I replaced my kitchen scale recently. The new one has every feature I was looking for — negative tare being the big upgrade. But it also has two features that I never asked for and do not want. Nobody wants them. I’m talking about the “millilitres” and “fluid ounces” settings, which are both complete bullshit.
You see, and I somehow feel crazy explaining this, a scale is a scale. It measures weight. I appreciate that my scale can measure things in grams, which I use most often, or in ounces. These are measures of weight. (OK, grams measure mass, but in typical use on a kitchen scale on planet Earth, this distinction is insignificant.)
Millilitres and fluid ounces, on the other hand, are measures of volume. A measuring cup holds eight fluid ounces. A disposable water bottle is typically 500 millilitres. Those numbers tell you how much space exists inside the cup or bottle.
Knowing the volume of something does not tell you how much it weighs. If you empty your water bottle and fill it back up with Everclear, or with maple syrup, or with olive oil, or with sand, or with molten lead, or with helium, it’s not going to weigh the same amount as when it was full of water. The whole reason we have different units for volume and weight is that they are not the same thing!
What is my scale doing when it says it’s measuring volume?
So what the heck does the fluid-ounces feature think it’s doing? Well, it’s just weighing whatever you put on top, and then assuming that that object is the density of water. I measured out a fluid ounce of water (using the little measuring cup that came with a bottle of cough syrup) and the scale told me that it was 1.0 fluid ounces, 28.35 g, 29 millilitres, and 29 grams.
That all checks out — the density of water is 0.997 g/mL, meaning that one gram of water takes up 0.997 millilitres of space. Rounding, one gram is roughly equivalent to one millilitre, and one fluid ounce is roughly equivalent to one (weight) ounce. If you need to add four fluid ounces of water to a recipe (half a cup), go ahead and weigh out four ounces. But you don’t need a “fluid ounces” setting to do that; you can just weigh out four regular (weight) ounces.
If you are measuring water or something similar, the fluid ounce and millilitre settings are redundant: they just duplicate what is communicated by the regular ounce and gram settings. If you are measuring something that does not have the same density as water — like alcohol, oil, or syrup, to name a few — the ounce and millilitre settings are worse than useless. They can lead you to put entirely the wrong amount of the ingredient into your recipe.
How big a difference does this make in the real world?
For a test, I measured out one “fluid ounce” of whiskey, and one “fluid ounce” of Log Cabin maple-flavoured pancake syrup.
Alcohol has a much lower density than water, at 0.789 g/mL. That means it’s lighter than water, which explains why you need to mix your mixed drinks — otherwise the alcohol will float on top. My cheap whiskey was 90 proof (45% alcohol), meaning its actual density is heavier than pure alcohol, but still significantly lighter than water.
Meanwhile, syrup has a higher density than water, because the sugar that’s dissolved in it adds weight without adding very much volume. If you’ve ever made simple syrup, you know this: one cup of water plus one cup of sugar yields something like one and a quarter cups of syrup — even though it contains the full mass (and thus weight) of the water and the sugar you combined. Pancake syrup contains even more sugar than simple syrup, so it’s going to be a lot heavier than water.
The image at the top of the page shows my results. Both cups were judged by the scale to contain one “fluid ounce” of liquid. They are clearly not the same volume. I measured them with my little medication cup: The whiskey (left) is 36 millilitres or 1.22 fluid ounces. The syrup (right) is 22 millilitres or 0.74 fluid ounces.
So if you were making a mixed drink for four people and decided to use your scale rather than a jigger to measure out six fluid ounces total (42.52 g for each person), you’d be making the drinks a good bit stronger than expected — 51.88 g for each person.
On the other hand, if you were following a cake recipe that calls for 3.5 fluid ounces of maple syrup and used your scale instead of a measuring cup, you would end up putting only 2.55 fluid ounces of syrup in the cake. The result wouldn’t be anywhere near as sweet as intended.
(I know the maple cake recipe just says “ounces,” but the British version calls for 100 millilitres, which is about 3.4 fluid ounces. I also used the density of actual maple syrup, 1.37, in my calculation, although it’s very similar to the fake syrup, 1.31, I measured in my own experiment. Just in case any of my old chemistry professors are out there checking my work.)
Syrup and alcohol are extreme examples, which is why I chose them. To name a few other common liquids that will offset your measurements:
- Oils are a bit lighter than water (0.917 mg/mL for olive oil)
- Milk is very slightly heavier than water (1.04 mg/mL)
- Peanut butter is a bit heavier than water (1.1 mg/mL)
- Honey is a lot heavier than water (1.4 mg/mL)
So, what should you do? Well, if you’re measuring water, any setting will work. If you’re measuring a different type of liquid, use volume measurements. A measuring cup, a jigger, or even a tablespoon (one tablespoon is half a fluid ounce) will do.