Cooking is not an exact science. Good food can't happen in a kitchen without the skills and talent of the cook producing it. Whenever it is reduced to a science that machines can produce without the intervention of a human, you get the kind of food that foodies love to hate -- the same foodies who might try to argue that cooking is an exact science.
Almost all recipes are going to survive a close conversion estimate.
If you were to give really precise measurements, such as "3 mm thick onion slices", a recipe reader would think you were just a little too hard-core. And, no one can measure the kind of weird metric conversions that many recipes give you -- 267g of cheese. But then, when dealing with recipes in Imperial, no one would have ever told you to use 9 7/16 oz, either.
For kitchen conversions "close enough" really is "good enough." Yes, the over-achiever in the crowd will leap to point out that, actually, 1 cup equals 236.588238 ml, not 250ml (as is the standard conversion given), but 250ml is only 13ml off (okay -- 13.411762 ml), which is less than a tablespoon. And in a kitchen, there are going to be just as many other variables that you could pick on: how dry / moist is the flour, how accurate is any given kitchen measuring cup to start with, when measuring something like water how closely does anyone really eyeball the "meniscus" (the curved surface at the top of a cup or glass of liquid -- you're supposed to measure based on the bottom of the curve.)
If ever you "calibrated" the measuring cups that were handed down to you from your grandmother -- tested them to find out how accurate they really are -- you may find out that all of them have been off in one way or another all these decades.
And besides, it's true that many good home cooks barely measure anything.
The definition of measurements -- whether Imperial, Metric, or American -- was all done by men and focussed on commerce. Even today, the tools of weights and measures in the UK and in North America are only inspected if they are for commercial use. Household ones are not, and the consumer has to ascertain for him or herself how accurate they are. There's no legal requirement that a set of home scales be accurate. If they can't tax what you're making, they don't seem to be interested.
In the UK, commercial measurements -- pints, pounds -- just got applied to the kitchen. It took a woman, Fannie Merritt Farmer, to come up with standardized measurements for the kitchen in North America.
American MeasurementsKitchen scales are very uncommon in North America; in general, only foodies or people following diets tend to have them.
Before the American Revolution, America had welcomed settlers from England, Scotland, Ireland, Wales, the Netherlands, France, Switzerland, Germany, Spain, etc. People would settle amongst their own in various settlements and various colonies and of course would keep on using the measurements that they knew. There was no central authority; each yet-to-be-state was its own colony. Consequently, a bushel of oats in one colony often ended up weighing more or less than a bushel of oats from another. And there was no universal metric system to turn to, as it hadn't even been invented yet -- though efforts at official standards in France evolved slowly through the 1600 and 1700s, official standards were only passed in France on 10 December 1799 (529 years after they were passed in England, in 1270.)
America based its gallon on the British "wine gallon" -- a smaller gallon measurement that was used in the wine trade. Thus the divisions of a gallon -- a quart, a pint -- ended up smaller in America. An American pint of 16 oz is what the Brits used to call a "wine pint" (a proper Imperial pint is 20 oz.)
In the first half of the 1800s, it was common to see American recipes using weighed dry ingredients, and liquids measured in pints. For instance, to make pound cakes, Americans used to measure by weight 1 pound each of butters, eggs, flour and sugar.
Fannie Farmer and her Boston Cooking School firmed up the convention in North America of measuring by volume in cups. But even Fannie Farmer gave Americans recipes that measured in weight a lot of things that Americans now measure in cups instead.
There is actually an internal logic to North American measurements:
- 3 teaspoons = 1 tablespoon
- 4 tablespoons = 1/4 cup
- 4 x 1/4 cup = 1 cup
- 1 cup = 1/2 pint
- 2 x 1/2 pint = 1 pint
- 1 pint of water weighs 1 pound, and is 3 cubic inches (e.g. 3 x 3 x 3)
Imperial MeasurementsImperial measurements are used in the United Kingdom, Canada, Australia and other commonwealth countries.
