Ordinal numbers

Spell out all ordinal numbers in text.


Not recommended: 1st, 5th, 12th, 43rd

Recommended: first, fifth, twelfth, forty-third

Numbers as words

In general, spell out the following:

  • Numbers from zero through nine, except as noted below in Numbers as numerals.
  • A number that starts a sentence.

    Recommended: Fifteen directories are created.

    In some cases it's better to rearrange the sentence so that the number appears later.

    Not recommended: 164 MB is generally considered too large a file to send as an attachment.

    Recommended: In general, avoid sending files larger than 164 MB as attachments.

    Exception: It's okay, but non-optimal, to begin a sentence with a four-digit year.

  • A number that is followed by a numeral.

    Recommended: This procedure creates fifteen 100,000-byte files.


    Recommended: This procedure creates 15 of the 100,000-byte files.

  • Indefinite and casual numbers.

    Recommended: You can specify thousands of combinations.

    Recommended: The API might return a list of a million songs.

Numbers as numerals

In general, use numerals for the following:

  • Numbers 10 and greater.

    Recommended: The link expires in 24 hours.

    Exceptions: Always use numerals for the following items, even if they're less than 10:

    • Version numbers.
    • Technical quantities, such as amounts of memory, amounts of disk space, numbers of queries, or usage limits.
    • Page numbers.
    • Chapter numbers, sections, pages, and so on.
    • Prices.
    • Numbers without units, such as numbers used in mathematical expressions.
  • Numbers less than 10 when they appear in the same sentence with numbers greater than 9.

    Recommended: The menu contains 15 options but 6 of them are deselected.

  • Negative numbers.
  • Most fractions.
  • Percentages.
  • Dimensions.
  • Decimals.
    • For decimal numbers less than one, place a zero in front of the decimal point.

      Recommended: 0.3 inch

  • Measurements.
  • Numbers in a range.

Numbers as Roman numerals

In general, avoid using Roman numerals when possible. Instead, use Arabic numerals because they are easier to scan.

You can use Roman numerals for sub-steps in procedures.


  • Express fractions as decimal numbers, when possible.
  • When expressing fractions as words, connect the numerator and denominator with a hyphen unless one of them is already hyphenated.

    Recommended: two-fifths

    Recommended: five sixty-fourths


In general, use numerals and the percent sign (%), without a space between them.

Exception: If the percentage starts the sentence, then spell out both the number and the word percent.

Recommended: Forty percent of the files

Ranges of numbers

Use a hyphen with no space either side of it. Do not use an en dash (–).

Recommended: 2012-2016

Also see:

Suspended hyphens

When two or more hyphenated compounds that start with numbers modify the same word, use suspended hyphens.

Recommended: You can set up the system to scan for new files at one-, two-, or three-hour intervals.


Make sure it's clear what country's currency you are describing. See the currency section in Units of measurement.

For US dollars, use a comma to delineate the thousands place of whole currency. Use a period to delineate whole currency and fractions of currency. Always include the dollar sign ($) at the beginning of the currency. Do not use any punctuation or spaces to the right of the decimal.

Not recommended: The price is $0.006,653 per vCPU hour.

Recommended: The price is $0.006653 per vCPU hour.

Not recommended: $10 000 in fees is out of reach for many developers.

Recommended: $10,000 in fees is out of reach for many developers.

Commas and decimal points in numbers

Use commas and decimal points in accordance with standard American number-formatting.

Specifically: in numbers four or more digits long, use commas to set off groups of three digits, counting leftward from the decimal point, in the standard American style. For long decimal numbers, do not use any digit-group separators to the right of the decimal point.

Use a period for a decimal point, also in the standard American style.

Examples of comma grouping

Not recommended: The limit is 1532784 bytes per day.

Recommended: The limit is 1,532,784 bytes per day.

Not recommended: The API supports up to 2000 vertices.

Recommended: The API supports up to 2,000 vertices.

Not recommended: $0.031 611/vCPU hour

Recommended: $0.031611/vCPU hour

For more about decimal points and digit group separators, see Wikipedia's decimal mark entry.


Use numerals for dimensions.

Use a lowercase x between the numerals in the dimensions, with no space between the numerals and the x.

Not recommended: 192 x 192

Recommended: 192x192


Use standard mathematical notation. Don't put a space between the base and the exponent.

Recommended: 23

More examples


  • two-day total
  • version 3
  • four options
  • five minutes
  • 6 queries per second
  • 8 pixels
  • nine developers
  • 10 chapters
  • 18 years old
  • 27 minutes
  • 50 Mbps
  • 64-bit
  • 102 degrees
  • 728 shipments
  • 18,000,000 users
  • 40%