Mass Percent Calculator
Calculate the mass percent (weight percent) of a solute in a solution, plus mole fraction and parts per million (ppm) conversions.
Mass Percent Formula
Mass Percent (%) = (Mass of Solute / Mass of Solution) × 100
Mass of Solution = Mass of Solute + Mass of Solvent
Alternative forms:
- w/w% (weight per weight): most common in chemistry labs
- g per 100 g of solution: the literal meaning of mass percent
Worked Example, Saltwater
25 g of NaCl dissolved in 100 g of water:
Mass of Solution = 25 g + 100 g = 125 g Mass Percent = (25 / 125) × 100 = 20.0%
This means every 100 g of this solution contains 20 g of NaCl and 80 g of water.
Watch the denominator. It is the mass of the solution, not the mass of the solvent. Dividing 25 by 100 gives 25%, and that mistake is by far the most common one on this calculation. The dropdown above exists precisely so you can say which of the two numbers you actually weighed.
Real-World Mass Percent Examples
| Solution | Component | Mass % |
|---|---|---|
| Seawater | Total dissolved salts | 3.5% (of which NaCl is about 2.9%) |
| White vinegar | Acetic acid | 5% |
| Household bleach | NaOCl | 3% to 8% |
| Isopropyl (rubbing alcohol) | Isopropanol | 70% to 99% |
| Hydrogen peroxide (pharmacy) | H₂O₂ | 3% |
| Hydrogen peroxide (industrial) | H₂O₂ | 30% to 35% |
| Blood plasma | Proteins | ~7% |
| Baking soda solution | NaHCO₃ | varies |
Mole Fraction
Mole Fraction (χ) = Moles of Component / Total Moles in Solution
Moles = Mass / Molar Mass
Mole fraction is dimensionless and ranges from 0 to 1. It is used in thermodynamics and vapor pressure calculations (Raoult’s Law).
Concentration Units Comparison
| Unit | Formula | Best used for |
|---|---|---|
| Mass % (w/w) | (mass solute / mass solution) × 100 | General chemistry, commercial products |
| Volume % (v/v) | (volume solute / volume solution) × 100 | Liquids (alcohol content in beverages) |
| Mole fraction | moles solute / total moles | Thermodynamics, gas mixtures |
| Molarity (M) | moles solute / liters solution | Lab reactions (most common in labs) |
| ppm (mass) | mass % × 10,000 | Trace concentrations in water, air |
| ppb | mass % × 10,000,000 | Ultra-trace concentrations |
Parts per Million (ppm) and ppb
For trace concentrations (pollutants, heavy metals, nutrients in water):
ppm = Mass Percent × 10,000
ppb = Mass Percent × 10,000,000
Example: the EPA action level for lead in drinking water is 0.015 mg/L, which is 0.015 ppm, or 15 ppb. Lead has no maximum contaminant level in the usual sense: its goal is set at zero, and the 15 ppb figure is the point at which a water system has to act.
Mass % vs. Volume %
These are NOT interchangeable unless the densities of solute and solvent are equal. For dilute aqueous solutions they are close enough to swap. For alcohol and water they are not: “40% alcohol by volume” on a vodka label is about 33% by mass, because ethanol is less dense than water. Bottles in Europe sometimes carry both figures, which is worth knowing before you assume two labels disagree.
Pro Tips
- Always specify whether concentration is mass% or volume%: ambiguity causes laboratory errors.
- For dilute aqueous solutions (<10%), 1 ppm ≈ 1 mg/L because water density ≈ 1 g/mL.
- Mole fraction is useful when comparing solutions where the solute and solvent have very different molecular weights.
- When preparing solutions, always add solute to solvent (not solvent to solute) to control the process safely.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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