MolDraw › Chemistry tools › Molecular Weight Calculator

Molecular Weight Calculator

In short: Molecular weight calculator: type any formula (CuSO4·5H2O, Ca(OH)2) and get g/mol, element breakdown and % composition instantly. It is free, works in your web browser and needs no download.

Type a chemical formula and get its molecular weight (molar mass) in g/mol, with a per-element breakdown and percent composition. Works with parentheses, brackets, hydrates and all 118 elements.

Quick answer: molecular weight = sum of (number of atoms × atomic weight) for every element. For example, H2O = 2 × 1.008 + 15.999 = 18.015 g/mol.

Try an example

Hydrates: use · , * or a dot (CuSO4.5H2O). Element symbols are case-sensitive: Co is cobalt, CO is carbon monoxide.

Enter a formula to calculate molecular weight.

How to use the molecular weight calculator

  1. Type the chemical formula, for example Ca(OH)2 or C6H12O6. Use correct capitalisation for element symbols.
  2. Use parentheses or brackets for groups (Al2(SO4)3, K4[Fe(CN)6]) and a dot or · for hydrates (CuSO4·5H2O).
  3. Read the molecular weight in g/mol. The table shows the atoms of each element, their mass contribution and mass percent.
  4. Click Copy result to paste the answer into your notes, or open a related tool to convert grams to moles.

Worked examples

Water, H2O

2 × 1.008 + 1 × 15.999

= 18.015 g/mol

Glucose, C6H12O6

6 × 12.011 + 12 × 1.008 + 6 × 15.999
= 72.066 + 12.096 + 95.994

= 180.156 g/mol

Copper(II) sulfate pentahydrate, CuSO4·5H2O

CuSO4 (159.60) + 5 × H2O (18.015)

= 249.677 g/mol

Molecular weight formula

The molecular weight (MW) of a substance is the mass of one mole of it, in grams per mole (g/mol). It is found by adding the standard atomic weights of all the atoms in the formula:

MW = Σ (ni × Ai) where ni = number of atoms of element i, Ai = standard atomic weight

Atomic weights come from the IUPAC table of standard atomic weights (abridged values, e.g. C = 12.011, H = 1.008, O = 15.999, N = 14.007). For elements with no stable isotope (Tc, Pm and everything from Po onwards) the mass number of the longest-lived isotope is used, as on most periodic tables.

Molecular weight vs molar mass vs formula weight

In everyday chemistry the three terms give the same number. Molar mass has units of g/mol. Molecular weight (relative molecular mass, Mr) is strictly unitless but numerically equal. Formula weight is used for ionic compounds such as NaCl that do not form discrete molecules. For mass spectrometry you usually want the monoisotopic exact mass instead.

Percent composition

Each element's mass percent is its mass contribution divided by the molecular weight × 100. In glucose, carbon contributes 72.066 / 180.156 = 40.00% of the mass. Going the other way, from percentages to a formula, is what the empirical formula calculator does.

Have a structure rather than a formula? Draw it in the free MolDraw editor and get the formula, molecular weight and exact mass automatically.

Open MolDraw editor

Frequently asked questions

How do you calculate molecular weight?

Multiply the number of atoms of each element by its atomic weight and add the results. For CO2: 12.011 + 2 × 15.999 = 44.009 g/mol.

What is the molecular weight of water (H2O)?

18.015 g/mol: two hydrogen atoms (2 × 1.008 = 2.016) plus one oxygen atom (15.999). It is often rounded to 18 g/mol.

Is molecular weight the same as molar mass?

Numerically yes. Molar mass is expressed in g/mol, while molecular weight (relative molecular mass) is a unitless ratio with the same value.

How do I enter hydrates or complex ions?

Use a dot, asterisk or middle dot for water of crystallisation (CuSO4.5H2O) and brackets or parentheses for groups (K4[Fe(CN)6], Ca3(PO4)2).

Why does my formula say unknown element?

Element symbols are case-sensitive. Write Co for cobalt, Cl for chlorine and Mg for magnesium; CO and CL are read as carbon + oxygen and an invalid symbol.

How do I find the molecular weight of a protein?

Use the protein molecular weight calculator, which sums amino-acid residue masses from a one-letter sequence and adds one water.

Related chemistry tools

Molar mass calculatorStep-by-step molar mass workingEmpirical formula calculatorPercent composition to formulaGrams to moles calculatorConvert mass and molesExact mass calculatorMonoisotopic mass for MSProtein molecular weight calculatorSequence to kDa and extinction coefficientSMILES to molecular weightMW straight from a SMILES stringMolarity calculatorMoles, volume and concentrationLimiting reagent calculatorLimiting reactant and theoretical yieldIsotope pattern calculatorM+1 and M+2 peaks for any formulaBatch chemical converterConvert a whole CSV of SMILES or names

Need to sketch molecules for a report? Draw chemical structures online with MolDraw, a free online ChemDraw alternative (see the ChemDraw vs MolDraw comparison).

Last updated: