Weights and Measures
In American cooking, volumes and weights are sometimes used interchangeably, which can be confusing. This usually does not present a problem, since most recipes written for home kitchens use quantities small enough to cancel small measuring errors. Few home kitchens have scales, and those that do will have them stored out of the way as they are used only occasionally, if at all. This doesn't mean that the weight of an ingredient isn't important -- it is. It's just that we have a convenient substitute for an ingredient's weight, and that is its volume.
For example, many recipes call for certain ingredients in ounces (or grams in metric). Yet you don't see cooks using a scale. We use the convenient fact that one cup of water is equivalent to eight (U.S.) fluid ounces. And since other liquids weigh almost the same as water, we can easily estimate using a measuring cup, tablespoon, or other volumetric device.
But what about dry ingredients, such as flour, sugar, salt, or pasta? In the U.S., recipes usually call for volume equivalents, especially for flour. For example, my bread machine was designed for 580 grams, or about 20 ounces of flour to work properly. This is because Americans measure flour in volume rather than weight, and 580g equals four cups on an average day. Every recipe can then call for exactly four cups of flour.
Weight-per-volume estimation works fine under most conditions. But under very humid or very dry conditions, the volume of flour will vary considerably, and the bread dough will become either too dry or too moist. Humidity makes flour expand in volume, and four cups of humid flour will not only contain less than 580g of flour, but will contain more moisture, and the dough will be gooey and will not rise properly.
Bakeries, therefore, always measure dry ingredients by weight. And in metric countries (just about the rest of the world), recipes usually call for solid ingredients by weight (g and kg) and liquid ingredients by volume (ml). Even so, home cooks usually estimate by volume and adjust as they go along. In Italy, where they make a lot of homemade pasta, cooks use a large measuring cup that has two graticules, one in grams (weight) for flour and the other in ml (volume) for liquids.
An advantage to American cooks is the fact that it just happens that the density (weight per volume) of sugar, salt, butter, and oil (but not flour, powdered sugar, or cornstarch) is very nearly that of water; in other words, volume measurements can be used in recipes which may have originally specified weights. That's why American recipes can specify ¼ cup of oil when the original recipe may have specified 60g or 2oz.
(Another advantage Americans have is their stubborn refusal to abandon the avoirdupois system of weights and British measures and switch to metric. Metrication has advantages in science, medicine, and engineering, but is cumbersome in the kitchen and many other places. Also, now that the slide rule has been replaced by computers, what is the reason for metrication anyhow?)
Flour, cornstarch, confectioners' sugar, and other powdered ingredients (such as dried spices) need a separate rule of thumb, which I call the 3/5 rule. For these ingredients, the density is appoximately 9 grams (1/3 ounce) per tablespoon, as opposed to 15g (½ oz) per tablespoon for most other denser ingredients. So for powdered ingredients, remember the 3/5 rule to convert dry to liquid volume (or the 5/3 rule to convert liquid to dry), and also keep a scale handy in case of large quantities (as in bakeries and ships) or extreme days.
Example of 3/5 rule: a cup equals 8 fluid ounces, so a cup of flour equals 3/5 times 8, or about 5 ounces. Inversely, if you want half a pound (8 ounces) of flour, divide the volume by 3/5 (multiply by 5/3) to get 5/3 cups. So half a pound of flour will measure 1 and 2/3 cups, and a pound of flour measures 3 and 1/3 cups, etc.
Exercise: you downloaded a bread recipe from an Italian web site calling for half a kilo (500 grams) of flour. Using the Conversion Calculator, you calculate that 500g equals 2.11 cups liquid. Since we're converting liquid to dry, we multiply 2.1 by 5/3 to get 3½ cups. (Or if you did this enough times you would remember that a kilo of flour equals 7 cups.)
Who said the metric system was simple? Not in the kitchen.