I was taught that small numbers should be written with letters and not numerals. For example "5" would be "five". I've heard conflicting rules and am wondering what the largest number that should be spelt out and not written with digits is? Is it really a matter of opinion or is there a general consensus?
Also, when writing ordinal numbers what is the largest that should be written as opposed to spelt? For example 1,000th vs one-thousandth.
Chicago Manual of Style gives the general rule of writing in full all whole numbers from zero through one hundred. However, there are many exceptions and special cases detailed in the manual. American Psychological Association's Publication Manual indicates writing in full numbers from zero through nine, using numerals for anything 10 and above. Follow the style guide for the discipline in which you are writing.
According to Turabian's A Manual for Writers (a compendium of CMOS), the rules for determining whether words or numerals should be used are as follows:
Spell out numbers from one through one hundred; if the number has two words (such as thirty-five), use a hyphen.
Spell out round numbers like thousand, hundred thousand, million, etc.
Never begin a sentence with a numeral; always spell an initial number out.
Use numerals for percents and fractions (except at the beginning of a sentence; use the word percent instead)
In a series of large and small numbers, use numerals (e.g., 153 children and 5 adults...)
Spell out currency units (e.g., twelve dollars and three cents)
These are just general rules, and only for CMOS. If you require a specific style guide, consult that instead. Additionally, these rules do not apply for finance/math/statistics papers, or other settings involving many numbers or tables.