When do you write out numbers (and why do they call it “maths” in England)?
How to Write Fractions
Fractions are even more complicated than whole numbers.
First, you have what some sources call the “simple fractions.” These include ½, ¼, ¾, and so on, and most sources want you to spell out these fractions in text. In most cases, you use a hyphen with the words, so you’d write “two-thirds” and “three-quarters.”
However, style guides still differ once we go beyond simple fractions, and this time it’s the Associated Press that wants you to spell out everything. They recommend spelling out all fractions less than one, but the Chicago Manual of Style wants you to convert anything that isn’t a simple fraction to decimal form. So you’d write “five-sixteenths” for the Associated Press, but you’d convert it to 0.3125 for the Chicago Manual of Style.
Of course, the Associated Press also says that if you’re using lots of unusual fractions, you should consider converting them to decimals. So one or two “five-sixteenths” and “seven-fifteenths” are fine, but if you have lots of them, convert them all to decimals.
Again, there are lots of exceptions. For example, even though you should write out fractions in text according to the Guardian, you still use the figures in tables and recipes, and the Chicago Manual of Style also notes that the rules can be different for technical documents.
The Associated Press says if you’re writing the numerals for a whole number with a fraction—like 2 ½—you leave a space between them. The Chicago Manual of Style says no space.*
One final word on fractions: the Guardian style guide makes the excellent point that you should try to avoid mixing fractions and percentages in the same story. Stick with one format.
“Math” Versus “Maths”
I can’t give you an answer about why it’s “math” in the US, but “maths” in the UK, but I can give you some history.
Of course, both “math” and “maths” are a short version of the word “mathematics.” It comes from the Latin “Mathematica.” It’s plural in Latin, which is why it has the “s” on it in English, but we treat it as singular.
That change from plural to singular happens sometimes with Latin words that come into English. For example, “agenda” is plural in Latin, but we treat it as singular in English. So it’s not that unusual that “mathematics” is singular.
In both British and American English, the early uses of “math” and “maths” were followed by a period, indicating that they were abbreviations for “mathematics.” It looks like Americans abbreviated it first, because the first example of “math” in the Oxford English Dictionary is from 1891, and the first use of “maths” comes later in 1911.
It was only over time that they both became words in their own right instead of being considered abbreviations and started appearing without the period at the end.
*Note: This paragraph originally incorrectly stated that Chicago requires a space between a whole number and a fraction.