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Molecular Weight Calculator

Chemistry

Calculate the molecular weight (molar mass) of any compound by entering the number of atoms of each element. Get the result in g/mol with an element-by-element breakdown.

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Molecular Weight (g/mol)

18.015

This calculator computes your Molecular Weight (g/mol) from the values you enter.

Inputs
Hydrogen (H) atomsCarbon (C) atomsNitrogen (N) atomsOxygen (O) atomsSodium (Na) atomsChlorine (Cl) atomsCalcium (Ca) atomsSulfur (S) atoms
Outputs
Molecular Weight (g/mol)

What is a Mol. Weight?

The Molecular Weight Calculator computes the molecular weight (molar mass) of a compound from the number of atoms of each element in its chemical formula. Molecular weight — measured in grams per mole (g/mol) — is the sum of the atomic masses of all atoms in one formula unit of the substance. It is the single most-used physical property in quantitative chemistry, required for every calculation that relates the mass of a substance to the number of moles it contains.

To use the calculator, read the subscript numbers in your chemical formula (for example, H₂SO₄ has H = 2, S = 1, O = 4) and enter those counts for each element. The calculator multiplies each count by the standard atomic mass from IUPAC tables, sums all contributions, and returns the molecular weight in g/mol together with an element-by-element breakdown.

The tool currently covers eight elements — H, C, N, O, Na, Cl, Ca, S — which account for the majority of compounds in general, inorganic, and introductory organic chemistry. This includes water (H₂O), NaCl, CaCO₃, H₂SO₄, glucose (C₆H₁₂O₆), urea, ethanol, amino acids such as glycine, and hundreds of other common compounds.

In the Indian school curriculum, molecular weight (called formula mass in NCERT) appears in Class 11 Chemistry Chapter 1 (Some Basic Concepts of Chemistry) and is applied in every subsequent quantitative chemistry topic. It is the required input for the Mole Calculator and the Molarity Calculator, and is typically the first value students need to look up or calculate before solving any stoichiometry or solution problem.

Beyond academics, molecular weight is used by pharmaceutical chemists calculating drug doses, food scientists converting ingredient masses in formulations, and industrial chemists scaling up reactions from lab to production.

How to use this Mol. Weight calculator

  1. Identify your compound's formula — write out the chemical formula and count the atoms of each element. For H₂SO₄: H = 2, S = 1, O = 4. For CaCl₂: Ca = 1, Cl = 2. For glucose (C₆H₁₂O₆): C = 6, H = 12, O = 6.
  2. Enter Hydrogen (H) atoms — type the number of hydrogen atoms in the Hydrogen (H) atoms field. Leave at 0 if the compound contains no hydrogen.
  3. Enter atoms for each element — fill in Carbon (C) atoms, Nitrogen (N) atoms, Oxygen (O) atoms, Sodium (Na) atoms, Chlorine (Cl) atoms, Calcium (Ca) atoms, and Sulfur (S) atoms as applicable. Leave unused elements at 0.
  4. Read Molecular Weight (g/mol) — the highlighted output shows the total molecular weight. This is the molar mass value to use in all subsequent calculations.
  5. Check the steps panel — expand the steps to see each element's contribution to the total. Verify that each line looks correct before using the result.
  6. Use the molecular weight downstream — enter it as the Molar Mass field in the Mole Calculator, Grams to Moles Calculator, or Molarity Calculator to complete your calculation.

Formula & Methodology

Molecular weight formula:

> MW = Σ (nᵢ × Aᵢ)

Where:
- MW = molecular weight (g/mol)
- nᵢ = number of atoms of element i in one formula unit
- Aᵢ = standard atomic mass of element i (g/mol, from IUPAC 2021 values)

Atomic masses used:

| Element | Symbol | Atomic Mass (g/mol) |
|---|---|---|
| Hydrogen | H | 1.008 |
| Carbon | C | 12.011 |
| Nitrogen | N | 14.007 |
| Oxygen | O | 15.999 |
| Sodium | Na | 22.990 |
| Chlorine | Cl | 35.453 |
| Calcium | Ca | 40.078 |
| Sulfur | S | 32.06 |

Worked example — Sulfuric acid (H₂SO₄):

- H: 2 × 1.008 = 2.016
- S: 1 × 32.06 = 32.06
- O: 4 × 15.999 = 63.996
- MW = 2.016 + 32.06 + 63.996 = 98.072 g/mol

Worked example — Glucose (C₆H₁₂O₆):

- C: 6 × 12.011 = 72.066
- H: 12 × 1.008 = 12.096
- O: 6 × 15.999 = 95.994
- MW = 72.066 + 12.096 + 95.994 = 180.156 g/mol

One mole of glucose (180.156 g) dissolved in water to make 1 litre of solution gives a 1 M glucose solution — the standard concentration for a glucose infusion in a clinical setting.

