Mnemonics for the S.I. Prefixes

The S.I. System of Weights and Measures may be a bit of a dog’s dinner, but at least it’s a dog’s dinner prepped, cooked, served and — more to the point — eaten by scientists.

A brief history of the Système international d’unités

It all began with the métre (“measure”), of course. This was first proposed as a universal measure of distance by the post-Revolutionary French Academy of Sciences in 1791. According to legend (well, not legend precisely — think of it as random speculative gossip, if you prefer), they first proposed that the metre should be one millionth of the distance from the North Pole to the equator.

When that turned out to be a little on the large side, they reputedly shrugged in that inimitable Gallic fashion said: “D’accord, faisons un dix millionième alors, mais c’est ma dernière offre.” (“OK, let’s make it one ten millionths then, but that’s my final offer.”)

Since then, what measurement-barbarians loosely (and egregiously incorrectly) call “the metric system” has been through many iterations and revisions to become the S.I. System. Its full name is the Système international d’unités which pays due honour to France’s pivotal role in developing and sustaining it.

When some of those same measurement-barbarians call for a return to the good old “pragmatic” Britsh Imperial System of inches and poundals, I urge all fair-minded people to tell them, as kindly as possible, that they can’t: not now, not ever.

Since 1930, the inch has been defined as 25.4 millimetres. (It was, so I believe, the accuracy and precision needed to design and build jet engines that led to the redefinition. The older definitions of the inch simply weren’t precise enough.)

You simply cannot replace the S.I. system, you can, however, dress it up a little bit and call a distance of 25.4 millimetres “one inch” if you really wanted to — but, in the end, what would be the point of that?

The Power of Three (well, ten to the third power, anyways)

For human convenience, the S.I. system includes prefixes. So a large distance might measured in kilometres where the prefix kilo- indicates multiplying by a factor of 1000 (or 10 raised to the third power). The distance between the Globe Theatre in London and Slough Station is 38.6 km. Longer distances such as London and New York, NY would be 5.6 megametres (or 5.6 Mm — note capital ‘M’ for mega [one million] to avoid confusion with the prefix milli- ).

The S.I. System has prefixes for all occasions, as shown below.

The ‘big’ SI prefixes.
Note that every one of them, except for kilo, is represented by a capital letter.

Note also that one should convert all prefixes into standard units for calculations e.g. meganewtons should be converted to newtons. The sole exception is kilograms because the base unit is the kilogram not the gram, so a megagram should be converted into kilograms, not grams. I trust that’s clear. (Did I mention the “dog’s dinner” part yet?)

For perspective, the distance between Earth and the nearest star outside our Solar System is 40 petametres, and current age of the universe is estimated to be 0.4 exaseconds (give or take a petasecond or two).

A useful mnemonic for remembering these is Karl Marx Gives The Proletariat Eleven Zeppelins (and one can imagine the proletariat expressing their gratitude by chanting in chorus: “Yo! Ta, Mr Marx!” as they march bravely forward.)

Karl Marx Gives The Proletariat Eleven Zeppelins (“Yo! Ta, Mr. Marx!)

But what about the little prefixes?

Milli- we have already covered above. The diameter of one of your red blood cells in 8 micrometres and the time it takes light to travel a distance equal to the diameter of a hydrogen atom is 300 zeptoseconds.

Again, there is an SI prefix for every occasion:

The ‘little’ SI prefixes.
(Handily, all of them are represented by lower case letters — including micro which is shown the lower case Greek letter ‘mu’)

A useful mnemonic would be: Millie’s Microphone Needs a Platform For Auditioning Zebra Yodellers.

For the record, GCSE Physics students are expected to know the SI prefixes between giga- and pico-, but if you’re in for a pico- then you’re in for anything between a yotta- and a yocto- in my opinion (if you catch my drift).

Very, very, very small to very, very, very big

The mean lifetime of a Z-boson (the particle that carries the Weak force) is 0.26 yoctoseconds.

According to our current understanding of physics, the stars will have stopped shining and all the galaxies will dissipate into dissassociated ions some 315 yottaseconds from now.

Apart from that, happy holidays everyone!

2 thoughts on “Mnemonics for the S.I. Prefixes

  1. Hallouminatus May 27, 2021 / 4:26 pm

    “foot-poundals (the force exerted by a mass of one pound dropped from a height of one foot, in case you didn’t know)”
    I’m afraid this definition of the foot-poundal makes no sense. One might even say it is “not even wrong”. What force does a mass of one pound dropped from a height of one foot exert? what does it it exert it on?
    A foot-poundal is a unit of energy not of force. If you had said the kinetic energy of a mass of one dropped from a height of one foot, or equivalently the potential energy of a mass of one pound at a height of one foot, you would have described another unit of energy, the foot-pound, which is equal to just over 32 foot-poundals (32.174049 to be precise).
    One foot-poundal is defined as the work done by a force of one poundal (the force required to accelerate a mass of one pound by one foot per second per second) over a distance of one foot. It is also equal to the kinetic energy of a mass of one pound moving at a speed of two feet per second,
    I do agree with your response to the “measurement-barbarians” (mostly Americans as far as I can see).

    • e=mc2andallthat May 27, 2021 / 8:35 pm

      Oops (again). The poundal is the Imperial unit of force, not the foot-poundal. I really should stop trusting my all-too-fallible memory. Thanks for the correction 🙂

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