MULTIPLY BY DOUBLE
DOUBLE, MULTIPLY BY 2
If you are talking about making a number twice as large, the expression is “double” or “multiply by 2”: “double your sales to multiply your income by 2.”
You could properly say “increase by a 100%” to mean the same thing, but lots of people won’t understand that.
And definitely do not confuse people by saying “multiply by double.”
See also “divide by half.”
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