Mass percent, also called weight percent (% w/w), is the mass of solute divided by the total mass of solution, times 100: mass % = (mass of solute ÷ mass of solution) × 100. The mass of solution is the solute plus the solvent. For example, 10 g of salt dissolved in 90 g of water makes 100 g of solution, giving (10 ÷ 100) × 100 = 10% salt by mass.

Mass percent compares masses and is temperature-independent, since mass does not change with temperature. Molarity compares moles of solute to litres of solution and shifts slightly with temperature because liquid volume expands when heated.

Mass percent is common on consumer labels (a "3% hydrogen peroxide" solution) and when only a balance is available, while molarity is preferred for reaction stoichiometry where the number of particles matters.

To make 200 g of a 5% w/w solution, first find the solute mass: 200 × 5 ÷ 100 = 10 g. Then the solvent mass is 200 − 10 = 190 g. Weigh 10 g of solute and 190 g of solvent (for water, 190 g ≈ 190 mL) and mix. Because you are measuring masses, no volumetric flask is needed — a balance is enough.

Parts per million (ppm) and parts per billion (ppb) are the same kind of mass ratio scaled for very dilute solutions. 1% w/w equals 10,000 ppm, and 1 ppm equals 0.0001%. For dilute aqueous solutions, 1 ppm is also about 1 mg of solute per litre of water. These units are used for trace contaminants — for example, a drinking-water limit of 10 ppm — where percent would be an inconveniently small number.