Writing Mechanics: When should I write numbers in word form?

The rules for writing numbers in a sentence or paragraph haven’t changed over the years, but for some reason, I still feel the need to double-check them.   I’m not sure why numbers are such a stumbling block, but any time I need to communicate a quantity, dollar amount, percentage, measurement, or date, I wind up questioning how to write it correctly.  I can never remember if I’m supposed to write the numbers in words, or if I’m supposed to write them in figures?

If you have the same questions, here’s a quick guide to help you know when numbers should be written as words:

Numbers From 1 Through 10:

The numbers 1 through 10 should be written in words when used in isolated instances.

Example:  Each of the four students has one hour to complete the exam.

Numbers That Begin Sentences:

Any number that begins a sentence should be written in words.

Example:  Twenty-four hours is a long time to wait for an answer.  

When spelling out large numbers over a thousand, use the shortest form possible.

Example:  Fifteen hundred orders were received during the first week of business.

Fractions Standing Alone:

A fraction that stands alone without a whole number should be written in words.

Example:  Approximately one-half of the pie was eaten before dinner was served.

Ages:

Ages should be written in words unless they are considered significant statistics or technical measurements. 

Example 1:  Jackie began working for the company when she was nineteen years old. 

Example 2: Only employees who will have reached age 55 by January 1 of next year will be eligible for the new policy.

Periods of Time: 

General periods of time are usually written in words.

Example:  Although this textbook was written fifteen years ago, it still contains pertinent information.