In 1824, the British based their gallon on the volume of 10 pounds of water, and ended up with 20 oz per pint. Interestingly, at 34.7 cubic inches, this is very close to what used to be called the "ale pint" of 35.25 cubic inches, and 1/8th of an "ale gallon."
So you could say American pints got based on wine, British ones on beer.
There are two versions of British measurements:
Avoirdupois MeasurementsThe name comes from Old French "aveir de peis ", meaning "goods of weight."
- 1 grain = 64.798 91mg (the weight of a grain from the middle of an ear of barley)
- 1 oz = 437.5 grains (28g)
- 1 pound = 16 oz
- 1 stone = 14 pounds
- 1 quintal (hundredweight/ CWT) was 100 pounds in America; changed to 112 pounds in the UK
Troy MeasurementsTroy measurements came into use from trade fairs held at Troyes in Champagne, France. These used for measuring precious metals. Because they were aimed at measuring small quantities with precision, the Troy system came to be used by apothecaries for drugs as well.
- 1 grain = 64.798 91mg
- 1 oz (abbreviation: ozt) = 480 grains (31g)
- 1 pound = 12 oz
The Troy grain was the same as in the Avoirdupois scale. Thus 1 oz was 1/12th of a pound in this scale.
Apothecary measurements introduced two items to the Troy system:
- 1 scruple = 20 grains
- 1 dram = 3 scruples (there was also a dram in avoirdupois, but it was 1/8th oz, instead of 1/16th of an oz)
Food Measurements in CanadaFood measurements in Canada are a legally metric, but historically in practice a blend between Imperial and American measurements.
A pint of beer legally in Canada should be an Imperial pint of 20 oz (568 ml), but the customer is often given an American pint of 16 oz (473 ml) instead. In Canada, that 16 oz serving of beer should be called a "sleeve" or a "glass" instead. 
Percentages as MeasurementsGiving measurements as percentages is largely a baker's tradition.
Percentages are easy to scale (within reason -- when you get really small, it can be hard to translate into something measurable, such as 1g, unless you go metric), but it is easier to scale than half of 3/4 cups. It is easier to manage when firing up a batch of dough that has 2000 pounds of flour in it. Baker's percentages are proportional, not based on totals. Thus, if 100kg of flour is being used and the directions call for 65% water, that means to 100kg of flour, add 65 kg of water.
Baker's math is also taught at professional baking schools in North America.
It can be expressed in percent or, in America, "per CWT." CWT stands for "hundred weight", as in the American definition of it, 100 pounds, as opposed to UK 112 pounds. C comes from the Roman numeral for 100, C
Metric Kitchen Measurements TodayMetric conversion in North American has gotten very confused.
In the metric system, ml (millilitres) are used to liquid measurements; grams are used for dry measurements.
But, the US Metric Association (USMA) seems to be advocating measuring dry goods in mls, and those who are purporting to give recipes in metric often give dry measurements in ml, which is wrong, confusing and useless. Canadian government sites also will give 1 cup of flour as being 250 ml, which is wrong: it's 125g. With even respected sources giving these bizarre conversions, the people who are resisting switching might just as well keep on resisting. There seems little point in going through the effort of switching to the metric system to be standard with the rest of the world, if you're doing it wrong and putting in a non-standard system that the rest of the world still won't understand.
Some recipe writers, though, who do bother to give metric now seem to be doing it properly, in weight.
That being said, metric system countries aren't purely metric, either. Even people in metric countries aren't purists, and don't sweat the small stuff. They still use volume for very small measurements of dry ingredients, such as a few tablespoons. Which makes the North American approach even more bizarre. While giving weight measurements in liquid volume, they go all major about pretending tablespoons no longer exist, and direct North Americans to use 5ml of baking powder, which turns North Americans off. Whereas in metric countries, a recipe writer would just come right out and say "1 teaspoon."
