Do cooks in the US measure volume using 'traditional' or 'legal' units?

I took a look at some measuring cups and spoons available via amazon. For the vast majority, there is no information available regarding either the exact measure or the intended market (cup measures differ as much as 44ml between english speaking countries). Nevertheless, there are a few products where the milliliter conversion is provided.

To begin with some good news; every one of 20 measuring spoon sets used a 15ml measure for the tablespoon and 5ml for the teaspoon. This was the case even for brands that sold a traditional US cup measure.

The situation is a bit more messy for cups. Out of 20 cup measure sets, 6 products used the legal cup, 8 used the traditional cup and 6 were metric cup measures (250ml). It's doubtful that US consumers would knowingly purchase the 250ml cups. Remember though that most cups available online don't specify their exact volume, so it is possible that a meaningful percentage of online US consumers are unwittingly measuring in metric cups. Some angry comments in the amazon product reviews appear to confirm this.

It was clearly nonsensical to mention calibration in the question. Apart from the anecdotal evidence indicating the inaccuracy of measuring cups, I also note that the specified milliliter conversion for traditional measures ranged from 235ml to 237ml. The one measuring cup I saw claiming to be "precision engineered for accurate measurement" didn't even indicate whether it was for measuring a US cup.

My conclusion is that traditional, legal and metric cup measures are used in the US although I would expect the proportion of metric cups available in the shops to be much reduced to what I saw online.

What a great discussion. I don't think there is really an answer regarding whether most US cooks use "legal" or "customary" cups as I don't think that most cooks know that there is a difference. A legal cup is only 1.44% larger than a customary cup. This is a very small difference. I'm willing to bet that if a cook measured a cup of flour 10 different times with the same measuring cup that his results of each measurement would vary far more than 1.44%.

My other thought is that if a cook always uses the same set of cups, be they customary, legal, or metric, then their recipes would always turn out because all ingredients would be proportioned the same.

My answer to this problem is to throw away your measuring cups and start using a scale!

I’m a cook but also a trained engineer, a former math and science teacher and a practicing optician. Who cares, right? But let me explain why it’s relevant here.

First, as an educator, your question makes me proud because we emphasize precision and accuracy in the classroom. The comments about using a scale are very sound math and science. So is the comment about ratios—a foundation of good cooking. As an engineer, I’m deeply familiar with the realities of the rubber meeting the road. Even the most precise instruments don’t account for user biases. Thus, we allow tolerances or allowable errors and make allowances for this by design.

If industrial precision and consistency are your goal, having well know, and accurate measuring tools is required and an analytical balance (scale) is the standard. As an optician I refer to an ANSI standard to pass or fail eyeglasses based on fairly loose tolerances.

My cooking experience falls in line with these. Human beings mostly tolerate 10% sensory variations with ease. Those who don’t are unusual and difficult or even impossible to please. The bottom line is that it probably doesn’t matter UNLESS you are packaging something professionally/commercially/medically in which case...USE A SCALE! Bon appetit