Frequently Asked Questions

Molecular weight (also called molar mass or formula weight) is the sum of the atomic masses of all atoms in one molecule or formula unit of a substance, expressed in grams per mole (g/mol). For example, water (H₂O) has a molecular weight of 2 × 1.008 + 15.999 = 18.015 g/mol. It is one of the most fundamental properties of a chemical substance, used in virtually every quantitative chemistry calculation.
Molecular weight (MW) and molar mass are numerically identical and are used interchangeably in most chemistry contexts. Strictly speaking, molecular weight is dimensionless (a ratio relative to 1/12 the mass of carbon-12), while molar mass has units of g/mol. In practice, both terms describe the same quantity and both appear in textbooks depending on the author's convention.
Atomic mass is the mass of a single atom of an element, expressed in atomic mass units (amu) and listed on the periodic table. Molecular weight is the sum of atomic masses for all atoms in a compound's formula. For a monatomic element like Na, the molecular weight equals the atomic mass (22.990 g/mol). For a compound like NaCl, the molecular weight equals Na + Cl = 22.990 + 35.453 = 58.443 g/mol.
Molecular weight = Σ (number of atoms of each element × atomic mass of that element), summed over all elements in the formula. For glucose (C₆H₁₂O₆): MW = 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 72.066 + 12.096 + 95.994 = 180.156 g/mol. The Molecular Weight Calculator performs this summation automatically from the atom counts you enter.
Read the subscript numbers in the chemical formula to find how many atoms of each element are present. In H₂SO₄: H = 2, S = 1, O = 4. In Ca(OH)₂: Ca = 1, O = 2, H = 2 (multiply subscripts inside and outside brackets). In glucose C₆H₁₂O₆: C = 6, H = 12, O = 6. Enter these counts into the corresponding element fields in the calculator.
The current calculator supports the eight most common elements in general chemistry: H (Hydrogen, 1.008 g/mol), C (Carbon, 12.011 g/mol), N (Nitrogen, 14.007 g/mol), O (Oxygen, 15.999 g/mol), Na (Sodium, 22.990 g/mol), Cl (Chlorine, 35.453 g/mol), Ca (Calcium, 40.078 g/mol), and S (Sulfur, 32.06 g/mol). These elements cover the majority of compounds encountered in introductory and organic chemistry.
Molecular weight is the conversion factor between mass and moles: moles = mass ÷ molecular weight. It is required for preparing solutions of known molarity, calculating stoichiometric ratios in reactions, finding percent composition, determining empirical formulae from combustion analysis, and converting between mass and moles in any quantitative problem. Without molecular weight, no mass-based calculation in chemistry is possible.
Commonly needed molecular weights include: water (H₂O) = 18.015 g/mol, NaCl = 58.443 g/mol, glucose (C₆H₁₂O₆) = 180.156 g/mol, sulfuric acid (H₂SO₄) = 98.079 g/mol, ethanol (C₂H₅OH) = 46.068 g/mol, calcium carbonate (CaCO₃) = 100.086 g/mol, and urea (CH₄N₂O) = 60.055 g/mol. Enter the atom counts for any of these to verify the value.
Water (H₂O) has a molecular weight of 18.015 g/mol, calculated as 2 × 1.008 (H) + 1 × 15.999 (O) = 18.015. This means 18.015 grams of water contains exactly one mole (6.022 × 10²³ molecules). Enter H = 2, O = 1 in the calculator to confirm. The [Mole Calculator](/mole-calculator/) uses this value to convert any mass of water to moles.
Molecular weight (also called formula mass in NCERT) is introduced in Class 11 Chemistry Chapter 1 (Some Basic Concepts of Chemistry) and is prerequisite knowledge for all stoichiometry, mole concept, and solution chemistry topics. Students are expected to calculate MW from chemical formulas using the periodic table, and the value appears in every gram-to-mole and concentration calculation in both Class 11 and Class 12. It is also tested in JEE Main, JEE Advanced, and NEET.
Yes — for ionic compounds, use the formula unit (the simplest ratio of ions) rather than an actual molecule. For NaCl: Na = 1, Cl = 1 → MW = 58.443 g/mol. For CaCl₂: Ca = 1, Cl = 2 → MW = 40.078 + 2 × 35.453 = 110.984 g/mol. The term 'formula weight' is often used for ionic compounds instead of 'molecular weight' since ionic compounds do not exist as discrete molecules.
First, determine the moles of solute needed: moles = molarity × volume (L). Then convert moles to grams: grams = moles × molecular weight. For example, to prepare 500 mL of 0.1 M NaCl: moles needed = 0.1 × 0.5 = 0.05 mol; mass needed = 0.05 × 58.443 = 2.922 g. Weigh out 2.922 g of NaCl, dissolve in water, and make up to 500 mL. Use the [Molarity Calculator](/molarity-calculator/) to verify the final concentration.
Also known as
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