# Molecular Weight Calculator

Free molecular weight calculator. Find the molar mass of common compounds or calculate custom molecular weights by entering element counts.

## What this calculates

Calculate the molecular weight (molar mass) of any compound. Select from 15 common compounds or build a custom molecule by specifying element counts. The calculator shows the total molar mass and the contribution of each element.

## Inputs

- **Mode** — options: Common Compound, Custom (Element Counts) — Choose a common compound or enter element counts manually.
- **Common Compound** — options: Water (H₂O) - 18.015 g/mol, Sodium Chloride (NaCl) - 58.44 g/mol, Carbon Dioxide (CO₂) - 44.01 g/mol, Glucose (C₆H₁₂O₆) - 180.16 g/mol, Sulfuric Acid (H₂SO₄) - 98.079 g/mol, Sodium Hydroxide (NaOH) - 40.00 g/mol, Hydrochloric Acid (HCl) - 36.461 g/mol, Calcium Carbonate (CaCO₃) - 100.09 g/mol, Ammonia (NH₃) - 17.031 g/mol, Ethanol (C₂H₅OH) - 46.069 g/mol, Methane (CH₄) - 16.043 g/mol, Propane (C₃H₈) - 44.096 g/mol, Iron(III) Oxide (Fe₂O₃) - 159.69 g/mol, Potassium Permanganate (KMnO₄) - 158.03 g/mol, Sucrose (C₁₂H₂₂O₁₁) - 342.30 g/mol — Select a common compound.
- **Carbon (C) atoms** — min 0, max 100 — Number of carbon atoms. Atomic mass: 12.011 g/mol.
- **Hydrogen (H) atoms** — min 0, max 200 — Number of hydrogen atoms. Atomic mass: 1.008 g/mol.
- **Oxygen (O) atoms** — min 0, max 100 — Number of oxygen atoms. Atomic mass: 15.999 g/mol.
- **Nitrogen (N) atoms** — min 0, max 100 — Number of nitrogen atoms. Atomic mass: 14.007 g/mol.
- **Sulfur (S) atoms** — min 0, max 50 — Number of sulfur atoms. Atomic mass: 32.06 g/mol.
- **Chlorine (Cl) atoms** — min 0, max 50 — Number of chlorine atoms. Atomic mass: 35.45 g/mol.
- **Sodium (Na) atoms** — min 0, max 50 — Number of sodium atoms. Atomic mass: 22.990 g/mol.
- **Potassium (K) atoms** — min 0, max 50 — Number of potassium atoms. Atomic mass: 39.098 g/mol.
- **Calcium (Ca) atoms** — min 0, max 50 — Number of calcium atoms. Atomic mass: 40.078 g/mol.
- **Iron (Fe) atoms** — min 0, max 50 — Number of iron atoms. Atomic mass: 55.845 g/mol.
- **Phosphorus (P) atoms** — min 0, max 50 — Number of phosphorus atoms. Atomic mass: 30.974 g/mol.
- **Manganese (Mn) atoms** — min 0, max 50 — Number of manganese atoms. Atomic mass: 54.938 g/mol.

## Outputs

- **Molecular Weight** (g/mol) — The calculated molar mass of the compound.
- **Formula** — formatted as text — The molecular formula of the compound.
- **Mass Breakdown** — formatted as text — Contribution of each element to the total mass.

## Frequently Asked Questions

**Q: What is molecular weight?**

A: Molecular weight (also called molar mass) is the sum of the atomic masses of all atoms in a molecule, expressed in grams per mole (g/mol). It tells you how much one mole (6.022 × 10²³ particles) of the substance weighs.

**Q: How do I calculate molecular weight from a formula?**

A: Multiply each element's atomic mass by its subscript (number of atoms), then add all the results. For example, H₂O = 2(1.008) + 1(15.999) = 18.015 g/mol.

**Q: What is the difference between molecular weight and formula weight?**

A: Molecular weight applies to covalent (molecular) compounds, while formula weight applies to ionic compounds like NaCl that do not form discrete molecules. The calculation method is the same for both.

**Q: Where do I find atomic masses?**

A: Atomic masses are listed on the periodic table. They are weighted averages of all natural isotopes of each element. For example, carbon's atomic mass is 12.011 g/mol, not exactly 12, because it includes small amounts of carbon-13.

---

Source: https://vastcalc.com/calculators/chemistry/molecular-weight
Category: Chemistry
Last updated: 2026-04-21