A bronze market weight with the letters lb cast into it hangs from the hook of a Roman steelyard; a glint runs across it and it sways slightly. The beam first tilts and then settles as the counterweight slides along the scale to the mark 1. At the lower right, barley grains drop one after another into an iron pan — the grain, seven thousand of which make up a pound.
Quantity data sheet
mass · trade measure sheet 1/1 · rev. 2026-09Pound lb · avoirdupois
Unit outside the SI · quantity: mass; exactly 0.453 592 37 kg since 1959
The standard that burned with Parliament
In October 1834 workmen were burning old wooden Exchequer tally sticks in a furnace of the Palace of Westminster; the fire spread to the building, and by morning only the walls of Parliament were left standing, with the melted standards of the yard and the pound under the rubble. The commission charged with restoring them decided not to repeat the old troy pound but to make the trade pound the principal one, and in 1844 a new platinum cylinder was cast by which Britain weighed its goods for more than a century.
Since 1959 the pound no longer rests on its own cylinder: it is defined as exactly 0.453 592 37 kilogram, and the kilogram, since 2019, through the Planck constant, so every bag of flour marked “1 lb” is ultimately weighed by a quantum constant.
This sheet puts five pounds from different eras on the same scales, explains why an eight-digit number has such an odd tail, and why the troy ounce is heavier than the trade ounce while the troy pound is lighter.
The symbol lb is written in lower case and without a full stop, and it does not change in the plural: 5 lb, not 5 lbs. The letters come from the Latin libra, which also gave money its sign £; the force with which a pound presses on its support is written separately, lbf, and is not mixed up with mass.
The avoirdupois trade pound with the troy pound, still used to weigh gold: the troy pound is lighter, 373 grams, although its ounce is heavier than the trade ounce. Nor should it be confused with the metric “Pfund” of half a kilo, still used at markets in German-speaking countries, or with the pound sterling, which was born as a pound of silver but has long weighed nothing.
01 · Definition
kilogram · grain · ounceThe avoirdupois pound is a unit of mass equal to exactly 0.453 592 37 kilogram by the international agreement of 1959. It divides into sixteen ounces and into seven thousand grains, and fourteen pounds make a stone, in which people in Britain still give their body weight.
The word avoirdupois is Old French for “goods of weight”: that was the name for goods sold by weight, as opposed to precious metals and medicines, for which a separate troy reckoning was kept. The troy pound divides into twelve ounces and 5760 grains, and the grain is the same in both systems, which makes converting between them easy.
The pound is a unit of mass, not of force, though the two are confused in everyday life: the weight a spring balance shows in pounds is really a force, and in engineering it is written with a separate symbol, lbf, the pound-force, equal to 4.448 newtons at standard gravity.
How many grams are in someone else’s pound
A pound of the chosen era goes on the left pan of an equal-arm balance, and on the right go weights from a metric set adding up to the number of grams you set with the slider. The beam swings and settles for real, as if on a spring, and the pointer at the top shows which way it leans: until the pointer stands at zero, the pound and the weights are not equal.
Which is heavier: a pound of gold or a pound of feathers
An old trick question: a pound of feathers is heavier, because feathers are weighed by the trade pound of 453.6 grams and gold by the troy pound of 373.2. Yet an ounce of gold is heavier than an ounce of feathers, 31.1 grams against 28.3, because the troy pound divides into only twelve ounces and the trade pound into sixteen, and the larger share of the smaller whole turns out to be the heavier.
02 · Conversion
units · pounds of nations · the troy systemFrom ounces and stones to kilograms
Enter a mass in any unit, and the sheet will show it in kilograms, pounds, ounces, stones and grains, convert it into the pounds of different countries and eras, and compare the trade system with the troy one. The mass is the same in every tab; only the measure changes.
Rule of thumb: a kilogram is two pounds plus ten per cent, and a pound is a little less than half a kilogram.