Dutch recipes still use teaspoons (koffie lapel), tablespoons (eet-lapel) and sometimes cups. In the Netherlands, people still order a "pintje", a "small pint", which in metric is 250 ml (1/2 American pint / 8 oz) But when Europeans use teaspoon, tablespoon, cups, it's actually less precise than in North America, because they never had Fannie Farmer to standardize them all. They just use their favourite cup or spoon out of the cupboard.
Some purist Brits, though, get shirty if you specify 4 tablespoons of butter -- they think that's too much to have to measure and would rather weigh it.
There are still some old fractional oddities in metric countries -- jam jars in Belgium, for instance, that at the bottom say "3/8 litre" (instead of spelling it out in decimal as .375.)
Today only four countries still don't use the metric system exclusively as weights and measures: Burma, Liberia, the United Kingdom, and the United States of America (the UK continues to use miles, as of 2007.) And in countries that have officially switched "recently", such as Canada, Australia and New Zealand, people are still in their kitchens measuring exactly as they used to. Weighing in pounds, or measuring in cups. Given the botched conversion job being managed in Canada, for instance, a wag might say they are just as well to keep on ignoring it all.
Some US packaged commercial food now gives its weight / volume with a metric equivalent e.g. 12 fl. oz / 354 ml.
Imprecise Measurement Terms"Butter the size of an egg" is an old favourite measurement often quoted with amusement. It would have been very imprecise, given that different breeds of hens lay different sizes of eggs, and that even the same hen can lay eggs of different sizes.
But, some very imprecise measurements still survive, both in metric and Imperial measurement recipes:
- can numbers
- half a container
- hot oven
- knob of butter
- moderate oven
- stick of butter
In Norway, 1 pound (pund) was 498g (after 1683.)
In Sweden, 1 pot was close to a quart (966 ml.) A skålpund (scale pound, indicating that this pound could be measured on a hand-held balance scale), was 425g (15 oz.)
Measurements in AmericaThe only official legal measurement system in America is actually the metric system -- passed by Congress on 28 July 1866.
The law made the metric system official, but not mandatory: "It shall be lawful throughout the United States of America to employ the weights and measures of the metric system; and no contract or dealing, or pleading in any court, shall be deemed invalid or liable to objection because the weights or measures expressed or referred to therein are weights or measures of the metric system."
Congressman John A. Kasson of Iowa hoped that by making the metric system legal, its usage would spread to the point that the public would accept its being made mandatory: "The interests of trade among a people so quick as ours to receive and adopt a useful novelty, will soon acquaint practical men with its convenience. When this is attained — a period, it is hoped, not distant — a further act of Congress can fix a date for its exclusive adoption as a legal system. At an earlier period it may be safely introduced into all public offices, and for government service." (Report of the Committee on Coinage, Weights, and Measures. 39th Congress, 1st Session, H.R. Report No. 62, 17 May 1866.)
In 1893, the "Mendenhall Order" in the United States (coming into effect 5 April 5 1893 on the initiative of Thomas Corwin Mendenhall, Superintendent of Weights and Measures) gave the legal definition of American pounds, feet, yards, inches, etc, in terms of their metric equivalents.
Measurements in the United Kingdom1270. Edward 1 of England issues the "Assize of Weights and Measures", creating standards that would last, with few modifications, until 1824. Scotland adopted the same measures in 1707. Standardization, though, never meant rationalization.
Measurements in FranceIn France, surprisingly, despite the early French passion for a centralized, rigidly-controlled state, there remained no standardization up until the Revolution. Measurements differed from province to province, and even from district to district within a single province. French upper-classes, though, never placed the emphasis on commerce that the Brits did, leading to Napoleon famously dismissing the British as a "nation of shopkeepers."
