For example, writing down "two" instead of "2". Which one is better to use throughout a book. Plus, which one should be used in the following situations. Age Height (six feet one vs 6 ft 1) Grade (school grade. e.g. gr1, gr2, gr3...) Date Money Number of something Measurement (20 cm vs twenty cm) Weight (200 lbs vs two hundred pounds) Plus if you could add any other situation I might have missed (I know I missed a lot), it would be greatly appreciated. Thanks.
This is the sort of thing that style guides specify. If you're formatting for an agent, publisher, contest, or similar audience, you should seek out any style rules that they provide, and follow them. If you're not or none are provided, you could follow a standard style guide. The context also matters - is it fictional narrative, fictional dialogue, nonfiction, technical nonfiction, paragraphs and sentence, tables? If your cases are all fictional narrative or dialogue, and if I were king and could use my own style guide, I'd probably choose: Age: Spelled out. "John? Oh, he's forty-six. Maybe forty-seven. I don't know, I just know he's old.") Height: Spelled out. "He's old but he's big. He's got to be three inches taller than Joe, which would make him six foot four, minimum." Grade: Spelled out. "But he's in the second grade. Yes, second. Grade two, as opposed to grade three? Are you having trouble hearing me?" Date: Hmm. I guess I'd use numbers for years, but is that right? "He was born on March fourth, 1962." "He was born on March fourth, nineteen sixty-two." Hmmm. Money: Hmm again. "The peanut butter was sixty-seven cents, leaving John with a bit over thirteen dollars." Yes, I'd prefer to spell it out, and now that I think about it, I just read one of those murder mystery reading-of-the-will scenes, and those amounts were spelled out. Number of something. Spelled out. "Reconsidering, he decided to spend most of the change, and he came home with fourteen jars of peanut butter and two loaves of bread." Measurement: Spelled out. "Unfortunately, fourteen jars of peanut butter require a shelf at least half a meter wide, and the available shelf space was less than six centimeters." Weight: "He tried to stack it all on the available shelf space, but fourteen and a half pounds of peanut butter was more than the shelf could take." I should add that sometimes the direction is to spell out numbers below a certain number, and use digits for numbers above that number. So you'd spell out "three" but use digits for, "4,389". I'm not at all sure whether this works for fiction, though. I'd rather see, "Four thousand, three hundred, and eighty-nine." Or, really, "Somewhere over forty-three hundred."
I would only use numbers for the date because it looks horrible writing June, twenty-third, two-thousand and twelve. Whatever rules you pick stay with them in everything you write. If you type five-hundred yen as ¥500, than always type it like that, stay consistent.
In fiction you should use words for numbers, up to a certain level. I'm not sure what the cut-off is, but there's probably room for choice once you're over a hundred. Judge it according to context and common sense. Ten seems to be seen as "a common convention", but personally I don't think that's high enough and higher numbers are often written out. (I would write "Five hundred soldiers stood at the gate", because "500" in the text would seem clunky, but equally: "He showed me the result of his calculation: 347" when the character is seeing it on paper, because writing that out would seem unnecessary.) I think the point is that you want to reproduce the sense of the number in a character's head, most of the time. And most of the time we experience the concept of a number by hearing it in our minds rather than visualising it - unless we're thinking in a mathematical way, when the visual shorthand of the number might be closer to it. So actually in literary / experimental work we should probably vary it according to context, within the work. Especially if we're writing about a mathematician. But that's just me going off on a tangent and not good advice. Better advice is to be consistent within the work. In terms of the "done thing", always use words for numbers up to ten. (And judge the rest according to taste.)
In addition to what ChickenFreak pointed out, ALL numbers in dialogue should be spelled out fully, regardless of length. But the magnitude of the number is not what determines whether you should spell it out, but rather the number of significant digits. Twenty-three trillion, not 23,000,000,000,000, for example.
To add on to what everyone is saying, never use actual numbers as a start of a sentence. They must be written out.
