How does mass percent differ from mass per volume percent?

Question

Sheet Reserve · Accepted Answer

Mass percent divides solute mass by total solution mass and multiplies by 100. Mass per volume percent conventionally expresses grams of solute per 100 milliliters of solution. The denominators are different, so 5 percent by mass cannot be assumed to mean 5 grams per 100 milliliters. Converting between them needs appropriate volume or density information.

Five grams of solute plus 95 grams of solvent makes 100 grams of solution and a 5 percent mass fraction. The final volume is not established by those masses alone.
Where is mass fraction defined?
The IUPAC Gold Book defines mass fraction using constituent mass and total mixture mass. A spreadsheet should keep those mass inputs typed and labeled with consistent units.