Understanding Units

Understanding Units

One teaspoon of sugar is around five grams.

So it is obvious that one gram is quite a small amount – it’s not difficult to imagine a fifth of a teaspoon and it is not a lot.

But when we are talking about medicines and tests, we often see much smaller quantities mentioned. The standard way of handling these smaller amounts is to keep dividing the units by one thousand.

So if we want units smaller than one gram, the next step smaller would be one thousandth of a gram – which is called a milligram. It gets harder to visualise this – a single crystal of granulated sugar might be a fiftieth of a gram – or twenty milligrams. Standard abbreviation is mg.

The next step is the microgram – which is one thousandth of a milligram. Or you could say it is one millionth of a gram. And any attempt to visualise it starts to fail – but maybe one fifth of the ink required for a single full stop. Standard abbreviation in medicine is mcg (but sometimes you see µg or ug).

When we go beyond medicines and into tests, we need to go much further. And the next step is the nanogram. One billionth of a gram. Standard abbreviation is ng.

Beyond that we have the picogram. One trillionth of a gram. An unimaginably small amount. But something that we deal with all the time. Standard abbreviation is pg.

There are many more steps – but this covers what we usually see when discussing thyroid issues.

In summary, we have:

o... Gram

o... Milligram (1/1000)

o... Microgram (1/1,000,000)

o... Nanogram (1/1,000,000,000)

o... Picogram (1/1,000,000,000,000)

This same system of naming is used not only for weights, but for volume, as in litre, millilitre, nanolitre, picolitre. And mole, millimole, nanomole and picomole (which are effectively counting the number of atoms or molecules).

International Units

Some substances are measured by how much effect they have rather than absolute amount in terms of weight or volume. This is often expressed using the International Unit (IU) - or often simply Unit (U).

Very often, International Units were created before anyone knew what substance was having the effect. And International Units are strictly related to single substances. There is absolutely NO relationship between, say, an IU of vitamin D and an IU of vitamin A.

For vitamin D, one microgram is 40 international units.

But for vitamin A it varies greatly depending on which form of vitamin A you are talking about.

International Units are also used for some blood tests such as TSH concentration are in mIU/L (milli-International Units per litre).


In medicine there have been deviations from science in how some units are printed. These are usually to try to avoid ambiguity or confusion. Some examples:

A litre is usually abbreviated l (lower-case letter ell), but in medicine we see a lot of L (upper-case letter ell).

The prefix micro- is usually abbreviated µ (Greek letter mu), but in medicine we see a lot of mc or u.

An international Unit is usually abbreviated IU, but in medicine we often see U.

The reason behind this is largely typewriters, computers, and typefaces. It is far too difficult to type characters like µ on typewriters or even computers. And the characters lower-case ell, number one and upper-case letter I are far too easy to mistake for each other. (As in Il1.) And how easy they are to distinguish varies depending on which typeface is used. What might be clear on-screen might not be when printed.

Some typical units

Levothyroxine and liothyronine are measured in micrograms (mcg). (For some bizarre reason, a lot of USA documents use milligrams for these. Which means that you need to use decimal points and leading zeroes. But sometimes they use micrograms. So it is a mess. Be careful on the internet.)

Aspirin and paracetamol tablets are measured in milligrams.

B12 blood tests measure in picomoles per litre or picograms per litre.


I want to make this blog as understandable as possible. If you think I could improve anything, feel free to send me a message.

15 Replies

  • Thanks Rod - that is really helpful as I lose the plot after mcg :D

  • I get in such a mess with mcg and mg, but someone pointed out a way to remember them which has stuck.... You can see millimetres. ( milligram) on a ruler, but you would need a microscope to see a microgram.

    No idea who put it this way, but it works for me!

    G x

  • Thank you soo very much Rod. You know (maybe you don't ;-) ) something that just won't go in and stay there... 'fraid this is one of them.

    I'm printing this off (hope you don't mind) so will now have no excuse: thank you kind sir!

    L x

  • Welcome.

    If it helps to avoid one significant mistake, the whole world can print it!

  • Hope your printer copes with the Greek characters!

  • As for Greek characters... ummm - I'll write them in! Thanks again.

  • You can get a lower case mu on most windows PCs with charmap - search for micro sign, select and copy it. In Word you can type 03BC and then press Alt+X (or look it up in Insert Symbol, under Basic Greek)

  • Thank you! U from up this way? ;-)

  • South of the river, east of Heworth

  • Right... there you go, never seems to be anyone from up here!

  • Indeed you can do things like that. But sometimes it changes to something else if you send the text to someone else - maybe in an email or as an attachment. Should be OK but no guarantees!

    Or press the Alt key, keep it pressed, type 0181 using the numeric keypad, leave go of the Alt key.

  • That's definitely true - it was advice for Linda (or anyone else) who wanted to copy or type up this article on their own PC and print it on their own printer. Email is something else entirely. It's also easy to do on your own website, but not when you are posting on someone else's.

  • Rod, your wealth of knowledge always impresses me! Thanks for that info- it does clarify things for an 'arty', non-scientific person like me!

  • Now I'm really messed up...how the 'eck do I measure an eighth of a teespoon (cream) as in serenity cream. Good grief, can it be that strong? Love the blog for all the other measurements, I get so mixed up with all the various ones.

  • What I would do is buy a small scale. There are several available, for example, on Amazon. The ones I mean can weigh a small fraction of a gram. Weigh an empty teaspoon,a full teaspoon, work out the difference. Then divide that by 8. That is how much you need to weigh out.

    Obviously, don't put thee cream directly on the scale - either use a teaspoon or a small piece of plastic film, or whatever is convenient.

    This is one reason many medicinal creams come in tubes - you can then measure them out by inches or centimetres of squeezed out contents! :-)


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