What's the rule or your preference on writing numbers?

[Dresdenus Prime]

Anyone know what the rule is on writing numbers? Is there a rule? I've been writing them as - If the number is lower than 10, then spell it out, example seven, but if it's 10 or higher, use the actual number, example 15.

Is that a right way to do it? And if so does that mean if one chapter i wrote out 5 percent, but in a future chapter I wrote 100%, is that correct or inconsistent. Thanks!

[Aminar]

I've always heard if it needs a comma show the numbers otherwise write it out.  But that was my school's rule, not a hard and fast one. 

Personally I would avoid putting huge numbers if at all possible.  What's the scenario?

[Dresdenus Prime]

i use numbers throughout the book pertaining to distance and authorization codes and such

[Sully]

I was taught numbers in a conversation get spelt, numbers of an item get written as numbers.

[Aminar]

i use numbers throughout the book pertaining to distance and authorization codes and such

In that case just play it by ear.  Your best bet is to find the shortest way to write it out.
.9C versus the Equivalent MPH.  Talk about space distance in light Days(assuming Sci-Fi.)  Etc.  The basic answer is keep it away from 9,465,321 and the like.  I would assume an editor won't turn you down for something like that, they'll just ask you to do what they prefer.

[Wordmaker]

Generally I use numbers for dates, codes, temperature, etc. For indicating numbers of people, etc, I tend towards using words.

[Quantus]

I think the rule I learned in school was spell numbers less than 100, but as this thread proves its more of a preference thing.  Personally I think it has more to do with how bulky the spelled out version seems.  For example, it is easier to understand "One Billion" at a glance than "1,000,000,000"  but a lot harder to get "One hundred and fifty seven thousand, four hundred eighty two" than it is to write "157,482" even though the former is larger than the latter. 

[Wordmaker]

Yeah, that's true. When in doubt, write whichever way is easier to read.