Ounces vs. fluid ounces


I was looking at the recipe for the Golden Pull-Apart Butter Buns, and I noticed something weird in the conversion between volume and weight. In terms of volume, the recipe calls for 2/3 cup of water; in terms of weight, it says the equivalent is 5 3/8 ounces of water. So my first question is, does "ounce" here refer to fluid ounces or dry ounces since the former is actually a unit of volume, not a unit of weight?

This conversion makes sense if by "ounce," you mean "fluid ounce." There are 8 fluid ounces in a cup, so 2/3 of a cup is equal to 5.333 fluid ounces, which is pretty close to 5 3/8=5.375. In terms of dry ounces, 2/3 of a cup of water weighs 5.563 ounces. So it appears that even when you ask the web site to give you weights and it appears to convert the amount to a weight, it's really still giving you a volume measurement.

But then what about other liquid ingredients, like honey? If a recipe calls for 3 ounces of honey, is that 3 fluid ounces of honey, 3 dry ounces of honey, or the amount of honey that weighs the same as 3 fluid ounces of water?

Finally, the KA Master weight chart equates 1/3 cup of water with 2 5/8 ounces and 2/3 cup of water with 5 1/4 ounces. Neither of these seems to be correct, regardless of whether you're using fluid ounces or dry ounces. (The differences are small amounts of water so it probably doesn't make much of a difference, but it would be nice if the conversions were consistent.)

badge posted by: placebo on November 22, 2010 at 5:50 pm in Q & A
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reply by: uninvited-guest on November 22, 2010 at 5:58 pm

I can only make a comment about the water weight/measure. A handy thing to remember is "A pint's a pound the world around." One pint of water, placed on a scale, should weigh 16 oz, or 1 pound.

reply by: frick on November 22, 2010 at 6:54 pm

It refers to fluid ounces and INMO, fluid ounces refer to both volume and weight (of water) and since a 'pint's a pound the world round', a cup weighs 8 ounces. 2/3 cup water does in fact weight 5.33 ounces or 5 3/8 if you are measuring in a pyrex cup.

I have always assumed that 1/3 cup of honey or molasses to be a liquid measurement, since they are liquids, so I measure them in a pyrex cup. That's always worked for me.

All this used to puzzle me until I had to explain it to a neighbor. Isn't that a great way to learn something, to explain it to someone else?

reply by: placebo on November 23, 2010 at 2:44 pm

That was my first thought, that for water, one fluid ounce weighed one dry ounce. It turns out, however, this isn't the case, though it's close. A fluid ounce of water weighs approximately 1.043 dry ounces, so there's about a 4.3% difference between the two. One cup of water, which is 8 fluid ounces, weighs 8.346 ounces, and a pint would weigh 16.69 ounces or 1.043 lb.

For water, the difference between the units doesn't amount to much. "A pint's a pound the world round" is a good enough approximation for water, but for other fluids with different densities, it could make quite a bit of difference. I did notice that for honey, one recipe on this site converted 1/4 cup to 3 ounces, so in this case, "ounces" referred to dry ounces.

reply by: Mike Nolan on November 23, 2010 at 3:30 pm
Mike Nolan

You don't see it used much any more (thankfully!) but an IMPERIAL pint is 20 fluid ounces.

I tend to remember that a quarter cup of canola oil is about 2 ounces by weight, a half cup of light corn syrup is about 5 1/2 ounces by weight, and quarter cup of honey is about 3 ounces by weight, because I use those quantities a lot, and measuring them by weight into the bowl saves cleaning another measuring cup.

The pecan pie recipe I've been using for the past week or so uses 3/4 cup of Golden Syrup, which is about 8 2/3 ounces by weight.

reply by: frick on November 23, 2010 at 9:47 pm

So Mike, are you weighing the syrup at this point, and if you did not have a scale, would you use a dry or liquid measure?

If I'm not with my scale, I use liquid measure for liquids like honey, syrup and molasses. I still measure the corn syrup for pecan pie, for instance, in a pyrex cup (sprayed with baking spray).

I just have not got in the habit of using the scale for all the old recipes for candy, etc. I would be nice to have the chart laminated and hanging on the wall, but pretty unsightly in my establishment.

reply by: pjh on November 23, 2010 at 10:35 pm

Dry weight is what we use in the KA test kitchen, and in our recipes. Set the measuring cup on the scale, pour in 1/4 cup honey, it weighs 3 ounces; 1/4 cup olive oil, it weighs 1 3/4 ounces, etc. We chose that method because most people with scales will measure the same way; our goal is to be as simple as possible, and consistently use regular dry weight, not fluid ounces. Hope this helps-

reply by: Mike Nolan on November 23, 2010 at 11:29 pm
Mike Nolan

I prefer to weigh sticky things, saves me on cleanup.

