The injuries we do and those we suffer are seldom weighed in the same scales.

AESOP (620–564 bc)



The kilogram is the only metric measure that still relies on a physical object - a metal cylinder called the International Prototype Kilogram (IPK). It was first made in 1799 and designed in London’s Hatton Garden to have the same mass as a litre of water. A litre is a thousandth of a cubic metre, and a metre is one ten-millionth of the distance from the Equator to the North Pole.
The current IPK dates from 1879. It’s made of a platinum-iridium alloy twice as dense as lead and kept under heavy security in Sèvres, on the outskirts of Paris.

By definition, the IPK always has a mass of 1kg. But, in practice, it has to be continually cleaned and compared to other replicas to make sure it hasn’t gained weight from dust or lost weight from wear and tear. The IPK is thought to have changed over the last 100 years by the mass of a small grain of sand. In a world increasingly dependent on precise measurements, this is problematic. In 2014 it is likely to be replaced by a definition based on Planck's Constant - the tiny but unchanging fundamental constant of quantum physics. 


A bird’s feathers weigh more than its entire skeleton.

ALBERT CAMUS (1913-60)

We come into the world laden with the weight of an infinite necessity.

The total weight of all the ants in the world is roughly three times more than the weight of all the human beings in the world.

How To Weigh Your Head

There is no easy way to weigh your head but you could try any of the following methods:

1. Cut it off and place on scales. (This is accurate, but you’ll be dead.)
2. Rest your head on the kitchen scales (Inaccurate - your neck still supports some of the weight.)
3. Archimedes’ solution is to stick your head in a bucket of water - the density of most peoples’ heads is very close to that of water. To measure your head you should fill a bucket brimful with water and set it in a large tray to catch any overspill. Then cut your hair off and submerge your head. Weighing the overspill of water will give you a fairly good approximation of the weight of your head. If you then do the same experiment with your whole body you can compare the amount of water displaced by your head to the amount displaced by your whole body, and work out what fraction of your total bodyweight your head is. (This method is still an - albeit pretty good - approximation. Bone is significantly denser than water, but the air in your head cavities is significantly less dense. Each of these will create an error in the opposite direction to the other; the result will only be accurate if the two effects happen to be equal as well as opposite.)
4. Use a seesaw. This uses another of Archimedes’ principles. Take a seesaw, a tape measure, and a large weight. Lie flat on your back on the seesaw and balance yourself with the weight. Mark opposite the middle of your head. Then bend your head forward on to your chest and measure, d, the distance the centre of your head has moved along the seesaw from the mark. The seesaw will tilt, so slide along until balance is restored. Measure the distance s you have to slide. Divide s by d and multiply by your own body weight to give an approximate weight of your head.


From birth, man carries the weight of gravity on his shoulders. He is bolted to earth. But man has only to sink beneath the surface and he is free.

Sensitive Scales

The world's most sensitive scales (at Caltech) can detect a cluster of xenon atoms a billion, trillion times lighter than a gram. A zeptogram (10-21g) is roughly the mass of a single protein molecule. But to identify proteins by weight, the scales will have to become another 1,000 times more precise, capable of weighing yoctograms (10-24g), or individual hydrogen atoms. If this can be achieved, such devices could be used to diagnose diseases very early by detecting single marker molecules found in a drop of blood.

A newborn blue whale puts on 14 stone in weight every day.

The International Space Station has cost more than 30 times its own weight in gold.