°C and C are not the same!

Sometimes one has to write to the papers!

Sometimes one has to write to the papers!


Sometimes I am unable to stop myself writing to the papers.

Some issues – such as people not using measurement units correctly  – are just too important to let pass.

And people referring to temperature units incorrectly induces apoplexy!

For the record, the degree Celsius is an SI unit for temperature: the degrees C********e and F********t are not.

Their use in everyday language is understandable – many people use the F-word occasionally – and in the correct context, it gives no offence.

But for newspapers and media outlets to do so is outrageous!

And using the abbreviation C instead of °C is just wrong.

As I wrote to The Guardian recently:

Dear Guardian,

The measurement system that underpins all of our physical measurements of the world around us is called the International System of Units, widely referred to as ‘the SI’.

It is a staggering achievement, used daily by hundreds of thousands of scientists and engineers.

It provides a standard way of comparing measurements around the globe and of referring to those measurements. So why has The Guardian invented its own system of units?

To refer to a temperature of 25 degrees Celsius, the standard abbreviation is 25 °C. However The Guardian routinely refers to this as 25C, using the symbol ‘C’ which refers to the SI ‘coulomb’, an amount of electric charge. Why?

You might argue that your meaning is clear in context. And generally it is. But why be wrong when you can be right so easily?


Michael de Podesta

National Physical Laboratory.

P.S. In MS Windows™ systems, the degree symbol is [ALT] + 2 + 4 + 8 on the number keypad and in MacOS the degree symbol is [ALT] + [SHIFT] + 8. In iOS, on numeric keypad use a long press on the zero key to reveal the degree symbol.

P.P.S. There should also be a space between the number and its unit, but I didn’t want to mention that in case you thought I was being pedantic.

More seriously, reporting measurements in the correct units aids clarity of understanding and establishes the basic competence of the author.

Reporting, as The Guardian did this week, that:

“the 2016 temperature is likely to be 1.25C above pre-industrial times, following a warming trend where the world has heated up at a rate of 0.18C per decade.”

merely establishes that the writer knows nothing about measurements.

This is not a matter of style, it’s a matter of just being wrong.


[October 5th 2016: Weight this morning 73.5 kg: Anxiety: Low. I don’t know why, but I just felt OK today :-)]

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6 Responses to “°C and C are not the same!”

  1. Mary Anne White Says:


    Mary Anne White, O.C., FRSC, PhD, DSc(hc) Harry Shirreff Professor of Chemical Research Department of Chemistry Dalhousie University 6274 Coburg Road P.O. Box 15000 Halifax NS B3H 4R2 Canada Tel: (902) 494-3894 Email: mary.anne.white@dal.ca Research Website: http://mawhite.chem.dal.ca/ See also: http://www.physicalpropertiesofmaterials.com

    Address for Courier: Chemistry Building, Room 212 6274 Coburg Road Dalhousie University Halifax, Nova Scotia B3H 4R2 Canada


  2. edaviesmeuk Says:

    Well said. The failure of even semi-technical publications in relevant areas to correctly distinguish kW and kWh is a major source of apoplexy for me.

    Pedantically, though: “Weight this morning 73.5 kg”. Shouldn’t weight be measured in newtons? Alternatively, “Mass this morning…”.

    • protonsforbreakfast Says:

      I sympathise with the kW and kWh issue – it is infuriating.

      But I disagree with the view that units of weight are newtons. I also disagree with Wikipedia and the National curriculum. Weight is measured in kilograms.

      Operationally you can verify this by tying to buy 5 newtons of potatoes. Let me know how you get on.

      Philosophically, the reason I think this is that ALL weighing is currently traceable to the mass of the international prototype of the kilogram. So expressing weight in kilograms is functionally correct.

      If I expressed my weight in newtons, you would not know whether I was ‘heavy’ or ‘light’ unless I also told you the local value of gravity. Since this varies by 0.7% around the globe – there would be real uncertainty in what I meant (about 0,5 kg in the case of my weight) .

      By telling you my weight in kilograms I communicate the physical state of my body – which is my aim. Alternative, for potatoes, weighing kilograms will let you know how much ‘potato matter’ you will receive whereas weighing in newtons does not.

      • edaviesmeuk Says:

        I think it’s much simpler than that: in colloquial English the word “weight” is used for the concept which physicists would call “mass”. I wouldn’t ask for 5 N of potatoes because it’s not usually particularly relevant what their weight (in the physicists’ sense) is.

        Even if they’re traced back to the kilogram, some scales measure weight and some measure mass. I.e., some rely on being in a particular gravitational field whereas others just need the field to be uniform enough. That spring and load-cell type scales would give slightly different “weights” in different parts of the world would be regarded as a (small) flaw by most people which supports the idea that they really mean mass.

        So, yes, your statement is perfectly good normal English but problematic in a technical context, which I think this blog is. Another context where it ought, in my opinion, to have been handled more carefully:


  3. Edmond Says:

    Did I hear about the gnome experiment from you chaps? If not, I think you really need to refer to this:

    I think Michael has been hoisted by his own petard, but it is a petard that belongs to all of us- if we want to leave stones and pounds behind, we have no choice but to use kg. The way I see it, Michael is just telling us what reading he’s getting from his scales, rather than the SI unit for his mass. It’s a bit like our car speedometers, which are as far as I know just measuring the rotational speed of the wheels without regard for their changing circumference as the tyres wear. But if he says he’s doing 50km per hr, we know what he means, near enough.

    Which brings me to a delightful thought experiment I saw on XKCD- the idea that fuel consumption expressed as litres per kilometre must have a meaning, since it’s a volume divided by a length. The fuel consumption of a car is expressed as an area!
    It’s the cross sectional area of the imaginary string of fuel that the car is consuming as it goes along, as if sucking on a very long piece of spaghetti! The thicker it is, the worse the fuel consumption.

    • edaviesmeuk Says:

      Cute little video. Was hoping for pictures of the gnome around the world, though 😉

      It’s interesting how it uses “weight” in both the colloquial senses: at one point saying that the gnome’s weight varies around the world (force) and another saying it’s constant (mass). OK, that’s from a (presumably) native German speaker but neither form would be surprising from an English person, either.

      However, I’d point out the difference in weight (force) could register on ordinary scales though it wouldn’t be easy to measure very accurately. E.g., the effective force of gravity in Singapore is 9.781 N/kg whereas in Helsinki it’s 9.819 N/kg [¹] so a difference of 38 mN/kg which would register on a load-cell type scale as just over 3.8 grams with a 1 kg test mass. My kitchen scales registered the 1 g increments as I put fussili pasta twists on top of a 972 g jar of rice.

      [¹] https://en.wikipedia.org/wiki/Gravity_of_Earth#Comparative_gravities_in_various_cities_around_the_world

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