWilcox Also, let's not forget the oddity that 1 US Pint = 16 US fl.oz, but 1US Pint = 19.2 UK fl.oz... but 1UK Pint = 20 UK fl.oz. Meaning, oddly, that 1UK Pint = 1.2US Pint and 1UK fl.oz = 0.96US fl.oz. Talk about a headache! – J... Nov 05 '15 at 18:50
  • Metric can be abused too, many countries use millilitres (ml) for small fluid volumes, but some use decilitres (dl), and others centilitres (cl), what? Just to save one decimal digit you make everyone have to double check what they are reading, stupid! Decimal is simple with all units three points apart, and a minimum of prefixes to remember. What next the US ql (quarter litre), or the fl cc (1/3 litre aka as a coke can)? – TFD Nov 05 '15 at 20:13
  • @J... It gets even worse. Wikipedia says that ounce can be a unit of force, a unit of mass, or a unit of length. If you look just at the ounces that are units of mass, there are the avoirdupois ounce, the troy ounce, the apothecaries' ounce, the Maria Theresa ounce, the Spanish ounce, the French ounce, the Portuguese ounce, the Roman/Italian ounce, the Dutch metric ounce, the Chinese metric ounce, and the English Tower Ounce. It's maddening! (Although not nearly as maddening as doing time-based math with different calendars and time zones. That's pure insanity!) – mrog Nov 05 '15 at 21:44
  • @J... With regards to the different pints/gallons between the US/UK, if you go back to the 18th century there were two liquid volume gallons in use. The wine gallon and the ale gallon. The US customary gallon is just the wine gallon renamed; I've never seen an explanation behind why we standardized on it. The UK Imperial gallon was defined by tweaking the ale gallon so a gallon of water weighted exactly 10 pounds (give or take all the gotchas involved) instead of just close to it. This was done when London suffered a rare case of Paris Envy (tm). https://en.wikipedia.org/wiki/Gallon#History – Dan Is Fiddling By Firelight Nov 05 '15 at 22:17
  • @DanNeely I always thought it was because the King of England couldn't unilaterally raise taxes on the colonies but could change weights and measures by decree... same price, smaller gallon. – J... Nov 05 '15 at 22:20
  • @ToddWilcox If you look at package sizes on canned/bottled/boxed products over an extended period, the portion that have an ugly number in US units followed by a nice number in metric units is steadily trending up. The changeover is slowly happening in retail while Joe Sixpack is watching TV on his lazyboy oblivious to what's happening. For anyone who has recipes that are aligned with traditional package sizes it can be a real pain now. – Dan Is Fiddling By Firelight Nov 05 '15 at 22:21
  • @J... just an urban legend. The wine gallon was in common use since at least the 14th century, and was defined in statute in 1707; well before colonial relations began to break down. https://en.wikipedia.org/wiki/Wine_gallon – Dan Is Fiddling By Firelight Nov 05 '15 at 22:23
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    The last paragraph is really all this answer needs—"fluid ounce" disambiguates with the unit of mass. All the stuff about "dry measure" (used in agriculture with bulk commodities) is irrelevant and pretty off-base. US measuring cups and spoons are measuring fluid volume, not dry volume. No one uses a "dry ounce" unit of volume. – Miles Nov 06 '15 at 06:42
  • god I hate when they use 'oz' and mean 'fl oz', especially in granular ingredients context, where the two are very different and telling the author's intent is about impossible. – SF. Nov 06 '15 at 13:55
  • A troy ounce is 2 grams more than a regular ounce. An ounce of gold is not the same weight as an ounce of sugar. – Neil Meyer Sep 19 '22 at 15:26
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As has been discussed in comments:

  • A fluid ounce is a measure of volume.
  • An ounce is a measure of mass or weight.

The exact sizes of these measurements vary by country and by historical period, as can be seen at the links.

The fact that these units have a similar name is due to historical accident, and you'd be better served by not thinking of them as related AT ALL. They simply measure different things.

In a recipe, the word "ounce" is ambiguous. Unless the recipe or source specifies, I would generally assume:

  • An "ounce" of anything liquid or roughly fluid (gravy, sauce, syrup) should usually be measured in a measuring cup by volume (regardless of weight).
  • For dry goods, usually an "ounce" refers to weight, not volume.
  • Recipes for baking sometimes also include weight measurements in addition to volume measurements (e.g., cups); in this case it is generally clear that an "ounce" refers to weight measurement.
  • Professional cookbooks for large quantities sometimes have recipes which are entirely given in weight measurements; again, in the case context should make that clear.

