Re: fl oz to grams

Re: fl oz to grams   (07/22/2009, 6:09 PM)

Converting between grams and fluid ounces is somewhat ambiguous because it depends on the substance you are measuring and the specific temperature and pressure. That may be why you're seeing different numbers on different sites, it depends on the assumptions each site makes.

On our site, we assume that you are measuring a gram of pure water at 4 degrees Celsius, which then has a density of exactly 1.0 g/cm^3 or 1 gram per cubic centimeter.

If you take that assumption, then 1 fluid ounce = 29.57 cubic centimeters, so:

1 fl oz = 29.57 cm^3 x (density of 1.0 g/cm^3) = 29.57g of pure water at 4 deg C

One cubic centimeter is the same as one milliliter, so it's really 29.57ml in 1 fl oz, which you may have seen.

If you're looking at measurements for a cooking recipe, you'll often see 28.35 grams. I believe that's referring to the international avoirdupois ounce, or "ounce-force", also called the "mass ounce". There are many definitions of the word "ounce" including the troy ounce and other variations, so that can lead to confusion. The avoirdupois ounce is 28.35 grams, since both of these measure force, but this is different from the liquid ounce or fluid ounce. Even the fluid ounce has different definitions in the United States and the United Kingdom.

This page may give you more information:

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

For the fluid ounce, if you're measuring liquids, you can see this page:

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

This U.S. customary fluid ounce is equal to 29.57 ml, so assuming water with a density of 1.0 g/cm^3, you end up with 29.57g in 1 fl oz.