Spell out or write out numbers?

This ought to be a simple question.
When is it appropriate to spell out a number versus writing it out? aka: four and 4

 

It seems there are rules for that

 

I write out one to twelve. From 13 onwards I just write numbers, except for the full tens, which I write out again. So 19, twenty, 21...99, one hundred, 101...
More or less...

 

After reading the page twice I figured it out. I am bookmarking that site for future reference.

 

Supplanter7 has contacted me after I commented on their story, & I noticed they seem to be inconsistent on the same chapter, so perhaps rules do not exist, or at the least, are not hard-set.

 

Personally I'm going with the weird rule.

Does it feel weird to have a number here? Yes I spell it out. No I keep the number.

And I do the same thing with the opposite proposal.

But that is just me.

 

My preference is to write out numbers when they are vague (the number isn't specific just an estimate or exaggeration) and put in the number when it is specific.