Imperial Measures of Weight

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	Abbrev.	Metric	Table	Information
ounce	oz	28.35 gram	16 oz = 1 lb	'Ounce' comes from the Latin uncia or twelfth part. The ounce is a sixteenth part of a pound avoirdupois, but it used to be a twelfth part of a pound troy. The apothecaries ounce and troy ounce are now different weights. The abbreviation "oz" comes from 15th century Italian, an abbreviation of "onza". "Oncia" seems to be the modern Italian for ounce (although they use metric measures now, of course) and I suppose that "onza" is a variant of this. The British tend to weigh cooking ingredients rather than use cups (a volume measure). Old fashioned scales had metal weights which were balanced against what you were weighing (see right and below). While we're talking about cooking, oven temperatures are on this website here.
pound	lb	453.59 gram	16 oz = 1 lb 14 lb = 1 stone	A pound is always written as "lb" to prevent confusion with pound money "£". It is very old, traced back to the Roman libra, which explains its abbreviation. It was defined in England since Ethelred the Unready (968-1016). In fact, a pound (money) was originally a pound (weight) of silver, and the symbol for pound (money) £ is a stylised L. Winnie the Pooh wrote a poem about Tigger:           But whatever his weight in pounds, shillings and ounces,           He always seem bigger because of his bounces. Piglet didn't think that the shillings ought to be there, but Pooh explained "They wanted to come in after the pounds, so I let them. It's the best way to write poetry, letting things come." The old sterling money (pre-decimal) was £.s.d or "Pounds, shillings and pence". Pooh sensed that the rhythm required an extra (poetic) foot. The American pound cake was originally so-called, because it used a pound each of its main ingredients, flour, sugar, butter, eggs. It is sometimes baked in a tin with a hole in the middle, called a Bundt pan, or a loaf tin, or a round tin. The Victorians in Britain also baked pound cakes, but added a pound of dried fruits, such as raisins. Now no-one in Britain makes pound cakes but Victoria sponges are common, both as cakes and puddings such as Eve's Pudding. The ingredients are 4oz (quarter pound) of self-raising flour, sugar, butter and 2 eggs. The eggs also weigh 4oz, so the raw ingredients weigh a pound in total. There is a troy pound which is a different weight.
stone	st	6.35 kg	14 lb = 1 stone	The British weigh themselves in stone and the Americans weigh themselves in pounds. There are more advanced conversions on the tables page, but here is a simple conversion table (or see bottom of this page). Stone 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Pounds (lb) 14 28 42 56 70 84 98 112 126 140 154 168 182 196 210 224 238 252 266 280 The stone was originally used for weighing agricultural commodities. Historically the number of pounds in a stone varied by commodity, and was not the same in all times and places even for one commodity. Cities in England would have official standard weights and measures. Merchants weights and measures would be checked against this to make sure they weren't trying to cheat their customers. This is a standard weight from Manchester both dated 1754 and made of cast brass. It is now in the Museum of Science and Industry in Manchester. The plural of stone is also stone. I weigh 10 stone, not 10 stones.

Rough conversion between Imperial and Metric

Britain is supposed to use metric measures. Volumes are not so much of a problem, as we buy bottles or packs of this or that, and we have been allowed to keep our pints of beer! But we can buy meat, and fruit and vegetables, by weight, and so we really ought to learn how to do this. Formal conversions are too precise, so here is a rough-and-ready guide that you might be able to keep in your head.

Below are rough conversion charts. Work out what weight of meat or fruit or vegetables you normally buy, and memorise the metric equivalent.

This is a pair of scales with some apothecaries' weights. These are, left to right, ounce, half ounce, two drachms, two scruples and five grains. 'S' means a half (semi), '¡' means one ('i') and 'ij' means two ('ii'). The ounces are marked with a funny 'Z', to mean 'oz', and they seem to be apothecaries' rather than avoirdupois. I suppose that '3' means drachm and the back-to-front Greek 'E' means scruple. I deduce that the 5 weight is grains, as nothing else is small enough.

This is a more conventional set of scales. I think it is a butcher's scales. You would weigh meat by putting on the flat slab on the left, and the weights on the right. The heavier side would go down, and you would change the weights until you get the right weight. The weights are laid out in front.

These sets of weights are in the Science Museum in London. The description says "Council inspector's testing weights, 1890. Local authorities need a way to check that shops are not short-changing their customers. This case of testing weights was used by London County Council officials who carried out inpsections on shops and market stalls/ Inspectors would call on shopkeepers unannounced to test their weighing scales. Any machine found to be showing short measwures would be seized. Up to 30,000 sets of scales were tested in London each year. The work still goes on today."

