What is the difference between molar mass and monoisotopic mass?

Molar mass averages each element over its natural isotopes and is what you weigh out in the lab. Monoisotopic mass uses each element’s most common isotope and is what a mass spectrometer shows for the main peak.

Does it count charges?

Yes. The formula shows the net charge. As is usual, the mass is the sum of the atoms, with no correction for the electrons gained or lost.

What about a salt or a hydrate?

Every piece of the structure is counted together, and the result says how many pieces there are.

Is my structure uploaded?

A drawn or pasted structure is not. A name or CAS number is sent to PubChem to find the structure.

Molar mass calculator

Free, no sign-up. A structure you draw or paste is worked out in your browser; a name or CAS number is looked up in PubChem.

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Type a name, a CAS number or a SMILES, or draw a structure, and its formula and masses appear here.

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Type a name, a CAS number or a SMILES, or draw the structure.
The formula and masses appear at once, and follow every change to the drawing.
Read the composition by mass, or carry the structure on to the 3D viewer or the full editor.

How the numbers are worked out

The structure is counted atom by atom, implicit hydrogens included, and each element is weighted by its standard atomic weight: carbon 12.011, hydrogen 1.008, nitrogen 14.007, oxygen 15.999. The molar mass is the sum, in grams per mole. The monoisotopic mass instead uses the mass of each element’s most common isotope (carbon-12 is exactly 12), which is what a mass spectrometer measures for the main peak.

For caffeine, C₈H₁₀N₄O₂: 8 × 12.011 + 10 × 1.008 + 4 × 14.007 + 2 × 15.999 = 194.19 g/mol. The calculator was checked against RDKit on 1,222 structures and agrees on every formula and every mass.

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