Numbers and Units

Introduction

As science communicators, you will often have to include highly specific information in your written materials. For example, you might be writing a lab report in which you provide numerical details about the method you used in your experiment. There are some rules to follow if you want to do this effectively and achieve your basic goal of enhancing the readability of your work.

In a few cases, you might have to make a judgment call as to which rule should be followed; when working with numbers especially, there are sometimes occasions when rules from different style guides clash. Having said this, if you plan your work with clarity in mind, most sentences can be simplified to follow the important, universally accepted rules. When this is not possible, you should follow the one golden rule: Always be consistent in your style.

Some Basic Rules for Working with Numbers

Rule Example
Spell out small numbers (one to nine). I performed three experiments yesterday.
Use numerals for larger numbers (10 +),except when beginning a sentence. Mike performed 12 experiments.

Fifteen days later, he collected the data.

Use numerals for counts, percentages, decimals, magnifications, and official scales. We found 8 mice, 12 rats, and 37 rabbits. Mammal populations here have grown by about 30% in the last five years.
Use commas to make large numbers easier to interpret (one comma separates each three figures),

but round numbers up/down when very large and use a combination of numerals and words.

There are 5,194 new species of insect discovered each year.

There are approximately 7.5 million insect species on Earth.

Avoid having two distinct numbers written next to one another, most simply by rearranging a sentence. We tested 15 different 19-year-olds [not ‘we tested 15 19-year-olds].
Spell out names and nouns. The First Law of Thermodynamics.
Use numerals for dates. On March 4 [or 4th], we have an exam.
Use numerals for times,

except when writing ‘o’clock’

The exam begins at 9:00 am,

and finishes at one o’clock in the afternoon.

Use numerals for currency references. My lunch cost $4.35, but the chips were only 85 cents.


Always remember the one golden rule of being consistent in your style. If two rules clash in one sentence, you will have to favour one over the other. Make sure you continue to favour that one over the other throughout your text.

Some Important Rules for Working with Units

You should abbreviate units of scientific measurement in your writing, but remember that it is very important to use the correct abbreviations. At best, erroneous abbreviations give the impression that you don’t care about your work, but they also have the potential to confuse your readers.

The Table below shows the correct abbreviations for many commonly used scientific measurements, as well as some of the most important and commonly used rules governing their use in science writing.


Scientific Measurement Abbreviation
Mass (gram, kilogram) g, kg
Volume (millilitre, litre) ml, l
Temperature (Kelvin, Celsius) K, ˚C
Time (millisecond, second, minute, hour) ms, s, min, h
Length (millimetre, centimetre metre, kilometre) mm, m, km
Current (ampere, coulomb, electron, millivolt, volt, ohm) A, C, e, mV, V, Ω
Light Intensity (candela, lux, lumen) cd, lx, lm
Amount of Substance (millimole, mole) mM, mol
Rules for Appropriate Use Example
Do not pluralize unit abbreviations. The chemicals only weighed 46 g.
Only use a period after abbreviations if they end a sentence. The chemicals only weighed 46 g. Compound A was 30 g lighter than Compound B.
Put a space between numerals and unit abbreviations, unless using Celsius, Fahrenheit or percentages The chemicals only weighed 46 g. Compound A was approximately 20% as heavy as Compound B before burning at 21˚C.
Do not capitalize abbreviations

Unless they are named after people (e.g. Kelvin, Celsius) or begin sentences.

The chemicals only weigh 46 g.
Absolute zero (0 K) is around -273˚C.
Grams are abbreviated to ‘g’ in writing.

Video Resource

For a recap and for some extra information about the importance of using numbers and units correctly in your science writing, please watch Grammar Squirrel’s video on the UBC Science Writing YouTube channel.

We then suggest you complete the quick quiz (below) to see whether you have mastered some of the important skills relating to the use of numbers and units in your writing.

Numbers and Units

  • Read the following four statements (A, B, C and D below) and rank these in order from best to worst in the way that they deal with numbers and units. Hint: Think about which of those that contain errors are more problematic than others.

    A) Please pass me those 10 10 ml pipettes.
    B) I need to shake the reagents within 10 after they first mixed.
    C) If we wait longer, there is a 50% chance that the data will be inaccurate.
    D) Inaccuracies would be a problem, especially because these results might influence a decision that affects over 600000 people.

    For questions 7, 8, 9, and 10, simply input the number that indicates your ranking of this statement (1 = best, 2 = second best, 3 = second worst, 4 = worst). Hint: Only assign each ranking score once (e.g. Q7 and Q8 cannot both be ranked 1)