Percent sign spacing

To remember the space, treat units like words. You would write twelve compounds not twelvecompounds hence, you would write 12 mh not 12mL. A notable exception to this rule occurs with percentages in this case, there is no space between the number and the percent sign (%) ... 

Leave a space between a numeral and its unit of measure, unless the unit of measure is a percent sign (%) ... 

Mathematical operations have specific rules for the use of mathematical symbols with SI units. A space or a half-high dot represents the multiplication of units a negative exponent, horizontal line, or slash represents the division of units, and if these mathematical symbols appear in the same line, parentheses must differentiate them. The percent sign (%) denotes the number 0.01 or 1/100, so that 1%= 0.01, 30% = 0.30, and so forth. Arabic numerals with the appropriate SI or recognized unit indicate the values of quantities. Commas are not used to separate numbers into groups of three. If more than four digits appear on either side of the decimal point, a space Table 3. Prefixes. separates the groups of three. 

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