Kitchen scale

Why Your Kitchen Scale’s Fluid Ounce Measurements Make No Sense

Two cups of liquid, the one on the left (whisky) being much fuller than the one on the right (syrup).

Left: A purported “fluid ounce” of whiskey (actually 1.22 fl oz). Right: A purported “fluid ounce” of syrup (actually 0.74 fl oz)
Photo: Beth Skwarecki

I replaced my kitchen scale recently. The new one has all the features I was looking for – negative tare being the big upgrade. But it also has two features that I never asked for and don’t want. Nobody wants it. I’m talking about the “milliliters” and “fluid ounces” parameters, both of which are complete bullshit.

You see, and I feel a little crazy explaining this, a ladder is a ladder. It measures weight. I like that my scale can measure things in grams, which I use most often, or ounces. These are weight measurements. (Okay, grams measure Massbut in typical use on a kitchen scale on planet Earth, this distinction is insignificant.)

Milliliters and fluid ounces, on the other hand, are measurements of volume. A measuring cup contains eight fluid ounces. A disposable water bottle is typically 500 milliliters. These numbers tell you how much space there is inside the cup or bottle.

Knowing the volume of something doesn’t tell you how much it weighs. If you empty your water bottle and refill it with Everclear, or maple syrup, or olive oil, or sand, or molten lead, or helium, it won’t weigh the same amount as when it was full. some water. The only reason we have different units for volume and weight is that they are not the same thing!

What does my scale do when it says it’s measuring volume?

So what does the fluid ounces feature think it is doing? Well, it’s just a matter of weighing everything you put on it and then assuming that object is the density of water. I measured a fluid ounce of water (using the little measuring cup that comes with a bottle of cough syrup) and the scale told me it was 1.0 fluid ounce, 1.0 ounce , 29 milliliters and 29 grams.

Everything checks out: the density of water is 0.997 g/mL, which means that 0.997 of one gram of water absorbs 1 milliliter from space. Rounded, a gram is roughly equivalent to a milliliter and a fluid ounce is roughly equivalent to an ounce (of weight). 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 ounce” setting to do this; you can just weigh four regular ounces (of weight).

If you are measuring water or something similar, the fluid ounce and milliliter parameters are redundant: they only duplicate what is communicated by the normal ounce and gram parameters. If you measure something that does not have the same density as water – like alcohol, oil or syrup, to name a few – the ounce and milliliter settings are worse than useless. They can cause you to put the wrong amount of ingredient in your recipe.

What difference does it make in the real world?

For a test, I measured out a “fluid ounce” of whiskey and a “fluid ounce” of Log Cabin Maple Flavored Pancake Syrup.

Alcohol has a much lower density than water, at 0.789 g/mL. This means it is lighter than water, which is why you need to to mix together your cocktails, otherwise the alcohol will float on them. My cheap whiskey was 90 degrees (45% alcohol), which means its actual density is heavier than pure alcohol, but still much lighter than water.

Meanwhile, the syrup has a upper density than water, because the sugar dissolved in it adds weight without adding much bulk. If you’ve ever made simple syrup, you know this: one cup of water plus one cup of sugar yields about one and a quarter cups of syrup, even though it contains the total mass (and therefore weight) of water. and sugar that you combined. Pancake syrup contains even more sugar than simple syrup, so it will be much heavier than water.

The image at the top of the page shows my results. Both cups were judged by the scale to hold one “fluid ounce” of liquid. It’s clearly not the same volume. I measured them with my little medicine cup: Whiskey (left) is 36 milliliters or 1.22 fluid ounces. The syrup (right) is 22 milliliters or 0.74 fluid ounces.

So if you were making a mixed drink for four people and you decided to use your scale rather than a measuring cup to measure six fluid ounces total (1.5 ounces for each person), you would make the drinks a bit stronger than expected— 1.83 ounces for each person.

On the other hand, if you follow 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 would not be as smooth as expected.

(I know the maple cake recipe just says “ounces”, but the UK version asks for 100 milliliters, or approximately 3.4 fluid ounces. I also used the density of real maple syrup, 1.37, in my calculation, although it is very similar to the fake syrup, 1.31, which I measured in my own experiment. Just in case one of my old chemistry teachers is over 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 little 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 much heavier than water (1.4 mg/mL)

So what should you do? Well, if you’re measuring water, any setting will work. If you are measuring another type of liquid, use volume measurements. A measuring cup, measuring cup, or even a tablespoon (one tablespoon equals half a fluid ounce) will suffice.