Using numbers in correspondence doesn’t have to be confusing, although because grammar and style manuals don’t always agree on the details, it often is. But there are a few basic rules, with the inevitable exceptions, that can help.

We usually use the figure style in ordinary business correspondence, technical documents, and informal communications. But even with the basic rules below, you may be following a style guide that differs slightly on one or more of these. If you have no other guide, however, these ideas should help.

1.  Spell out numbers from 1-10; use figures for those over 10.
           a.  We received more than 75 requests for information last month.
           b.  Fewer than ten people attended the conference!

2.  Use figures when they have technical significance (even with numbers under 10) or
need to stand out for quick understanding, as in #1 above.

 3. Spell out large numbers when they’re just a figure of speech, not meant to be
interpreted literally.

           a.  I told her a million times not to do that!
           b.  She gave me a thousand and one reasons for her mistakes.

 4. Spell out numbers when they’re the first word of a sentence, except for years, which        are always written as figures.
           a.  Eight hundred twenty-five people signed up for the conference.
           b.  There were 825 people at the conference.
           c.  2015 was another great year for our company!
           d.  The year 2015 was another great year for our company.

 5.  Use million or billion for fast comprehension in regular business correspondence rather
than adding a bunch of zeros that could be
misinterpreted; put a $ in front of such terms
to indicate American money.

           a.  We spent $21 million last year on our school system!
           b.  There are 12 million people in the system.
           c.  Did you realize there are more than 50 billion stars in our universe?

 6.  Drop the zeros with whole dollar figures unless they’re used in a column or with other
mixed numbers.

           a.  The check came to exactly $20.
           b.  The books will cost either $3.50, $3.75, or $4.00 each.

 7.  Use figures for similar items in a series, even if one is under or over 10.
           a.  Our families ate 24 hamburgers, 8 hot dogs, and 2 steaks at the barbecue!  
           b.  The product will last between 9 and 12 weeks.

8.  Treat other types of related numbers alike.
           a.  Last year 21,333,594 people applied for aid; in 2000, only 23,000,000 did.
           b.  This year, we have 15 new students in the music program, and 9 in the theater                        program.

9.  Follow the usual rules in #1, above, if the items are not related.
           a.  We painted all ten rooms in 13 weeks!
           b.  It will take us four days to drive the 1,000 miles on this trip.

10. Use figures for ages when you want them to stand out.
           a.  She’s a 25-year-old woman.
           b.  He’ll be 60 in 2015.
           c.  Joe’s only 42 years old — too young to retire!
           d.  Sarah has a 1-year-old nephew.

I know I’ll hear from those of you who DO follow a different style guide, and that’s fine. But many folks never even considered owning or checking one, so these are meant to help in the most commonly found situations.