The pounds of different countries are rounded to the gram: one town often had two or three pounds at once, for apothecaries, for spices and for livestock, and the standards changed from century to century. Tons differ too — the short American ton of 2000 pounds and the long British ton of 2240 — and before converting it is worth checking which one is meant.
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04 · Measuring instruments
steelyard · beam · spring · riderFour ways to find out how many pounds
One weight for every load
The Roman steelyard makes do with a single weight: the load hangs close to the pivot, the counterweight is moved along the long arm, and the farther it goes, the more it balances. The scale on the arm is marked straight in pounds, so a market needed one instrument rather than a box of weights, which made the steelyard the traders’ favourite from Pompeii to the nineteenth century.
Comparison, not measurement
An equal-arm balance measures nothing by itself: it only answers which is heavier, the load or the weights, but it answers the same at the equator and at the pole, because gravity acts on both pans equally. That is why the standards of the pound were compared this way, while the spring, which stretches differently at different latitudes, was trusted only for shop weighing.
A pound that depends on latitude
A spring responds to force, not mass, and one and the same pound stretches it more at the pole than at the equator, by about half a per cent. The scale of such a balance is marked in pounds only by convention, converting the force at standard gravity, which is why in engineering it has its own unit, the pound-force, and in trade it is verified where it is installed.
Fractions of a grain without small weights
On precise apothecary balances the smallest weights are replaced by a rider — a tiny wire hook moved with tweezers along a graduated beam. Moved by one division, it acts as a tenth of a grain, so the apothecary weighed six and a half milligrams without holding a single weight smaller than a grain.
05 · Writing rules
symbol · mass and force · the troy systemFive lb, not five lbs
The pound symbol takes neither a plural nor a full stop, mass and force are written with different symbols, and troy units are marked with the letter t so that gold is not weighed with a trade ounce.
The first two lines on the right inflect the symbol like a word and add a full stop, the third writes eleven stone four pounds as a decimal, although 11.4 stone is 11 stone 5.6 pounds, and the fourth weighs gold with a trade ounce and loses almost a tenth. The fifth calls a force a mass, making an engine look like a load, and the sixth rounds the exact value and is out by ten per cent.
06 · Neighbouring units
stone · grain · pound-forceAbove the pound come the stone of fourteen pounds and the hundredweights, below it the ounce and the grain, shared by the trade and troy systems, and to the side stands the pound-force, on which pressure in psi and torque in foot-pounds rest.
Fourteen pounds, in which the British still give their body weight.
A barley corn, shared by the trade, troy and apothecary systems.
The force of a pound in standard gravity, and pressure per square inch.
07 · Historical section
archive · from the libra to the Planck constantHow there came to be many pounds, and then one
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From the Roman libra to the metric Pfund
The height of each bar is the mass of a pound in grams, from zero to five hundred: the Roman libra, the Tower pound, the troy, the trade and the metric pound of the German lands. In two thousand years the pound grew half again as heavy; the frame visits the bars in turn.
The metric Pfund of half a kilo is the only one of the five with a round number of grams, and that is why it survived the metric reform at markets where traders did not want to part with the word.
A number divisible by seven
When the pound was tied to the kilogram in 1959, the measured ratio of the British and American standards gave a number of about 0.453 592 4, and it could have been rounded at any digit. The choice was 0.453 592 37 — eight significant figures, the last of which, the seven, is there not for accuracy but for arithmetic: only with it does the number divide by seven exactly.
And since a pound is exactly seven thousand grains, the grain came out as an exact finite number, 64.798 91 milligrams, and with it the troy ounce, the apothecaries’ dram and the scruple became exact too. One digit in the eighth place spared a whole family of old measures from endless fractions they would otherwise have had to convert with rounding.
After that the platinum cylinder of 1844 became a museum piece: its mass no longer defines anything, and if it loses a few micrograms the pound will not change, because it now rests on the kilogram, and the kilogram on the Planck constant.