Temperatures can present problems when writing an essay or research paper. Do you write them out as words or present them as numerals? And how do you format them? Truth be told, it all boils down to preference, clarity, and whether you’re using a specific style guide. But we do have a few general rules to help you when writing temperatures.
1. Measurement Systems: Celsius, Fahrenheit and Kelvins
One thing you need to know when writing temperatures is which scale to use. The three most common options are Celsius, Fahrenheit, and kelvins:
- Most countries, including Australia, use the Celsius scale to measure temperatures. This system measures temperatures in degrees Celsius (°C). On this scale, water freezes at 0°C and boils at 100°C. This system is also known by the older term ‘centigrade’, but ‘Celsius’ is now standard.
- The Fahrenheit scale was widespread in the first half of the 20th century, with temperatures measured in degrees Fahrenheit (°F). In this system, water freezes at 32°F and boils at 212°F. Nowadays, the Fahrenheit scale is mostly used in the US, although it is also used in a few other countries.
- The Kelvin scale is used in scientific writing. It differs from the systems above in that it begins at absolute zero (so it has no negative numbers). In addition, this system uses ‘kelvins’ or ‘K’, not ‘degrees kelvin’.
We’ll look more at how these systems differ in writing below.
2. Writing Out Numbers: Words or Numerals?
When writing numbers, it’s usually best to write out one through nine (e.g. one, two, three) and use numerals for 10 and higher (e.g. 12, 19, 521). This can vary depending on your style guide, though (e.g. CMoS suggests writing out numbers up to 100), so make sure to check.
So, how does this affect temperatures? As with any numbers, it depends on the context in which you use them. Generally, though, we suggest:
- Using numerals in scientific or technical writing, particularly if you’re also using the degree symbol or an abbreviated unit of measurement.
- Spelling out temperatures as words in literary or descriptive writing, especially for smaller or round numbers.
For instance, if you mentioned a low or round temperature in the dialogue of a novel, you’d usually write out the number and unit of temperature:
‘I think he’s lost his marbles,’ Darcy said with exasperation. ‘He keeps his house at only three degrees Celsius in the winter!’
And if you were dealing with larger or more complex numbers, you could combine a numeral (for the number) and words (for the unit of measurement) in a single sentence. For instance:
‘I wouldn’t touch that,’ she said. ‘It heats up to around 650 degrees Celsius.’
However, in scientific or technical writing, you’d normally use a numeral plus an abbreviated measurement (e.g. ‘°C’ instead of ‘degrees Celsius’):
In this experiment, the water reached 180°C.
The temperature hit a low of 6°F.
As shown, this applies even for temperatures below ten degrees.
In less formal writing, meanwhile, the choice of whether to use numerals or words is broadly a matter of preference. Numerals are clearer in most cases, though, so keep your reader in mind.
3. To Space or Not to Space?
In the examples above, we’ve written temperatures without a space between the numeral and the unit of measurement when using the degree symbol.
However, some prefer to write temperatures with a space. For instance, the Chicago Manual of Style and the APA Style Guide differ on this point:
Chicago Style: The temperature will reach 120°C.
APA Style: The temperature will reach 120 °C.
If you’re not using a specific style guide, either approach is acceptable. Just make sure to apply consistent spacing across all temperatures in a document.
4. The Kelvin Scale: Temperatures Without Degrees
The Kelvin scale is not measured in ‘degrees’. Thus, with this temperature scale, you will not need the degree symbol or the word ‘degrees’:
In this experiment, the water reached 453 K. ✔
In this experiment, the water reached 453°K. ✘
The ideal temperature is 295.15 kelvins. ✔
The ideal temperature is 295.15 degrees kelvin. ✘
Kelvins are often used alongside degrees Celsius in scientific writing, so make sure you don’t confuse how temperatures are written in each system.
4. Capitalising Temperature Units
When writing an abbreviated unit of measurement, whether for Celsius, Fahrenheit or kelvins, you will always need a capital letter:
Water boils at 100°C. ✔
Water boils at 100°c. ✘
The triple point of water is 273.16 K. ✔
The triple point of water is 273.16 k. ✘
However, when writing units of measurement as words, the rules vary:
- Always capitalise Celsius and Fahrenheit, both when referring to the scale (e.g. the Celsius scale) and the unit of measurement (e.g. degrees Celsius).
- Only capitalise ‘Kelvin’ when referring to the scale (i.e. the Kelvin scale). Do not capitalise ‘kelvin’ when referring to a measurement (e.g. 276.3 kelvins).
- Never capitalise ‘centigrade’ unless it is at the beginning of a sentence.
5. Temperatures at the Start of a Sentence
It’s best not to start a sentence with a number. If you find yourself doing this, then, you would usually want to rephrase the sentence.
If you really need to use a temperature at the start of a sentence, though, make sure to write the number out as words:
Original: 6°C is the desired temperature. ✘
Correction 1: The desired temperature is 6°C. ✔
Correction 2: Six degrees Celsius is the desired temperature. ✔
Of the ‘corrected’ sentences here, we would suggest the first in scientific or technical writing, since it sticks to the numeral + unit format.
5. Negative Temperatures
In Celsius and Fahrenheit, you can indicate negative temperatures with the words ‘negative’ or ‘minus’ (for temperatures written as words) or a minus sign (for numerals). For example:
Temperatures were as low as negative thirty-four degrees Celsius.
Temperatures were as low as –34°C.
This is not an issue with the Kelvin scale, which does not go below zero.
6. Adding the Degree Symbol in a Microsoft Word Document
In Microsoft Word, you can insert the degree symbol a few different ways. One option is to use the Symbol drop-down menu. Here are the steps:
- Select the Insert menu on the ribbon
- Click on the Symbol option
- From there, choose More Symbols
- Then, scroll through the options and select the ° symbol
Alternatively, you can use a keyboard shortcut.
If you’re using a PC with a numerical pad, there’s an alt code (shortcut) for the degree symbol: Alt + 0176. Simply hold ‘alt’ and type each number.
If you’re using a Mac, use the Option key instead and type Option + Shift + 8.
7. Consult Your Style Guide
Most style guides include information on temperatures and units of measurement. And if you’re writing for a specific organisation, such as a university or your employer, you’ll need to check their style guide for advice.
For instance, the AP Stylebook requires you to use numerals plus the word ‘degrees’. As such, if you’d been asked to follow AP rules for an article, you would not use the degree symbol in the document.
But however you choose to format temperatures in your writing, make sure to apply a consistent style in each document. And, when in doubt, you can always have an editing expert check your work for errors or inconsistencies.