The denominator changes the question
NIST’s Guide for the Use of the International System of Units distinguishes several quantities that are often shortened to concentration. Mass concentration divides a component’s mass by the mixture’s volume. Amount-of-substance concentration divides its amount, measured in moles, by the mixture’s volume. Mass fraction divides a component’s mass by the mixture’s total mass.
These are not interchangeable labels. mg/mL answers a mass-per-volume question. mol/L answers an amount-of-substance-per-volume question. A percentage must say what kind of ratio it represents; the percent sign alone does not identify the numerator or denominator.
Hypothetical arithmetic—not preparation instructions
Suppose a fictional measurement record reports 12 mg of substance A in a final mixture volume of 3 mL. Its mass concentration is 12 mg ÷ 3 mL = 4 mg/mL. This describes an already-defined measurement; it is not a direction to prepare, administer or consume anything.
A different fictional record contains 12 mg of A in 300 mg of total mixture. Its mass fraction is 12 ÷ 300 = 0.04, or 4% by mass. The two examples share the numeral four but describe different quantities. Converting mass concentration to amount-of-substance concentration additionally requires the correct molar mass for the exact chemical species; a product nickname is not enough.
Make the record checkable
Write the ingredient identity and units at every step. Keep total mixture mass separate from a single component’s mass, and final mixture volume separate from any volume mentioned elsewhere. In a multi-component record, each component needs its own numerator; a blend’s total mass does not reveal its individual composition.
S7’s calculator remains an arithmetic tool. Correct arithmetic cannot establish identity, purity, compatibility, stability or suitability for human use. A certificate’s analytical percentage is not automatically a recipe or a clinical amount. Where a record leaves the denominator or ingredient form unclear, the honest answer is that the conversion is not determined.