Home›Calculators›Chemistry›Molecular Weight Calculator

Molecular Weight Calculator

Chemistry

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

Reviewed by the thecalcu.com team · Last updated July 17, 2026

2 atoms
atoms
0 atoms
atoms
0 atoms
atoms
1 atoms
atoms
0 atoms
atoms
0 atoms
atoms
0 atoms
atoms
0 atoms
atoms

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.

Why Use a Molecular Weight Calculator?

Calculating molecular weight manually requires looking up atomic masses from a periodic table, multiplying each by the number of atoms, and summing, a process that is error-prone for larger molecules or when working quickly under exam conditions. The most common errors are misreading subscripts in bracket groups (e.g., Ca(OH)₂ has 2 O and 2 H, not 1 each) and using approximate atomic masses that introduce rounding errors.

This calculator:

  • Shows element-by-element contributions in the steps panel, making it easy to spot if you entered the wrong atom count for one element.
  • Uses IUPAC standard atomic masses (H = 1.008, not 1; O = 15.999, not 16) so the output is accurate enough for precise lab work.
  • Feeds directly into downstream calculations: once you have the molecular weight, use it in the Grams to Moles Calculator or Molarity Calculator without re-keying the value.

Who Should Use This Calculator?

Class 11 and 12 chemistry students calculating molecular weights for stoichiometry problems, mole calculations, and solution preparation. The step-by-step breakdown confirms which element is contributing what to the total, helpful when learning how molecular weight is built up.

JEE Main and Advanced aspirants who need molecular weight as an intermediate value in multi-step problems. Having an instant, accurate value prevents compounding errors in long calculations.

NEET students working with biomolecules, glucose, amino acids, fatty acids, where molecular weights of 100–200+ g/mol are common and manual summation is especially prone to error.

Undergraduate chemistry and pharmacy students preparing solutions, titrating samples, and calculating reagent masses in practicals. Pair with the Mole Calculator and Moles to Atoms Calculator for complete mass-to-particle workflows.

Lab technicians and quality control analysts who verify reagent molecular weights before use, especially when working with hydrated salts where the water of crystallisation adds to the formula weight.

What Insights Does the Mol. Weight Calculator Give You?

Molecular Weight (g/mol) is the sole output, the total molar mass of the compound. This number directly tells you: (a) how many grams are in one mole of your substance, and (b) what divisor to use when converting any mass to moles. A higher molecular weight substance is "heavier per mole", you need more grams to have the same number of moles compared to a lighter substance.

The steps panel (expandable) shows each element's contribution separately: H contribution, C contribution, and so on, with the final sum. This breakdown is the equivalent of showing your working in an exam, and it immediately highlights if you entered an atom count incorrectly (for example, entering S = 4 instead of S = 1 for H₂SO₄ would show a sulfur contribution of 128.24 g/mol instead of 32.06).

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.
Show formula & methodology ↓Show less ↑

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

What is molecular weight?
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.
What is the difference between molecular weight and molar mass?
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.
What is the difference between molecular weight and atomic mass?
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.
What is the formula for calculating molecular weight?
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.
How do I find the number of atoms in a chemical formula?
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.
What elements does the Molecular Weight Calculator support?
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.
What is molecular weight used for in chemistry calculations?
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.
What is the molecular weight of common compounds?
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.
What is the molecular weight of water?
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.
How is molecular weight taught in India's school syllabus?
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.
Can I calculate molecular weight for ionic compounds?
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.
How do I use molecular weight to prepare a solution of known concentration?
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
molecular weightmolar mass calculatorformula weightMW calculatorcompound molar mass