fiction - Should I write out numbers or use the actual numbers (ordinals)?

Born late in the year, Adam was the only kid in his small 4th grade class that had already turned 11. Every morning he got up at exactly 4:56, the latest he could rouse in the morning with sequential numbers and still meet Bus 9 on time.

-or-

Born late in the year, Adam was the only kid in his small fourth grade class that had already turned eleven. Every morning he got up at exactly four fifty-six, the latest he could rouse in the morning with sequential numbers and still meet Bus Nine on time.

Is there a hard rule in general fiction when to use numbers and when to spell them out?

Answer

Hard rule? No. Style guide? Yes.

As an aspiring author, you absolutely need to pick a style guide and use it or these problems will keep picking at you. I like The Chicago Manual of Style.

As such: Born late in the year, Adam was the only kid in his small, fourth-grade class that had already turned eleven. Every morning, he got up at exactly 4:56 a.m., the latest he could arise in the morning with sequential numbers and still meet Bus No. 9 on time.

There are a lot! of individual rules involved here, and some variation with the time of day and the bus number. It would also potentially change from narrative to dialogue.

In the above examples (in addition to the punctuation issues):

1. 
   

   Numbers are spelled out from one to one hundred, as well as most fractions. This is highly variable depending on the style guide. What you don't want to do is mix things like three-and-3/4. Numbers without ones or tens are often spelled out. Two hundred, one million, thirteen thousand, for instance.

   
   


2. 
   
   

   Times are written with arabic numerals (ordinals) if they contain minutes. You may use AM, PM, a.m., p.m. but Chicago cautions against A.M., P.M., am, pm (but they are not incorrect). Whole hours are often written out Three o'clock in the morning.

   
   


3. 
   

   An effort should be made to use the modern convention of what a bystander or person knowledgeable in the industry would see and recognize. A Boeing 747, Channel 5 news at 9, Bus No. 9 or Bus #9 seems better to me than bus number nine.

   
   


4. 
   

   However, in dialogue, the writer may spell out everything. Just be consistent. Brand names, such as Boeing 747 should maintain ordinals even in dialogue, but not if the brand is written out: Coke Zero.

   
   


5. 
   

   There will be times to break this pattern. It is best not to mix ordinals with roman text in the same sentence or short paragraph. In this case, ordinals are preferred. His 15-year-old dog is 105 in human years.