Often French recipes call for « un verre » of some liquid like water, wine, or oil. A verre is a glass, as in drinking glass. After 40 years of of cooking in France, I've never in my life understood what quantity of liquid that meant. At one time I convinced myself it meant approximately "one cup" or one standard U.S. cup, which is eight fluid ounces. I now think that was wrong.
I've also seen French recipes that call for, say, « une tasse à thé » of rice or of some liquid. It seems to me that a teacup would more closely approximate the American 8-oz. cup. As CHM wrote in a comment my blog post yesterday, the verre in French recipes is probably based on the size of the little glasses that Dijon mustard is sold in at the supermarket. Everybody in France had a collection of them at home. Some recipes actually call for « un verre à moutarde » of a given liquid.
Different French verres each holding 150 milliliters — a mystery for the ages
Another theory I've read on French internet forums is that the verre is the size of the little water glasses that used to be on the tables in all the school cafeterias in France. That can be called « un verre de cantine ». Walt and I have a set of those glasses, made by the Duralex company, that we bought when we first got to Saint-Aignan years ago.
So this morning I set out to actually measure such glasses. People on the internet forum at the link above started by saying the verre might be 200 milliliters (ml). That would be very close to the 8 oz. U.S. cup. Then more people responded and a consensus developed around the idea that the volume of liquid represented by « un verre » is probably 125 ml, which is about ½ cup, or 4 fl. oz., in American terms.
Un verre à moutarde, a handy little measuring cup, and un verre de cantine
Somebody pointed out that that's the size of the standard yogurt container in France, and there are many recipes for cakes made with yogurt that use the yogurt "pot" as the measuring implement — 1 pot of yogurt, 2 pots of sugar, 3 pots of flour, etc. Somebody else pointed out that a bottle of wine contains 750 ml and supposedly you can get 6 verres or servings out of it. Do the math: 750 divided by 6 = 125!
When I measured my mustard glass, however, it came to 150 ml, not 125. So I'm now convinced that « un verre » is 150 ml, which is 5 fluid ounces in American terms. By definition, that's 5/8s of a cup. (By the way, do you know that the British fluid ounce and the American fluid ounce are not identical? In British terms, 150 ml = 5¼ fl. oz. Just to complicate things...) Both my « verres à moutarde » and my « verres de cantine » hold exactly that amount.
Un autre verre à eau, the same measuring glass as above, and a standard verre à vin from a French supermarket
So that's my theory and I'm sticking to it. From now on, when I see the measure « un verre » in a French recipe, I'm going to measure out 150 ml, whether in an American measuring cup that's graduated in millilliters or in a mustard glass or school cafteteria glass. My photos here illustrate this.