Numerical information must be understood correctly by the reader. Most rules for the use of numerical expressions, therefore, are based on the general principle that figures are easier and clearer for the reader to understand. The following rules cover most instances where the writer must choose between a number expressed in words and in figures:

1. Write out numbers less than ten except when they represent measurements:
   
   
   five sections
   5 ft.
   5 m
   
   
   Abbreviations are acceptable in the imperial system, and take periods.
   
   
   
2. Write out numbers or the word 'number' where they occur at the beginning of a sentence as well as any related numbers that follow closely:
   
   
   Three thousand line-kilometres must be flown this summer.
   
   Number 6 should not be included in the total; number 5 was the last in the series (not No. 6, or no. 5).
   
   Twenty-five of the three hundred samples were contaminated (or Of the 300 samples collected, 25 were contaminated).
   
   
   
   
3. Do not mix figures and numbers expressed in words in the same phrase:
   
   
   nine out of ten samples not nine out of 10 samples
   
   
   
4. Generally, where one numerical expression directly follows another, write out the smaller number to avoid ambiguity:
   
   
   300 six-inch core samples
   ten 43 cent stamps
   but
   one hundred twenty 1 g samples
   
   
   
   
5. Write out the word million and similar terms, unless the number represents a metric or imperial measurement, or is in a table:
   
   
   $25 million
   4.1 million people
   but
   2 000 000 t of ore
   1 000 000 km²
   
   
   Use figures for any number less than one million:
   
   
   250 000
   
   
   
6. To make long numbers easier to read, insert a space between three-digit groups:
   
   
   15 000
   250 000
   1 250 000
   
   
   Do not use a space with a four-digit number:
   
   
   1500
   
   
   except in tables, in order to preserve the alignment of columns:
   

   250 345

   45 678

   5 485

   335 798

   
   
7. Use figures for specific numbers, as follows:
   
   
   GSC locality 3
   report 680
   unit 4
   pages 99 to 146
   sample 224
   
   
   
   
8. Write out indefinite expressions:
   
   
   the mid-sixties
   the seventies
   but 1990s (not 1990's)
   
   
   
   
9. When a number is written out, do not repeat it in figures.
   
   
10. Use figures for measures and quantities:
    
    
    2.1 m
    4 cm
    20/20 vision
    8 by 12 cm (not 8x12 cm)
    a 7 km course
    250°C
    latitude 60°13'14"N
    longitude 85°22'10"W
    
    To express a tolerance, the form should be
    30 ± 2°C
    
    
    To express an error range, the form should be
    65 ± 3 Ma
    65 +2/-1 Ma
    
    
    
    
11. Write out fractions standing alone. A fraction in figures should not be followed by by a or of an. Do not use with SI units. Hyphenate fractions used as modifiers and that are written in full, except when the numerator or denominator already contains a hyphen:
    
    
    one-half inch
    one half of the sample was lost
    the long axis is 0.25 cm (not ¼ cm)
    twenty-nine fiftieths calcium (29/50 is preferable)
    
    
    
12. Always write decimals as figures. The decimal marker is the point. Place a zero before the decimal point for numbers less than one:
    
    
    2.75 cm
    3.5 kg
    0.12 m
    
    
    
    
13. Use figures to express clock time, but write out duration of time or time of day unless emphasizing an exact or precise time.
    
    
    10:00 h
    The helicopter left at four o'clock
    Our day starts at half-past seven (or seven-thirty, or 07:30)
    We worked an eight-hour day
    
    Use figures for the year and day in calendar dates, but write out the month in full:
    
    
    21 March 1987
    March 21, 1987
    
    
    The GSC style is to use the ISO all-numeric date standards. Thus, an all-numeric date shall be written in arabic numerals, in the following order: year - month - day. It shall consist of four digits to represent the year (Note: Two digits may be used when no possible confusion can arise from the omission of the century, however, four digits should be applied especially in correspondence and for documentation purposes to indicate clearly that the descending order is used.), two digits to represent the month, and two digits to represent the day. A hyphen is used between year and month, and between month and day. Thus, the 10th of July 1999 shall be written in the following way: 1999-07-10.
    
    
    
14. Join consecutive numbers by to in the text and by an en dash (–) in parentheses:
    
    
    1 to 2 m
    (140–150 m)
    5 to 50%
    (5–50%)
    
    
    
    
15. Do not hyphenate the numeral and unit in compound adjectival expressions:
    
    
    a 100 m high cliff
    (not a 100-m high cliff)
    the 200 ft. level
    (not the 200-ft. level)