To answer the question about why we use a "fluid ounce" rather than other volume measurements, originally the "fluid ounce" referred to a volume of a specific fluid that weighed an ounce, usually wine, ale, or water. An "ounce" would thus be a different size depending on the substance being measured. A couple centuries ago, both the U.S. and Britain decided to standardize the "fluid ounce" as a volume measurement equivalent in all contexts, regardless of the substance. Britain chose water as its standard; the U.S. chose the old British wine measure as the standard size for its ounce. Regardless, all fluid ounces are now defined as having a particular volume, with no necessary relationship to the density or weight of the material.

Despite the confusion it causes, the name stuck. For some reason it become adopted as a standard volume measure, probably due to its size and standardization. (Pints were too large and potentially inaccurate for labeling consumer goods; drams and minims were useful for pharmacy but were too small for labeling volumes of any size. Intermediate units like tablespoons and cups tended to vary in size historically. And no one seemed to like using the gill outside of alcohol measures.)

More details on use below.

To clarify a previous answer: If the term "dry ounce" occurs (only generally in culinary contexts), it is ambiguous and could refer to:

  • Measuring something as an ounce by weight (1/16th of a pound)
  • Measuring a solid -- usually a powder, finely chopped materials, or a semisolid like butter -- using a fluid ounce volume (1/16th of a liquid pint, equivalent to 2 tablespoons)

Large quantities of dry goods in the U.S. are measured in bushels, pecks, dry quarts, and dry pints (which are about 16.4% larger than liquid pints). A "dry ounce" (as 1/16th of a dry pint) is NOT used as a volume measurement. (They definitely don't exist officially in the U.S., which is the only place that still has "dry measure" volume units.)

There's no reason for it to exist. The dry pint is the smallest official unit of dry goods; the FDA does not acknowledge the possibility of "dry ounces" for food labeling (see regulation 101.105(b)(3)). Anything solid smaller than a dry pint must be measured by weight or using fluid volume measurements. Smaller units in culinary purposes (cups, tablespoons, teaspoons, and yes, fluid ounces) are all liquid volume measurements which are derived from (and equivalent to) the liquid system, not related at all to the dry goods units.

Also, while we're clearing this up, contrary to popular lore, a "pint is NOT a pound the world around," even with water. In the traditional Imperial system, a fluid ounce of water does weigh exactly an avoirdupois ounce. But an Imperial pint is 20 ounces, not 16, so a pint weighs more than a pound (1.25 lbs., actually). In the U.S. customary system, the fluid ounce was defined as 1/128th of a wine gallon. Thus, a fluid ounce of wine should weigh approximately a U.S. ounce. But since water is denser than wine, a fluid ounce of water weighs more than an ounce (by weight); similarly, the U.S. pint of water (16 fluid ounces) weighs more than a pound (about 1.044 lbs.).

For that matter, the other place of confusion occurs with butter, which is assumed to have 16 ounces in a pound, according to the way each stick is generally marked in the U.S. (with 8 tablespoons = 4 (fluid?) ounces). But butter is about 5% less dense than water and 16 fluid ounces of butter can't weigh the same as 16 fluid ounces of water. A pound of butter thus usually contains somewhere around 33-35 tablespoons, despite being marked as containing 32 tablespoons. This discrepancy is usually not significant in culinary practice and also applies approximately to oils. (To add to the confusion, a U.S. tablespoon for labeling purposes is often defined as 15 milliliters, rather than the 14.787 milliliters which actually make up 1/2 of a customary U.S. fluid ounce. A fluid ounce is also therefore defined by the FDA in the U.S. to be 30 milliliters when referring to nutritional information -- see point 9 here.)

Anyhow -- other than for butter/oil and water/wine, though, these sorts of relationships between volume and weight measurements don't hold AT ALL.

In general, I would avoid trying to equate these two units in any meaningful way, since they only relate in very specific cases and only imprecisely even then. A fluid ounce refers to volume (and is related to cups and pints and quarts and gallons and liters); an ounce is a measure of weight (and is related to pounds and kilograms).

For culinary purposes, it's simple:

  • if you're using a cup to measure something, you're measuring volume (fluid) ounces (whether the material is liquid or not)
  • if you're using a scale, you're measuring ounces
  • if you're measuring butter or shortening in a stick, just accept what the package says and don't think too hard about it
Athanasius
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Actually a US Tbsp is aprox.14.78 ml, 1 ml of butter weigh 0.959 gr 1 U.S.tbs=0.5 fluid oz then 0.959*14.78*32=454 gr=1 pound=16 fluid oz JF

JF Dore
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