The first time I make a recipe with a quantity of an ingredient I don't already have memorized, I look it up (usually on the recipe analyzer on about.com) and make a note of the proper weight on the recipe.

In the absence of a scale or any comments in the recipe that make it clear, I would assume eight ounces is a cup, not half a pound. Is that 'dry' or 'liquid' weight, when measuring water? :-)

I've done a quarter cup of oil or honey so many times I don't need the scale, I just use it to double check my accuracy.

When we were first married, my wife asked me to measure out a quarter cup of milk for something she was making. I just poured it into the bowl direct from the carton. She had me do it three more times into a glass, all of them were so close to a quarter cup the difference wasn't important.

I reminded her that I worked in a soda fountain for 7 years (in my grandfather's drug store), and when making a malt we would measure out a quarter cup of milk twice, once before adding the syrup, once after adding the ice cream, so I'd measured out that amount of milk hundreds of times. (Milk, syrup, ice cream, syrup, ice cream, milk was the proper order for most malts.)

reply by: silkenpaw on November 24, 2010 at 3:06 pm

I think that's only approximately right. 1 cup is 237 ml and that much water weighs 237 g. 1 pint is 2 cups or 474 ml; that volume of water weighs 474 g, while 1 lb is 454 g.

I have to say that this (Imperial?, British?, US?) system makes me dizzy, so if I made a mistake somewhere in my calculation, someone please point it out.

I did weigh 2 cups of water and got 1 lb 7/8 oz,which is 479 g, reasonably close to my calculation.

The whole mess is a great argument for converting to metric, IMO.

reply by: silkenpaw on November 24, 2010 at 3:12 pm

Oops, I just wrote the exact same thing before I saw your comment.

IMO, the "pint's a pound the world round" is good enough for government work but I like my baking to be more precise. I make enough mistakes without any help from my starting measurements.

Love the kitty in your photo, by the way.

reply by: Mike Nolan on November 24, 2010 at 3:47 pm
Mike Nolan

How did you measure the 2 cups of water? In my experience, you're far more likely to have measuring cups that measure a bit inaccurately than a digital scale. With a digital scale, the roundoff error (when it goes to the next increment) is usually less than the error in measuring liquids.

To complicate matters more, you need to consider what kind of water (because the impurities have a different mass) and what temperature it is at, since pure water expands slightly as it approaches freezing. (If it didn't expand as it starts to freeze, life as we know it would probably be impossible, since the bottom of the ocean would be frozen solid.)

A gallon of distilled water at its densest temperature (around 40 degrees F) weighs 8.345404 pounds, so a cup of water weighs 8.345404 ounces, since there are 16 ounces in a pound and 16 cups in a gallon. See, sometime the math DOES work out. :-)

reply by: silkenpaw on November 24, 2010 at 6:10 pm

The math ALWAYS works out, just not necessarily to what you want :)

To answer your question, I did not have a volumetric flask handy :), so I used a measuring cup and I did the measurement only once. Tsk tsk. And I used room temperature, not 4ºC water. My room temp is 80ºF, water density per Wolfram Alpha 0.9966 g/ml. Probably not a source of significant error at the level I'm able to measure in my kitchen.

But in your example, a pint of water still does not weigh a lb but 16.79 oz, which is pretty close to the 16.89 oz I got in my admittedly flawed experiment.

I don't think there are enough impurities in tap water to make a significant difference in weight (at least to the degree of accuracy we are discussing).

And I do agree that volume measurements are less accurate than weight because they depend on judgment in reading the meniscus.

You still haven't convinced me that a system in which you have to deal with numbers like 8.345404 makes sense. I like my powers of 10, thank you! :)

reply by: Mike Nolan on November 25, 2010 at 11:03 am
Mike Nolan

Yeah, well 1 cc = 1 gram only works for water, not other ingredients.

Is it really easier to remember that 60 CC's of honey weighs 85 grams than to remember that 1/4 cup of honey weighs 3 ounces? :-)

reply by: silkenpaw on November 25, 2010 at 11:35 am

True about the water and remembering weights. But I think it's easier to remember that 1 g = 1000 mg and 1 kg = 1000 g than 1 cup = 8 oz and 1 gal = 16 c. Or did I get that wrong, as usual? :)

I guess a lot of it is whatever you are used to. My brain works better in metric, I guess yours works better in British.

One more point, though, is the confusion between liquid and avoirdupois ounces, which is how this thread started. It would be useful to have another name for one of them, instead of always having to specify ounces by volume or ounces by weight.

reply by: Mike Nolan on November 25, 2010 at 11:54 am
Mike Nolan

Given what it costs, maybe saffron should be measured in troy ounces or carats!

reply by: placebo on November 25, 2010 at 8:59 pm

Thanks! Unfortunately, she had kidney problems, common for cats her age, and we had to put her down a few years ago.

reply by: silkenpaw on November 26, 2010 at 2:50 am

So sorry. Been there myself and know how hard that is.