Mass v. weight

Mass and weight are not the same thing. Weight is a force, and varies throughout the universe. Mass remains the same. If you were on the Moon, your weight is less, because gravity is less. If you jumped, you would go higher. If you are in free-fall, for example in orbit, then you become weightless. People get confused as an object's mass (in pounds mass) and weight (in pounds weight) have the same numerical value (approximately) when you are on Earth.

In metric measurements, the unit of mass is the kilo and the unit of force is the newton. You would have thought that Imperial units (or U.S. Customary systems) would have sorted this problem out, but no! I was criticised, in my opinion rightly, by an 11 year old correspondent, who said that pound (lb) measures mass, not weight, and so my webpage ought to be titled Mass. The Imperial unit of weight is pound weight (or lb wt). I remember learning this distinction in my Physics class (in Britain) back in the 1960s. However, I defended myself as saying that these pages were really documenting history rather than science, as they were inspired by the tables on the back of my exercise books in the 1950s and 1960s, and these were always described as Weights and Measures (and had lb, not lb wt).

However, another correspondent said "Pounds measure force, not mass (a weight is a force). The unit of mass in the Imperial system is a ridiculous unit called a 'slug', which weighs 32 pounds avoirdupois under standard conditions." There does seem to be a confusion here. Scienceworld.wolfram.com says "A pound is a unit of weight in the British engineering system equal to 4.448 newtons that is commonly used in the United States. Unfortunately, there is a great deal of confusion about the definition of the pound, with many authors using the unit to denote the mass-equivalent of 4.448 newtons, namely 453.592 grams. The use of the unqualified term 'pound' is therefore discouraged, with the more descriptive terms pound-mass (abbreviated lbm) and pound-force (lbf) being preferred. The confusion between pound-mass and pound-force alone is a good reason to use metric units wherever possible."

I suspect that there is a difference between British and American use. The British obviously decided that if they were going to buy apples on the Moon, then they wanted a pound of apples to contain the same number as on Earth, so it should be mass. The Americans said rather that they wanted a pound of apples to feel the same on the Moon as the Earth, so it should be force. Personally, I refuse to buy a slug of apples wherever I am!

A further American comment: "In my experience, the folks who insist that pounds are always a unit of force tend to be the physics students (probably to make F=m*a easier on first-semester students) while the engineering students will use pounds more as a mass. Ultimately, Congress defined the pound in terms of grams (not newtons) in the 1890s, and in the 1950s the remnants of the foot/pound using world got together and standardized on the same number of grams, so 'pound' is definitely a unit of mass. Of course, there's still 'pounds of force' used, abbreviated 'lbf', but that has to be differentiated from 'regular pounds.' The slug, as I mentioned before, is simply a convenient unit to use so that it takes 1 lbf to accelerate it 1 ft/s². " I must point out that Physics students in Britain did not use pound as a unit of force! Nowadays, they all use metric.

Mass can be measured with a balance scales (as long as there is any weight present at all) since this compares the object against known masses (called, unfortunately, weights!) A spring balance will measure weight, as this is what presses against the spring, and will push less under lighter gravity. Old fashioned scales are balance scales and modern scales are spring balances, so that doesn't help us, since they are both marked in pounds!

A poundal is the unit of force required to give acceleration of one foot/sec² to a one pound mass. One pound weight gives acceleration of 32 feet/sec² to a one pound mass on Earth, which is 32 poundals.

One last point: The newton is the metric unit of force. It is roughly the weight of an apple on Earth. Who says that scientists don't have a sense of humour!

Which is heavier, a pound of lead or a pound of feathers? There is an old joke "Which is heavier, a pound of lead or a pound of feathers?" Of course, they weigh the same. But if you said "Which is heavier, an ounce of gold or an ounce of feathers?", then you could claim that the gold is measured in troy and the feathers in avoirdupois, and so the gold is indeed heavier. But a troy pound of gold is lighter than a pound (avoirdupois) of feathers! The weight of feathers in a vacuum is different than those same feathers at STP (buoyant force, Archimedes' Principle). In fact, if you took a pound (mass) of lead and feathers, and weighed them on scales (on Earth, which has an atmosphere), the lead would be heavier. This is because the feathers displace more air (for non-scientists, something similar to what makes things float.) The original joke depends partly on getting weight and volume muddled up. However, we also know that if you dropped a feather, and a piece of lead, the feather would take longer to drop as it would float off to one side then the other. This is because of air resistance. One of the experiments when men went to the Moon (which has no atmosphere, and so no air resistance) was to drop a feather and a piece of lead together. It looked strange to see them drop at the same speed!

What is your weight in bees? Or elephants?

The British TV programme (which stands for Quite Interesting) gave this fact:

Using their conversion ratio, I have added 'bees' as a unit in the convertor at the top of this page. But here is a convertor covering the three typical ways to describe your weight. (It can be used as a useful convertor, ignoring the bees!) I have also added an elephant for comparison, because it seemed a good idea at the time.

© Jo Edkins 2009 - Return to units index