Molecular Weight Calculator
Enter any chemical formula to get its exact molar mass, plus a full element-by-element breakdown of how much each atom contributes to the total.
Results from this calculator are estimates provided for general informational purposes only, based on formulas, rates, and standards commonly accepted as of 2026. Figures may differ slightly from other calculators or professional sources due to rounding methods, differing assumptions, or regional regulations, and rules may change over time. Always consult a qualified professional — such as a financial advisor, healthcare provider, or other relevant specialist — before making decisions based on these results.
Molecular Weight of H2O
18.015 g/mol
Example
H2O has a molecular weight of 18.015 g/mol, made up of O (88.81% by mass), H (11.19% by mass).
Percent composition by mass
What is a Molecular Weight Calculator?
Molecular weight (also called molar mass) is the mass of one mole of a substance, expressed in grams per mole. It's calculated by adding up the atomic weights of every atom in the chemical formula — so water's molecular weight is simply two hydrogen atoms plus one oxygen atom, each weighted by how many appear in the formula.
This calculator parses standard chemical notation directly — element symbols with counts, parenthetical groups like Ca(OH)₂, and hydrate notation like CuSO₄·5H₂O — and computes the exact molar mass using standard atomic weights for all 118 elements.
Element-by-Element Breakdown
| Element | Count | Atomic Weight | Total Weight | % by Mass |
|---|---|---|---|---|
| O | 1 | 15.999 | 15.999 | 88.81% |
| H | 2 | 1.008 | 2.016 | 11.19% |
How Molecular Weight Is Calculated
Every element's contribution is its atomic weight (from the periodic table) multiplied by however many times it appears in the formula. The molecular weight is the sum of every element's contribution:
For example, glucose (C₆H₁₂O₆) has 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms — (6 × 12.011) + (12 × 1.008) + (6 × 15.999) = 180.156 g/mol.
Reading Parentheses and Hydrates
Parenthetical groups like Ca(OH)₂ mean the entire group inside the parentheses is repeated — here, one calcium atom plus two full hydroxide (OH) groups, for a total of 2 oxygen and 2 hydrogen atoms. Hydrate notation like CuSO₄·5H₂O (copper sulfate pentahydrate) means 5 full water molecules are attached to each formula unit, adding their combined weight to the total.
Molecular Weight vs. Molar Mass vs. Formula Weight
These terms are often used interchangeably in practice. Strictly, "molecular weight" applies to true molecules (like H₂O or glucose), while "formula weight" is the more technically correct term for ionic compounds like NaCl, which don't exist as discrete molecules. "Molar mass" is the most general term and always carries units of grams per mole (g/mol) — this calculator computes molar mass regardless of which name applies to your compound.
Example — Your Current Formula
H2O has a molecular weight of 18.015 g/mol, made up of O (88.81% by mass), H (11.19% by mass).
Additional Example — Table Salt (NaCl)
Sodium chloride is one sodium atom (22.990) plus one chlorine atom (35.45), giving a molecular weight of 58.44 g/mol — meaning one mole of table salt (about 6.022 × 10²³ formula units) weighs 58.44 grams.
About This Parameter
- Chemical Formula
- Use standard element symbols with capital first letters (Na, not NA) followed by an optional count. Parentheses group atoms that repeat together, and a period or middle dot before a trailing formula (like .5H2O) adds that many water molecules for hydrates.
Frequently Asked Questions
Why do element symbols matter for capitalization?
Chemical symbols are case-sensitive — "Co" is cobalt while "CO" is carbon monoxide (a carbon atom plus an oxygen atom). Always capitalize only the first letter of a two-letter symbol.
What atomic weights does this calculator use?
Standard atomic weights for all 118 known elements, consistent with the periodic table. For radioactive elements without a stable isotope, the mass number of the most stable known isotope is used as a conventional approximation.
Can I calculate the weight of a hydrate like a crystal with water attached?
Yes — write it with a period before the water count, e.g. CuSO4.5H2O for copper sulfate pentahydrate. The calculator adds the full weight of 5 water molecules to the base compound's weight.
Does this work for ionic compounds, not just molecules?
Yes — the math is identical whether the formula represents a true covalent molecule or the repeating formula unit of an ionic compound like NaCl or CaCO₃.