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Mass Percent Calculator

Chemistry

Calculate mass percent (% w/w) of a solute in a solution from solute mass and solution mass. Also find mass fraction, parts per hundred, and grams per kg.

10 g
g
100 g
g

Mass Percent (% w/w)

10
Mass Fraction
0.1
Grams per Kg (g/kg)
100

This calculator computes your Mass Percent (% w/w), Mass Fraction, Grams per Kg (g/kg) from the values you enter.

Inputs
Mass of SoluteMass of Solution (Solute + Solvent)
Outputs
Mass Percent (% w/w)Mass FractionGrams per Kg (g/kg)

What is a Mass Percent?

Mass percent (also written as % w/w, meaning weight per weight) is one of the most fundamental ways to express the concentration of a solution. The Mass Percent Calculator determines what fraction of a solution's total mass is contributed by the dissolved substance, expressed as a percentage. Because it is based entirely on mass ratios, mass percent remains constant regardless of temperature or pressure — a property that makes it indispensable in analytical chemistry, pharmaceutical manufacturing, and industrial quality control.

When a chemist prepares a 5% w/w sodium chloride solution, it means that every 100 g of solution contains exactly 5 g of NaCl, with the remaining 95 g being water (the solvent). This expression is more precise than volume-based concentrations for solid solutes because volumes change with temperature and are difficult to measure directly for powders. In India, mass percent is routinely specified on chemical reagent labels, Bureau of Indian Standards (BIS) formulations, and the Indian Pharmacopoeia — making it essential knowledge for chemists, pharmacists, and QC analysts alike.

For solutions that are nearly pure (such as concentrated acids), mass percent can climb close to 98–99%, while trace solutions may have mass percent values in the thousandths of a percent, better expressed as the mass fraction (the decimal form) or in units of g/kg. The calculator outputs all three representations so you can choose the most suitable one for your context.

Unlike Molality Calculator (which uses moles per kg of solvent) or Mole Fraction Calculator (which uses mole ratios), mass percent requires no molar mass data — you only need a balance to weigh the solute and the total solution.

How to use this Mass Percent calculator

  1. Weigh your solute on a balance and note the mass in grams. Enter this value in the Mass of Solute field.
  2. Prepare the solution by adding the solute to the solvent and mixing until dissolved. Weigh the total solution (or calculate it as solute mass + solvent mass) and enter that value in the Mass of Solution (Solute + Solvent) field.
  3. The calculator displays Mass Percent (% w/w) instantly — this is your reportable concentration.
  4. Note the Mass Fraction value for use in thermodynamic equations or when cross-referencing molality using the Molality Calculator.
  5. Read the Grams per Kg value for food or environmental reporting where g/kg or ppm notation is required.
  6. Verify that the solute mass you entered is less than the solution mass — if not, the inputs are physically inconsistent and the calculator will clamp to the maximum valid value.

Formula & Methodology

Core formula:

Mass Percent (% w/w) = (m_solute ÷ m_solution) × 100

Where:
- m_solute = mass of the dissolved substance (g)
- m_solution = total mass of solute + solvent (g)

Derived outputs:

Mass Fraction = m_solute ÷ m_solution g per kg = Mass Fraction × 1000

Worked example:

Preparing a 15% w/w potassium nitrate (KNO₃) solution for a soil chemistry experiment:

- Required: 15 g of KNO₃ dissolved in enough water to make 100 g of solution
- Solvent mass = 100 − 15 = 85 g of water
- Mass Percent = (15 ÷ 100) × 100 = 15.0000% w/w
- Mass Fraction = 15 ÷ 100 = 0.1500
- g/kg = 0.1500 × 1000 = 150.0 g/kg

For a commercial 37% w/w HCl stock (density ≈ 1.19 g/mL), to find molarity you would need the molar mass of HCl (36.46 g/mol):

Molarity = (% w/w × density × 10) ÷ Molar Mass          = (37 × 1.19 × 10) ÷ 36.46          ≈ 12.1 mol/L

Use the Concentration Calculator to perform the full molarity conversion once you have the density.