Measurements varied throughout France and over time, but as a generalization:
1 litron (litre) was approximately 28 US oz (830ml) or about 75% of what came to be defined in 1824 as an Imperial quart/
1 livre (pound) was 480g
1 quintal was 100 of those pounds
It is perhaps because of the confusion in France that out of France was born the dream for a universal and rational system. In April 1789, just a few months before the French Revolution started, a man named Joseph-Jérôme Lalande proposed to that National Assembly that national standards be established based on the existing weights and measures used at the time in Paris (quelle surprise.) In March 1790, though, the National Assembly decided instead to strike up a commission to investigate the metric system (see below.)
Measurements in the Netherlands1 oz was approximately (30.881g, compared to 28.34g), with 16 oz making a pound of 17.4 oz 494.09g.
But, in Amsterdam, a pound was 404g, and in Den Haag, it was 469g.
1 pint was 600ml (20 US fluid oz), the same as a British pint.
Napoleon compelled the Dutch to invite his brother, Louis I Napoleon Bonaparte (2 September 1778 to 25 July 1846) to become their king, which Louis modestly accepted on 6 June 1806. For an imposed King, he doesn't appear to have been that bad. He attempted to learn Dutch, and even changed his name to Dutch, King Lodewijk I, and brought in a series of reforms that actually promoted Dutch interests (in fact, only 4 years later, in 1810, Napoleon booted him out for putting Dutch interests above French ones.) He introduced the metric system in the Netherlands. It was discontinued in 1813, but in 1820 officially introduced again.
Measurements in ItalyThe word for a pound in Italian is "libbra."
Unsurprisingly, weights and measures varied wildly everywhere throughout the peninsula before the introduction of the metric system:
- 1 Pound (Rome) 339g
- 1 Pound (Tuscany) 348g
- 1Pound (Ferrara) 346g
Everywhere, though, pounds were divided into 12 oz.
People working from very old Italian recipes, unless they are sure of which pound is meant, tend to estimate 350g as a compromise.
- oz: 1/12th pound (approximately 29g based on a 350g pound)
- dragma ("dram", aka "ottavo"): 1/8th of an ounce (approximately 3.6g)
- scrupulo: 1/3 of a dragma (approximately 1.2g)
- grano (grain, aka "pizzico"): 1/24 of a dragma (approximately .05g)
Other old Italian measurements included:
- boccale (bottle): about 1 litre
- fiasco (flask): 2 "boccali" (2 litres)
- mezzetta (halves, aka "foglietta"): 1/2 of a "boccale", about 1/2 litre
- quartuccio (quarter): 1/4 of a "boccale", about 1/4 litre
Metric HistoryIn France, the metric system was made compulsory in 1795. On 12 February 1812, Napoleon abolished the use of the metric system (he had also abolished the French revolutionary calendar, which was "close" to being metric, in 1806.) Still, the metric system had been spread to some other countries by French conquest, and the French abandoning it made it actually more acceptable to those countries.
The Low Countries (Netherlands and Belgium) made it mandatory in 1820; in 1830, when Belgium became independent, it made the metric system obligatory as well.
- 1840 Metric reinstated as compulsory in France in 1840.
- 1862 Mexico switched to metric.
- 1864 The metric system became legal in Great Britain, but a subsequent bill to made it mandatory as well never made it through the final stages in Parliament
- 1866 America made the metric system legal but not mandatory
- 1868 Germany unified and made metric mandatory. It was made mandatory in the same year in Bolivia and Switzerland as well.
- 1875 The International Metric convention was signed by 17 countries: United States of America, Germany, Austria-Hungary, Belgium, Brazil, Argentina, Denmark, Spain, France, Italy, Peru, Portugal, Russia, Sweden and Norway, Switzerland, Ottoman Empire, and Venezuela. Called the "Treaty of the Metre" and signed in Sèvres, France, it also established the International Bureau of Weights and Measures (BIPM.)
- 1889 Sweden switched to metric.
- 1907 Denmark switched to metric.
- 1929 China switched to metric.
Some metric terms that just never got used are "myriametre" (10,000 metres), centare (square metre) and "quintal" (100 kg.)
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-- William Shakespeare (26 April 1564 – 23 April 1616. Twelfth Night. Act II, Scene 5.)