Molecular Weight Calculator - Calculate Molar Mass of Chemical Compounds

A molecular weight calculator (also called a molar mass calculator or formula weight tool) is an online utility that computes the mass of a molecule based on its chemical formula. For each element in the formula, the calculator multiplies its atomic weight (from the periodic table) by the number of atoms present, then sums the contributions of all elements to give the final molecular weight in grams per mole (g/mol).

Molecular weight is a cornerstone concept in chemistry. It bridges the microscopic world (atoms and molecules) with the macroscopic world (grams, liters, and moles) - for example, by knowing the molecular weight of a substance you can convert between a weight you can measure on a lab balance and the number of molecules needed for a reaction, making stoichiometry possible.

This free online molecular weight tool operates entirely within your browser - no data is sent to any server, ensuring your privacy and enabling offline use. It supports two input methods: directly typing a formula (e.g., `H2O`, `C6H12O6`, `(NH4)2SO4`) or manually building the formula by selecting elements from a periodic table dropdown. The calculated molecular weight can be copied instantly, and the tool includes detailed explanations and reference tables to help you understand the underlying chemistry.

Using this molar mass calculator is simple. Follow these steps:

1. Enter a chemical formula in the input field. The calculator recognises formulas such as `H2O`, `NaCl`, `C6H12O6`, `Fe2(SO4)3`, and even parentheses like `(NH4)2SO4`. Click "Parse" to automatically extract the element symbols and their subscripts.
2. Alternatively, build the formula manually by selecting an element from the periodic table dropdown, entering a count (optional), and clicking the plus button. Repeat for each element in your compound.
3. Adjust counts by editing the number fields next to each element in the "Formula Elements" list. Remove any element by clicking the minus button.
4. The molecular weight is automatically calculated as you type or adjust elements. No extra "Calculate" button is needed - the tool updates instantly.
5. Copy the result using the copy button, or use the Clear All button to start a new calculation. The tool saves your last‑used inputs locally for up to 30 days.

Important: Chemical symbols are case‑sensitive. Enter "Co" for cobalt and "CO" for carbon monoxide - the tool distinguishes them correctly.

The molecular weight is calculated by:

1. Identifying each element in the chemical formula and the number of atoms of that element.
2. Looking up the atomic weight of each element from a standard periodic table.
3. Multiplying the atomic weight by the number of atoms.
4. Summing the results for all elements.

Example Calculation - Water (H₂O):

• Hydrogen (H): atomic weight = 1.008 g/mol; subscript = 2 → contribution = 2.016 g/mol
• Oxygen (O): atomic weight = 15.999 g/mol; subscript = 1 → contribution = 15.999 g/mol
• Total = 2.016 + 15.999 = 18.015 g/mol

Example Calculation - Glucose (C₆H₁₂O₆):

• Carbon (C): 12.011 × 6 = 72.066 g/mol
• Hydrogen (H): 1.008 × 12 = 12.096 g/mol
• Oxygen (O): 15.999 × 6 = 95.994 g/mol
• Total = 72.066 + 12.096 + 95.994 = 180.156 g/mol

Although the terms are often used interchangeably, there is a subtle but important distinction:

In practice, the numerical values of molecular weight (in amu) and molar mass (in g/mol) are identical for a given compound - for example, water is 18.015 amu per molecule and 18.015 g per mole. Because a mole contains Avogadro’s number of particles, the mass of one mole (molar mass) equals the mass of one molecule (molecular weight) multiplied by Avogadro’s number. The two terms are therefore used interchangeably in most chemistry contexts, and this calculator reports the result in g/mol (the standard lab unit).

The calculator includes 26 of the most common chemical elements used in organic, inorganic, and biochemistry. For compounds containing other elements, you can manually add them (future update). Below is a quick reference table of the included elements with their standard atomic weights (IUPAC) in g/mol:

The molecular weight (or molar mass) is the bridge between the amount you can weigh on a balance (grams) and the amount used in a balanced chemical equation (moles). The relationship is given by:

Once you know the number of moles, you can use the coefficients of a balanced equation to determine the moles of other reactants or products - the heart of all stoichiometry problems.

Example - Using Molecular Weight in a Stoichiometry Problem:

You weigh out 10.00 g of sodium chloride (NaCl). The molecular weight of NaCl is 58.44 g/mol. How many moles is that?

If the equation requires 2 moles of NaCl to produce 1 mole of product, your 0.1711 mol would produce half that amount of product.

The reverse conversion (from moles to grams) uses m = n × MW.

• Completely free, no registration. Unlimited calculations with no hidden costs.
• Privacy‑first design. All calculations happen locally in your browser. Your data never leaves your device.
• Supports formula parsing for common formulas (e.g., `H2O`, `C6H12O6`, `(NH4)2SO4`).
• Manual element‑by‑element builder for custom or complex formulas.
• Auto‑calculation as you type - no separate "Calculate" button to press.
• Local save of recent inputs for up to 30 days.
• Works offline after initial page load.
• Fully responsive design with resizable panels on desktop for custom workflow.