Molecular Weight Calculator
Find relative molecular or formula mass and average mass in daltons from a formula, with transparent atom counts and standard atomic weights.
Calculate the relative mass of a molecule or formula unit by adding its constituent atomic weights. This is useful when checking chemical labels, comparing formulas or preparing for a mass-to-moles calculation. The result describes an average isotope composition rather than one exact isotopic species.
How to use this tool
- Enter a case-sensitive chemical formula, including every subscript.
- Use parentheses for repeated groups and · for hydrate components.
- Check the expanded formula and atom counts.
- Read the dimensionless relative mass and the corresponding average mass in daltons.
Formula
Mᵣ = Σ(nᵢ × Aᵢ); average formula-unit mass = Mᵣ Da
Aᵢ is a standard relative atomic weight and nᵢ is an atom count. Relative molecular mass has no unit. A dalton, Da, expresses particle mass; g/mol expresses mass per amount of substance.
Example
For glucose, C6H12O6, calculate 6 × 12.011 + 12 × 1.008 + 6 × 15.999. The sum is 180.156. Its relative molecular mass is 180.156 and its average molecular mass is 180.156 Da using this reference table.
Frequently asked questions
Is molecular weight the same as molar mass?
They are different quantities. Relative molecular mass is dimensionless, while molar mass is usually given in g/mol. Their numerical values agree at the precision of these abridged atomic weights, which explains their frequent interchange in laboratory shorthand.
Can I enter an ionic solid?
Yes. NaCl gives a relative formula mass of 58.44. An ionic lattice is described using formula units; calling that value a discrete molecule’s mass would be misleading.
Does this predict mass-spectrometry peaks?
No. Isotope-specific masses, ion charges, adduct ions and isotope envelopes need a different calculation. Charges and isotope labels are rejected here.
What formula notation is supported?
Round and square groups, Unicode digit subscripts and hydrate dots are supported. Leading reaction coefficients, decimal subscripts and variable polymer counts are rejected. The local parser allows 500 characters, twelve nested groups and one million expanded atoms; it does not validate chemical stability.
Sources
Last updated: 2026-10-30