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Decimals

Decimal numbers are a linear way to represent fractions based on multiples of 10. The decimal 0.45 represents the following fraction:

  • 45/100

See FRACTIONS.

The decimal point (period) is the mark dividing the whole number on the left from the decimal fraction on the right:

  • 504.678

In some countries, writers use a comma for the decimal point:

  • 504,678
  1. Use figures for all decimals and do not write the equivalent fractions:

    • 4.5 (not 4 5/10)
    • 0.356 (not 356/1000)
    • 0.5 (not 5/10)
    • 0.4690 (not 4690/10,000)
  2. If the decimal does not have a whole number, insert a zero before the decimal point:

    • 0.578 (not .578)
    • 0.2 (not .2)

    NOTE: This rule has a few exceptions, including:

    • Colt .45
    • A batting average of .345
    • A probability of p =.07
  3. Retain the zero after the decimal point or at the end of the decimal number only if the zero represents exact measurement (or a significant digit):

    • 0.45 or 0.450
    • 28.303 or 28.3030

    NOTE: Also retain the final zero in a decimal if the zero results from the rounding of the decimal:

    • 23.180 for 23.1789 (if the decimal number is supposed to be rounded to three digits in the decimal fraction)
  4. Use spaces but not commas to separate groups of three digits in the decimal fraction.

    In the metric system, the decimals may be broken into groups of three digits by inserting spaces:

    • 56.321 677 90
    • 707.004 766 321
    • but 567.4572 (not 567.457 2)

    You can use commas to separate groups of three digits that appear in the whole number part of the decimal:

    • 56,894.65
    • 500,067.453 467

    However, do not use commas to separate groups of three digits in the decimal fraction:

    • 4.672 34 (not 4.672,34)
    • 2344.000 567 (not 2344.000,567)

    See METRIC SYSTEM.

  5. In columns, line up the decimal points:

    •   56
    •     0.004
    • 115.9
    •   56.24445
    •     0.6

    NOTE: Whole numbers without decimals (e.g., 56 above) do not require a decimal point.

  6. Do not begin a sentence with a decimal number:

    • this
    • The timer interrupts the processor 14.73 times a second.
    • not this
    • 14.73 times a second, the timer interrupts the processor.

    See NUMBERS.

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