Article — Molecular Weight Calculator
Molecular Weight Calculator: Atomic Sum and Mass-Mole-Particle Conversions
Molecular weight is the mass of one mole of a substance in grams per mole, or equivalently the mass of a single molecule in daltons. It equals the sum of atomic weights of every atom in the molecule. Water is 18.015 g/mol, aspirin 180.16 g/mol, caffeine 194.19 g/mol. The calculator parses any chemical formula and returns molecular weight plus the conversions between grams, moles, and number of particles.
Molecular weight is the single number you reach for most often in chemistry. Solution preparation, dosing, combustion engineering, biochemistry binding studies, every quantitative calculation begins with a molecular-weight lookup.
What molecular weight measures
Atomic weights have been tabulated since the early 1800s. Molecular weight is the natural extension: add up the atomic weights of every atom in the molecule, with proper multiplicities. The result tells you how much one mole (6.02 × 10^23 molecules) of that substance weighs.
A useful mental check: hydrogen weighs about 1 unit, oxygen 16, carbon 12. Water (H2O) is 1 + 1 + 16 = 18. CO2 is 12 + 16 + 16 = 44. Round atomic weights make excellent approximations; the calculator uses the more precise IUPAC 2021 values.
How to calculate molecular weight
H2O 2(1.008) + 15.999 = 18.015NaCl 22.990 + 35.45 = 58.44C6H12O6 6(12.011) + 12(1.008) + 6(15.999) = 180.156Ca(OH)2 40.078 + 2(15.999) + 2(1.008) = 74.09The parser handles nested parentheses. (NH4)2SO4 expands to 2 nitrogens + 8 hydrogens + 1 sulfur + 4 oxygens: 2(14.007) + 8(1.008) + 32.06 + 4(15.999) = 132.14 g/mol. Hydrates use a center dot; rewrite as a single formula: CuSO4·5H2O becomes CuSO4(H2O)5 = 249.69 g/mol.
Molecular weight versus molar mass
In practice the two terms mean the same thing. Strictly, molecular weight (relative molecular mass M_r) is a unitless ratio of average molecule mass to 1/12 of a carbon-12 atom, while molar mass (M) is in grams per mole. Numerically they are identical: water has M_r = 18.015 and M = 18.015 g/mol.
Biochemists often use daltons (Da) instead of g/mol; 1 Da = 1 g/mol. A 50 kDa protein has a molar mass of 50,000 g/mol. Pick whichever notation matches your audience.
The atomic-mass unit (u or Da) was defined relative to oxygen-16 until 1961. Physicists and chemists used slightly different conventions, with the chemists' value 0.027% larger. Adoption of carbon-12 as the new standard unified the two systems and changed every entry in the periodic table by about 0.003 percent.
Molecular weight and Avogadro's number
Avogadro's constant N_A = 6.02214076 × 10^23 mol^-1, defined as an exact value since the 2019 SI redefinition. One mole contains N_A particles, and one mole of a substance weighs its molecular weight in grams. The three conversions follow:
| From | To | Formula |
|---|---|---|
| grams | moles | n = m / M |
| moles | particles | N = n · N_A |
| grams | particles | N = m N_A / M |
| particles | grams | m = N M / N_A |
Examples: 18.015 g of water = 1 mol = 6.022 × 10^23 water molecules. One molecule of water weighs 18.015 / 6.022 × 10^23 = 2.99 × 10^-23 g.
Molecular weight of proteins and polymers
For biomolecules and polymers, atom counting is impractical. Mass spectrometry measures molecular weight directly by ionizing the molecule and timing its flight through a magnetic field. Typical proteins are 10-100 kDa; hemoglobin (4 subunits, each about 16 kDa) is 64.5 kDa; large molecular machines like ribosomes reach 2.5 MDa.
Polymers have a distribution of molecular weights, not a single value. The number-average M_n and weight-average M_w report different statistical means. Their ratio (polydispersity index, M_w / M_n) measures the breadth of the distribution. A monodisperse sample has PDI = 1; commercial polyethylene runs 2-10.
Common molecular weights
- H2O: 18.015 g/mol
- CO2: 44.01 g/mol
- O2: 32.00 g/mol
- NaCl: 58.44 g/mol
- Glucose (C6H12O6): 180.16 g/mol
- Sucrose (C12H22O11): 342.30 g/mol
- Aspirin (C9H8O4): 180.16 g/mol
- Caffeine (C8H10N4O2): 194.19 g/mol
- Cholesterol (C27H46O): 386.65 g/mol
- Penicillin G: 334.39 g/mol
- Insulin (51 amino acids): 5807 g/mol
- Hemoglobin (4 subunits): ~64,500 g/mol
Where molecular weight matters
Pharmacology. Drug dosing typically uses mg of compound, but receptor binding is described in nM molar concentration at the receptor. Translating one to the other (and back) requires molecular weight at every step. A 100 mg aspirin tablet contains 100 / 180.16 = 0.555 mmol of acetylsalicylic acid; in a 5-L blood volume, that gives a peak plasma concentration around 100 micromolar, well above the receptor affinity. Every pharmacokinetics calculation starts here.
Combustion. Burning 1 g of methane (CH4, MW 16.04) produces 1 / 16.04 mol = 0.0623 mol of CO2 (MW 44.01) = 2.74 g of CO2. The 2.74:1 ratio is what climate accounting tracks. Burning 1 kg of gasoline (about C8H18, MW 114.23) produces 3.09 kg of CO2 by the same logic. Vehicle emission standards convert between fuel mass and CO2 mass using molecular weight.
Forensic toxicology. Identifying unknown compounds in seized samples relies on mass-spec molecular weight matching against database entries. A few seconds of analysis time on an LC-MS instrument can reduce candidate compounds from millions to a short list, simply by matching the measured molecular weight to a few decimal places against PubChem entries.
Polymer chemistry. Polyethylene's tensile strength, melting point, and processability all depend on molecular weight. Ultra-high molecular weight polyethylene (UHMWPE) used in bulletproof vests has molecular weights of 3 to 6 million g/mol. Low-density polyethylene used in plastic bags is 50,000 to 250,000 g/mol. The chemistry is the same; the molecular weight is the entire engineering distinction.
Molecular weight mistakes
The 12 in C6H12O6 is not optional. Skipping a subscript (treating C6H12O6 as C+H+O) gives 29 g/mol instead of 180.16, a factor of 6 error. Every digit matters; double-check by counting atoms before computing.
Other regular slips: confusing atomic with molecular weight for diatomic elements (oxygen atom is 16 g/mol, O2 is 32), forgetting parentheses in Ca(OH)2 (should be 74, not 57), using integer rather than tabulated atomic weights (Cl = 35.45, not 35), and ignoring water of crystallization in hydrates.
Standard atomic weights are weighted averages over natural isotope abundances. For most chemistry the average is exactly what you want. For mass spectrometry, isotope-resolved peaks require the monoisotopic mass (using the most abundant isotope of each element), which differs from the average atomic mass by about 0.5 g/mol for typical organic molecules.