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The wikipedia article for the cup unit identifies two different measures used in the US: a legal cup of 240ml and a customary cup of ~236.588ml.

Similarly, the tablespoon and teaspoon articles describe traditional measures (~14.8ml and ~4.93ml respectively) and measures used in "nutritional labeling" (15ml and 5ml).

Are measuring spoons in the US calibrated to the traditional, or to the legal definitions of these units?

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both. (within acceptable tolerances, as I doubt they're that well calibrated, or that usage is that precise) –  Joe Nov 7 '12 at 11:41
    
In case it seems odd that I would need to known this; I'm doing calculations based on a large number of recipes and I want to introduce as little systematic bias as possible. –  Chris Steinbach Nov 7 '12 at 11:43
    
Then you likely need to look at when and where the recipes were created & tested, and how/if the 'legal' definitions made it into general (non-commercial) usage. –  Joe Nov 7 '12 at 11:50
    
To put things into comparison -- during my undergrad (civil engineering), there were lots of different ways to calculate the strength of a steel-reinforced concrete beam, with results that varied somewhere on the order of 5% of each other ... my professor showed that if you consider that the accuracy of steel placement is within 1" in many cases, that threw the strength by ~20%. So, my point is -- how someone fills the cup is likely of more signficance than which of the two sizes of cups they use. (especially for flour ... but then you also have regional differences in production). –  Joe Nov 7 '12 at 11:51
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Unless it's coming from a recipe to be used in bulk commercial processing, I doubt they would use the 'legal' measures. My only other suggestion might be to try contacting the various 'test kitchens', and ask them what they use. Among other things, the commercial test kitchens verify that cookbooks recipes work as listed, and so they might have to deal with this issue. –  Joe Nov 7 '12 at 12:40
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Great question Chris! I'm a US baker who has been baking for a LONG time and never gave it any thought whether my measuring utensils are "legal" or "traditional" - they say "1 cup", "1 Tsp", "1 TBSP", etc.

In practice, all this has meant to me is that when my US recipes call for 1 cup of flour, I measure 1 level measuring cup of flour. Same for teaspoons and tablespoons. My European cookbooks refer to ingredients in weight (250 grams, for example). In those cases, I would either convert those measurements into my more familiar cups, tsp's, TBSP's, etc., or use another set of utensils with measurements marked on the side.

While I've used the written measurements for a guideline, I've always also depended on the preparation instructions to determine the correct consistency of the dough or batter. This is particularly true for flour and liquids.

Regarding uniformity in the measurement tools, unfortunately for the home cook, there is no one monopoly. Every grocery store, dollar store and cooking store has an assortment of measuring cups and spoons. Some of the better measuring cups, like those from Pyrex, have volumes written on the side of the cup to use as guides.

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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.

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I wouldn't be confident the teaspoons and tablespoons were all actually 5 and 15 mL; they may just be rounding. The same might even happen for cups. –  Jefromi Nov 8 '12 at 5:27
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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!

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