Frequently Asked Questions

Mass percent (% w/w) expresses the mass of a solute as a percentage of the total mass of the solution. It is calculated by dividing the mass of the solute by the mass of the entire solution (solute plus solvent) and multiplying by 100. Unlike volume-based concentrations, mass percent does not change with temperature because mass is independent of thermal expansion.
The formula is: Mass Percent (% w/w) = (Mass of Solute ÷ Mass of Solution) × 100. Here, the mass of solution includes both the solute and the solvent. For example, if 10 g of salt is dissolved in 90 g of water, the mass of solution is 100 g and the mass percent is (10 ÷ 100) × 100 = 10% w/w.
Mass percent (% w/w) is based on the ratio of masses, while volume percent (% v/v) is based on the ratio of volumes. Mass percent is more reliable for solid solutes because their volume in solution is difficult to measure directly. Volume percent is commonly used when both components are liquids, such as in alcohol-water mixtures.
Mass fraction is the decimal form of mass percent — it equals the mass of solute divided by the mass of solution, without multiplying by 100. A mass fraction of 0.10 is equivalent to a mass percent of 10% w/w. Mass fraction is dimensionless and often preferred in thermodynamic calculations and equations of state.
Mass percent (% w/w) uses the mass of the solution in the denominator, while percent solution (% w/v) uses the volume of the solution in millilitres. For dilute aqueous solutions the difference is small because water's density is approximately 1 g/mL. For concentrated solutions or non-aqueous solvents, the two values can differ substantially.
Mass percent is more stable across temperature changes because it is based on mass ratios alone. Molarity depends on solution volume, which changes with temperature due to thermal expansion, making it less precise at very different temperatures. In pharmaceutical quality control and thermodynamic work, mass percent is often preferred for this reason.
First, add the solute mass and the solvent mass to get the total solution mass. Then apply the formula: Mass Percent = (Solute Mass ÷ Total Solution Mass) × 100. For example, dissolving 25 g of glucose in 225 g of water gives a solution mass of 250 g and a mass percent of (25 ÷ 250) × 100 = 10% w/w.
Enter the mass of your solute (in grams) in the 'Mass of Solute' field and the total mass of the solution (solute plus solvent combined) in the 'Mass of Solution' field. The calculator instantly displays the mass percent, mass fraction, and grams per kilogram. If you only know the solvent mass, add the two masses yourself before entering the solution mass.
No, mass percent cannot exceed 100% because the solute mass can never be greater than the total solution mass. A 100% mass percent would represent a pure solute with no solvent at all. If your entered solute mass is greater than your solution mass, the calculator clamps the result to prevent physically impossible outputs.
Yes, the Indian Pharmacopoeia (IP) and Bureau of Indian Standards (BIS) use % w/w extensively for expressing concentrations of active pharmaceutical ingredients, reagents, and chemical solutions. Labels such as '98% w/w sulphuric acid' indicate that 98 g of H₂SO₄ is present per 100 g of solution, which is the standard mass percent notation.
Grams per kilogram is simply the mass fraction multiplied by 1,000. It tells you how many grams of solute are present in every kilogram of solution. A 10% w/w solution contains 100 g/kg of solute. This unit is sometimes used in food science, agriculture, and environmental analysis when concentrations are low and ppm or mg/kg notation is more convenient.
Common household chemicals span a wide range of mass percent concentrations. Vinegar is typically 4–8% w/w acetic acid, hydrogen peroxide antiseptic is 3% w/w, and concentrated bleach is about 3–5% w/w sodium hypochlorite. Commercial sulphuric acid sold in laboratories is often 98% w/w, making it nearly pure.
Also known as
% w/w calculatormass percent solutionweight percent calculatormass fraction calculatorpercent by mass