Molecular Weight Calculator

Calculate the molecular weight of a compound from its chemical formula by summing atomic masses of each element.

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Supported elements

H, He, C, N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Fe, Cu, Zn, Br, Ag, I, Au. Use standard notation with parentheses for groups, e.g. Ca(OH)2.

Enter a chemical formula Type a formula like H2O, NaCl, or C6H12O6 to calculate its molecular weight with a full element breakdown.

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Chemistry

Molecular weight from a chemical formula

A molecular weight calculator finds the molar mass of a compound by parsing its chemical formula and summing the atomic masses of every element. Enter any formula — from simple molecules like H2O to complex organic compounds — to see the total molecular weight and element-by-element breakdown.

How molecular weight is calculated

The molecular weight (or molar mass) of a compound is the sum of the atomic masses of all atoms in one molecule. Each element's standard atomic weight is taken from the IUPAC periodic table, and multiplied by the number of times it appears in the formula.

For example, water (H2O) has two hydrogen atoms at 1.008 each and one oxygen at 15.999, giving a molecular weight of 18.015 g/mol. Parentheses in formulas like Ca(OH)2 indicate groups: the OH group appears twice, contributing 2 oxygen and 2 hydrogen atoms.

M = Σ (nᵢ × Aᵢ)

Sum of each element's count times its atomic weight.

Limitations of this calculator

This calculator uses standard atomic weights and supports common formula notation. It does not handle isotope-specific masses, hydrated salts (e.g. CuSO4·5H2O), or polymer repeat units. For precise isotopic calculations, use a mass spectrometry reference.

Frequently asked questions

What is the difference between molecular weight and molar mass?

They are numerically identical but expressed in different units. Molecular weight is dimensionless (in daltons or amu), while molar mass is in grams per mole (g/mol). For practical chemistry, the values are interchangeable.

 How do I enter a formula with parentheses?

Use standard chemical notation: Ca(OH)2, Mg(NO3)2, Al2(SO4)3. The calculator expands parenthesised groups and multiplies by the subscript.

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