Calculate mass percent of a component.
Mass percent, also written % w/w, is the mass of one component divided by the mass of the whole mixture. It is the composition unit of choice for alloys, ores, fertiliser blends and formulated products because it does not change with temperature the way volume-based units do. The same ratio expressed as parts per million is simply the percentage multiplied by ten thousand, which is how trace components are conventionally reported. Adjusting a blend to hit a target is the part people get wrong: adding component also increases the total mass, so the amount needed is (target fraction x current total - current component) divided by (1 - target fraction), not the naive difference.
Mass percent and blend adjustment
Mass percent = component mass / total mixture mass x 100; ppm = mass percent x 10,000; mass to add = (target fraction x total - component) / (1 - target fraction)
Because adding the component raises the total mass too, diluting your own addition. Chasing a target by adding the naive difference always undershoots — the formula here solves for it properly in one step.
Mass percent is temperature-independent; volume percent is not, because liquids expand. Mixing 50 mL of ethanol with 50 mL of water also gives less than 100 mL, so % v/v figures need the mixing behaviour stated.
Below roughly 0.1%, where percentages become strings of zeros. Contaminant limits, water hardness and trace metals in alloys are conventionally quoted in ppm for exactly this reason.