Numbers: how to write them in fiction

How do you write numbers in a story?

In most of the stories I write, this isn’t a massive issue: I apply the old rule of using words for one to nine and digits for anything higher, but, as with everything, this simple rule isn’t always enough.

I’m currently working on a story which is heavily dependent on numbers, so have had to do a bit of Googling to find the generally accepted rules. I’ve used this site, but their explanations are long, so for brevity and my own future reference, here’s an abbreviated list, along with a few additions of my own.

1. Cut off: Small numbers are written as words, large ones as digits. The commonly-known rule is one to nine as words and 10 and above as digits, though there is variability in the cut off.
2. Normalisation: If you have two or more related numbers in 1(!) sentence, write them in the same way, even if this breaks your cut-off rule.
   EXAMPLE: “The two women joined the party of nineteen.” OR “The 2 women joined the party of 19.”
3. Not normalisation: If you have two or more unrelated numbers in one sentence, don’t apply the normalisation rule (follow the cut-off rule).
   EXAMPLE: “The two women had 19 cats between them.”
4. Distinctive neighbours: When two numbers appear adjacent in a sentence, write one as digits and one as words.
   EXAMPLE: “The two 2-year olds…” OR “The 2 two-year olds…”
5. First word: When the first word in a sentence is a number, write it using words.
   ~However, if the number is awkwardly long (93,467), rejiggle your sentence so it doesn’t appear first.
6. Dialogue: In dialogue, write numbers as words, as far as possible (unless their length makes them ridiculous).
7. Percent: Use digits for percentages (but remember the first-word rule). This applies whether using the symbol (%) or the word (percent/per cent).
8. Units of measurement: Use the full word, not the abbreviation.
   EXAMPLE: Don’t use ‘m’ for ‘metre’ (or ‘mile’); don’t use ‘lb’ for ‘pound’.
9. Abbreviations of units of measurement: When using the abbreviated forms of the units of measurement (though you shouldn’t–see above), put a space between the number and the unit (as you would if you used the full word).
   EXAMPLE: The robot was 170 cm tall. NOT: The robot was 170cm tall.
10. Decimals: For decimals, use numbers. If the number is between zero and one, make sure you put the zero before the point.
    EXAMPLE: 0.1 not .1.