Your Kitchen Scale's "Fluid Ounces" Setting Is a Lie

If you have one of the fancy kitchen scales that have “milliliters” and “fluid ounces” settings, you’re being duped. If you look at the picture above, the glass of whiskey on the left measured on a kitchen scale as one fluid ounce, but it's actually 1.22 fluid ounces. The glass of maple syrup on the right measured as one fluid ounce, while it's actually 0.74 fluid ounces. A scale measures weight, whether it’s in grams or ounces. Milliliters and fluid ounces, on the other hand, are measures of volume. A measuring cup holds eight fluid ounces, while a disposable water bottle is typically 500 milliliters. Those numbers tell you how much space exists 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 fill it back up with maple syrup, olive oil, sand, 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 aren't the same thing. So, what does the fluid-ounces feature do? Well, it’s just weighing whatever you put on top and then assuming that the object is the density of water. 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 — measuring cup, a jigger, or even a tablespoon (one tablespoon is half a fluid ounce). Other liquids that will offset your measurements include oils — which are a bit lighter than water — and milk, peanut butter, and honey, which are all heavier than water. The best rule of thumb to use is: if measuring a liquid, use a glass measuring cup and forget the scales.