A tablespoon is a unit of volume. Namely, it is one-sixteenth of a cup.
Now, there are two distinct units called “ounces” that are commonly used in the United States. One is the avoirdupois ounce, also know as the United States customary ounce, which is a unit of weight; it is one-sixteenth of a pound. The other is the U.S. customary fluid ounce, which is a unit of volume; it is one-eighth of a cup.
One-sixteenth of a cup is a tablespoon. One-eighth of a cup is an ounce (fluid). One-eighth of one-half of a cup is a tablespoon. These are all measures of volume.
Butter, however, is sold by weight:
The 16-oz. package of butter in the photo above says that it contains four sticks. This is one stick:
The stick is divided into eight “tablespoons”.
But the “tablespoons” of butter depicted above are not one tablespoon each in volume. And there is no such thing as a unit of weight called the “tablespoon”.
So what is this? Well, one stick of butter is 4 ounces in weight. 8 ounces in volume is one cup. By analogy, if we think of 8 ounces in weight as a “cup” in weight (which is not actually a real weight unit!), then one-sixteenth of that weight is a “tablespoon” in weight (by analogy with one-sixteenth of a cup in volume being a tablespoon in volume). Neither cups nor tablespoons are real weight units! But if we call an 8-oz. weight a “cup”, then we can call a 1⁄2 oz. weight a “tablespoon”.
Sticks of butter are divided into metaphorical tablespoons of butter.
It sounds like 1⁄2 cup of butter (8 tbps) weighs 4 oz, so shouldn’t this actually work out so each of those sections actually is 1 tbsp in volume, and it’s just a coincidence (or not) that the density of butter is 1 oz / 2 fl oz?
This is almost true. Fat is less dense than water, so a tablespoon of butter weighs something like 10% less than a half ounce. Not enough to matter in practice for most cooking. Your toast and your average chocolate chip cookie don’t care. But, many approximations like this exist, and are collectively important enough that professionals use weight not volume in most recipes. And enough that the difference in fat content between butters (as low as 80% in the US but more often 85+% in European or otherwise “premium” butters) can matter in more sensitive recipes, like pie crust and drop biscuits. I used to add 1-2 Tbsp of shortening to my pie crust. I stopped when I switched to Kerrygold butter—no longer needed.
Edit to add: I think almost every concept we use in life is part metaphor, part not, and the difference is one of degree and not kind. I was definitely surprised to learn this, or at least to learn how deep the rabbit hole goes.
All of the sources I can find give the density as exactly 4 oz = 1⁄2 cup, although maybe this is just an approximation that’s infecting other data sources?
No, you’re misunderstanding. There is no 1⁄2 cup of butter anywhere in the above scenario. One stick of butter is 4 oz. of butter (weight), but not 1⁄2 cup of butter (volume).
The density of water (at refrigerator temperatures) is ~1 g/cm^3. 1 oz. = ~28.35 g; 1 fl. oz. = ~ 29.57 cm^3; thus the density of water is (1/28.35) / (1/29.57) = ~1.043 = oz./fl. oz. (This is, of course, equal to 0.95033293516 / 0.911, allowing for rounding and floating point errors.)
Note that the composition of butter varies. In particular, it varies by the ratio of butterfat to water (there are also butter solids, i.e. protein, but those are a very small part of the total mass). American supermarket butter has approx. 80% butterfat; Amish butter, European butters (e.g. Kerrygold), or premium American butters (e.g. Vital Farms brand) have more butterfat (up to 85%). Butterfat is less dense than water (thus the more butterfat is present, the lower the average density of the stick of butter as a whole—although this doesn’t make a very big difference, given the range of variation).
Given the numbers in the paper at the last link, we can calculate the average density (specific gravity) of butter (assuming butterfat content of a cheap American supermarket brand) as 0.8 * 0.9 + 0.2 * 1.0 = 0.92. This approximately matches our 0.911 kg/m^3 number above.
Tablespoons of butter.
A tablespoon is a unit of volume. Namely, it is one-sixteenth of a cup.
Now, there are two distinct units called “ounces” that are commonly used in the United States. One is the avoirdupois ounce, also know as the United States customary ounce, which is a unit of weight; it is one-sixteenth of a pound. The other is the U.S. customary fluid ounce, which is a unit of volume; it is one-eighth of a cup.
One-sixteenth of a cup is a tablespoon. One-eighth of a cup is an ounce (fluid). One-eighth of one-half of a cup is a tablespoon. These are all measures of volume.
