Molecular Weight Calculator
Compute the molecular weight (molar mass, g/mol) of a carbon-hydrogen-oxygen-nitrogen compound from its atom counts, with natural-abundance, monoisotopic, ¹³C-labelled or deuterated masses.
Last updated: October 2026
Molecular Weight
180.16 g/mol
Formula below · 3 sources (NIST, usgs.gov, iupac.org) · Updated Oct 2026
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About this calculator
Molecular weight (molar mass) is the sum of the atomic weights of the atoms in a molecule, in grams per mole:
MW = C × 12.011 + H × 1.008 + O × 15.999 + N × 14.007
These are the IUPAC abridged standard atomic weights, averages over the natural mix of isotopes. Glucose, C6H12O6, is 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.16 g/mol.
Isotope settings
Monoisotopic uses the mass of the main isotope of each element (¹²C = 12 exactly, ¹H = 1.007825, ¹⁶O = 15.994915, ¹⁴N = 14.003074), which is what a mass spectrometer reports for the main peak. The labelled settings replace every carbon with ¹³C (13.003355) or every hydrogen with deuterium (2.014102).
Limits
Only carbon, hydrogen, oxygen and nitrogen are covered, which handles sugars, amino acids without sulfur, alcohols, acids and many drugs. For compounds with sulfur, phosphorus, halogens or metals, add their atomic weights separately (S 32.06, P 30.974, Cl 35.45, Na 22.990) or use a formula-based molar mass tool.
| Element | Standard atomic weight | Monoisotopic mass |
|---|---|---|
| Carbon (C) | 12.011 | 12 (¹²C) |
| Hydrogen (H) | 1.008 | 1.007825 (¹H) |
| Oxygen (O) | 15.999 | 15.994915 (¹⁶O) |
| Nitrogen (N) | 14.007 | 14.003074 (¹⁴N) |
How to use
Enter the number of each atom from the molecular formula and pick an isotope setting. The default is glucose, C6H12O6: 180.16 g/mol. Caffeine, C8H10N4O2: 8 × 12.011 + 10 × 1.008 + 2 × 15.999 + 4 × 14.007 = 194.19 g/mol. With Monoisotopic selected, glucose gives 180.06, the [M] peak in mass spectrometry; fully deuterated glucose-d12 gives 192.23 g/mol.
Frequently asked questions
Is molar mass the same as molecular weight?
They have the same number. Molar mass is in grams per mole; molecular weight (relative molecular mass) is the same value without a unit. Water is 18.015 g/mol, and its molecular weight is 18.015.
Why are atomic weights not whole numbers?
Most elements are a mix of isotopes. Natural carbon is about 98.9% ¹²C (mass exactly 12) and 1.1% ¹³C (13.003), which averages to 12.011.
When should I use the monoisotopic setting?
For mass spectrometry. A spectrometer separates individual isotopic forms, so the main peak of glucose is at 180.063 (all ¹²C, ¹H, ¹⁶O), not at the average 180.156. For weighing chemicals and stoichiometry, use natural abundance.
What if my compound contains other elements?
Enter the C, H, O and N counts here, then add the other atoms by hand from their standard atomic weights. For sodium acetate, C2H3NaO2: the C, H, O part is 59.04 g/mol, plus 22.990 for sodium, 82.03 g/mol in total.
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Sources & references
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