What is molecular weight?

Molecular weight (also called molar mass or formula weight) is the sum of atomic weights of all atoms in a molecule, expressed in g/mol. For H2O: 2(1.008) + 15.999 = 18.015 g/mol. It tells you how many grams of a substance equal one mole (6.022 x 10^23 molecules). Our calculator uses IUPAC 2021 standard atomic weights.

How do I calculate molecular weight step by step?

Three steps: (1) Parse the formula into elements and counts. H2SO4 = H:2, S:1, O:4. (2) Look up each element's atomic weight on the periodic table. H=1.008, S=32.06, O=15.999. (3) Multiply count by atomic weight and sum: 2(1.008) + 1(32.06) + 4(15.999) = 98.07 g/mol. Our calculator does all three steps automatically.

What is the difference between molecular weight and molar mass?

They are the same number, just different names. Molecular weight is technically dimensionless (relative to 1/12 the mass of C-12). Molar mass has units of g/mol. In practice, scientists use them interchangeably, and the periodic table values are identical for both purposes.

How do I calculate the molecular weight of a hydrate?

Include the water in the formula. CuSO4.5H2O is copper(II) sulfate pentahydrate. Add 5 water molecules to the formula: Cu:1, S:1, O:4+5=9, H:10. Total = 63.55 + 32.06 + 9(16.00) + 10(1.008) = 249.68 g/mol. Our calculator accepts the dot notation directly.

How do I convert grams to moles using molecular weight?

Moles = grams / molecular weight. For 50 g of NaCl (MW 58.44): 50 / 58.44 = 0.856 mol. To convert moles to grams, multiply moles by molecular weight. Our calculator has a Mass (grams) field that automatically shows the corresponding moles when you enter a formula.

What is the molecular weight of common substances?

Water H2O = 18.015 g/mol. Salt NaCl = 58.44 g/mol. Glucose C6H12O6 = 180.16 g/mol. Sulfuric acid H2SO4 = 98.08 g/mol. Ethanol C2H5OH = 46.07 g/mol. Aspirin C9H8O4 = 180.16 g/mol. Caffeine C8H10N4O2 = 194.19 g/mol. Carbon dioxide CO2 = 44.01 g/mol. Our calculator handles all of these and any other valid formula.

Molecular Weight Calculator - Molar Mass from Formula

Parse the formula

Break the formula into elements and their counts. Handle parentheses, subscripts, and hydrates.

Look up atomic masses

Use IUPAC standard atomic weights from the periodic table. Multiply each mass by its count.

Sum the total

Add up all element contributions. The result is the molecular weight in g/mol.

Common Applications

Stoichiometry: Convert grams to moles or vice versa for any balanced reaction.
Solution prep: Calculate how many grams of a compound to dissolve to make a given molarity.
Percent yield: Compare actual product mass to theoretical mass from a balanced equation.
Elemental analysis: Verify a compound's composition matches the theoretical formula.
Drug dosage: Convert between dose (mg/kg) and moles per body weight.

Enter a chemical formula

Type the formula using standard notation: H2O, C6H12O6, Ca(OH)2, CuSO4.5H2O. We parse parentheses, nested groups, and hydrates automatically.

We fetch atomic masses

For each element, we look up the standard atomic weight from the periodic table. The calculator also shows the count of each atom in the formula.

Get the molar mass

See the total molecular weight in g/mol plus a per-element breakdown. Useful for stoichiometry, solution preparation, and chemistry homework.

Key things to know about our molecular weight calculator:

Common notations: Ca(OH)2 = calcium hydroxide, 74 g/mol. CuSO4.5H2O = copper(II) sulfate pentahydrate, 250 g/mol. The dot separates the hydrate water from the salt.
For solutions: To make 1 liter of 1 M solution, multiply molecular weight by 1 to get grams per liter. M = moles per liter, m = moles per kg solvent.
Use across chemistry: Molecular weight is fundamental to stoichiometry, limiting reagent problems, yield calculations, and titration math.

For related calculations, try our proportion calculator.

Frequently asked questions

Everything you need to know about this calculator. Can't find what you're looking for? Email our team.

A molecular weight calculator is a free chemistry tool that finds the molar mass of any chemical compound from its formula. Our molecular weight calculator uses the IUPAC standard atomic weights from the periodic table and supports parentheses, hydrates, and nested groups. The molecular weight calculator also shows the percent composition of each element.

How to calculate the molecular weight of a compound

Our molecular weight calculator lets you find the molar mass of any chemical compound from its formula, with full breakdown by element, in three steps.

1
Enter the chemical formula
Type the formula using element symbols and subscripts. Examples: H2O, C6H12O6, NaCl, H2SO4, Ca(OH)2, CuSO4.5H2O for hydrates. Element symbols are case-sensitive: Co is cobalt, CO is carbon monoxide. The Quick examples row gives common formulas.
2
Calculate the molecular weight
Press Calculate. The result shows the molar mass in g/mol, the total number of atoms, and the count of unique elements. The breakdown card shows each element's count, mass contribution, and percent of the total mass.
3
Optionally enter a mass in grams
Type a mass in grams to also see how many moles that mass represents. Useful for solution prep: enter 5 g to see 5/180.16 = 0.0277 mol of glucose. The share button creates a link with both the formula and the mass pre-filled.

Molecular Weight Calculator

Molecular Weight Calculator

How Molecular Weight Works

Parse the formula

Look up atomic masses

Sum the total

Common Applications

How our molecular weight calculator works

Enter a chemical formula

We fetch atomic masses

Get the molar mass

Key things to know about our molecular weight calculator:

Frequently asked questions

How to calculate the molecular weight of a compound

Enter the chemical formula

Calculate the molecular weight

Optionally enter a mass in grams

Related Articles

Mastering Mathematical Problem-Solving

Calculation Best Practices