Butter, however, is sold by weight:
The 16-oz. package of butter in the photo above says that it contains four sticks. This is one stick:
The stick is divided into eight “tablespoons”.
But the “tablespoons” of butter depicted above are not one tablespoon each in volume. And there is no such thing as a unit of weight called the “tablespoon”.
So what is this? Well, one stick of butter is 4 ounces in weight. 8 ounces in volume is one cup. By analogy, if we think of 8 ounces in weight as a “cup” in weight (which is not actually a real weight unit!), then one-sixteenth of that weight is a “tablespoon” in weight (by analogy with one-sixteenth of a cup in volume being a tablespoon in volume). Neither cups nor tablespoons are real weight units! But if we call an 8-oz. weight a “cup”, then we can call a 1⁄2 oz. weight a “tablespoon”.
Sticks of butter are divided into metaphorical tablespoons of butter.
It sounds like 1⁄2 cup of butter (8 tbps) weighs 4 oz, so shouldn’t this actually work out so each of those sections actually is 1 tbsp in volume, and it’s just a coincidence (or not) that the density of butter is 1 oz / 2 fl oz?
This is almost true. Fat is less dense than water, so a tablespoon of butter weighs something like 10% less than a half ounce. Not enough to matter in practice for most cooking. Your toast and your average chocolate chip cookie don’t care. But, many approximations like this exist, and are collectively important enough that professionals use weight not volume in most recipes. And enough that the difference in fat content between butters (as low as 80% in the US but more often 85+% in European or otherwise “premium” butters) can matter in more sensitive recipes, like pie crust and drop biscuits. I used to add 1-2 Tbsp of shortening to my pie crust. I stopped when I switched to Kerrygold butter—no longer needed.
Edit to add: I think almost every concept we use in life is part metaphor, part not, and the difference is one of degree and not kind. I was definitely surprised to learn this, or at least to learn how deep the rabbit hole goes.
Almost all human thinking is part metaphor.
Words have uses not meanings. Definitions are abstractions.
In otherwise, everything is (in part) a metaphor.
All of the sources I can find give the density as exactly 4 oz = 1⁄2 cup, although maybe this is just an approximation that’s infecting other data sources?
https://www.wolframalpha.com/input?i=density+of+butter+*+(1%2F2+cup)+in+ounces
No, you’re misunderstanding. There is no 1⁄2 cup of butter anywhere in the above scenario. One stick of butter is 4 oz. of butter (weight), but not 1⁄2 cup of butter (volume).
But 1⁄2 cup of butter weighs 4 ounces according to every source I can find: https://www.wolframalpha.com/input?i=density+of+butter+*+(1%2F2+cup)+in+ounces
Which means a 4 ounce stick of butter is 1⁄2 cup by volume.
The density of butter is reasonably close to 1 avoirdupois ounce per 1 fluid ounce, but is definitely not exactly equal:
https://kg-m3.com/material/butter gives the density as 0.95033293516 oz./fl. oz., or 0.911 kg/m^3.
(The link you provide doesn’t give a source; the data at the above link is sourced from the International Network of Food Data Systems (INFOODS).)
Further commentary:
The density of water (at refrigerator temperatures) is ~1 g/cm^3. 1 oz. = ~28.35 g; 1 fl. oz. = ~ 29.57 cm^3; thus the density of water is (1/28.35) / (1/29.57) = ~1.043 = oz./fl. oz. (This is, of course, equal to 0.95033293516 / 0.911, allowing for rounding and floating point errors.)
Note that the composition of butter varies. In particular, it varies by the ratio of butterfat to water (there are also butter solids, i.e. protein, but those are a very small part of the total mass). American supermarket butter has approx. 80% butterfat; Amish butter, European butters (e.g. Kerrygold), or premium American butters (e.g. Vital Farms brand) have more butterfat (up to 85%). Butterfat is less dense than water (thus the more butterfat is present, the lower the average density of the stick of butter as a whole—although this doesn’t make a very big difference, given the range of variation).
Given the numbers in the paper at the last link, we can calculate the average density (specific gravity) of butter (assuming butterfat content of a cheap American supermarket brand) as 0.8 * 0.9 + 0.2 * 1.0 = 0.92. This approximately matches our 0.911 kg/m^3 number above.
Plus, butter is churned, so it is a few percent air by volume when solid.