CalcaTools

Molecular Weight Calculator

A Molecular Weight Calculator is a free online tool that computes molar mass calculator using molar mass calculator, formula weight, atomic mass units. Tradespeople, planners, and curious owners use it to sanity-check estimates, plan projects, and answer everyday questions.

Last updated: June 2026 · Free · No sign-up required

Results

Enter values above and click Calculate to see your result instantly.

Quick reference

ElementAtomic mass (g/mol)
H1.008
C12.011
N14.007
O15.999
Na / Cl22.990 / 35.45

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Every tool is plain HTML + JavaScript with the calculation logic compiled in. No spinner, no "verifying browser" splash, no five-second wait while ads load. Hit the page, fill the form, get the answer. Slow tools waste your time and ours.

devices Works on every device

Responsive design tested down to a 320px-wide phone screen. The input fields use native number/date keyboards on iOS and Android, the calculate button is thumb-sized, and the result still reads big and bold on the smallest screen. No app to download, ever.

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No newsletter pop-ups, no "create an account to see the answer", no fake countdown, no fake stock counter, no upsell. Two ad slots per page (clearly labelled as Advertisement) keep the site free — that's the whole business model.

Interpretation guide

CompoundMolar mass
H₂O18.02 g/mol
CO₂44.01 g/mol
NaCl58.44 g/mol
C₆H₁₂O₆ (glucose)180.16 g/mol

Formula & methodology

Formula: Molar mass = sum over atoms of (atomic mass x count); moles = mass (g) / molar mass (g/mol)

How molecular weight is calculated

Molecular weight (molar mass) is the sum of the atomic masses of every atom in the formula, in grams per mole.

Example: H₂O

2 hydrogen (2 x 1.008) + 1 oxygen (15.999) = 2.016 + 15.999 = 18.02 g/mol.

Example: glucose C₆H₁₂O₆

6 x 12.011 + 12 x 1.008 + 6 x 15.999 = 72.066 + 12.096 + 95.994 = 180.16 g/mol.

Once you have the molar mass, convert any sample: moles = mass / molar mass. So 36 g of water is 36 / 18.02 = 2.0 mol.

Frequently asked questions

How do you calculate molecular weight?
Add up the atomic masses of all the atoms in the chemical formula. Multiply each element's atomic mass (from the periodic table) by how many of that atom appear, then sum the results. For water (H₂O): 2 x 1.008 + 15.999 = 18.02 g/mol.
What is the difference between molecular weight and molar mass?
Numerically they are the same for a given substance. Molecular weight (or molecular mass) is technically a relative, unitless ratio to one-twelfth of a carbon-12 atom, while molar mass carries the unit grams per mole (g/mol). In everyday chemistry the terms are used interchangeably.
How do I convert grams to moles?
Divide the mass in grams by the molar mass in g/mol: moles = mass / molar mass. For example, 58.44 g of NaCl (molar mass 58.44 g/mol) is exactly 1 mole. To go the other way, multiply moles by molar mass to get grams.
What is the molar mass of water?
Water (H₂O) has a molar mass of about 18.02 g/mol: two hydrogen atoms at 1.008 each (2.016) plus one oxygen at 15.999, totaling 18.015 g/mol. This means one mole of water — about 6.022 x 10²³ molecules — weighs roughly 18 grams.
Do I include the subscripts and parentheses in the formula?
Yes. Subscripts multiply the atom immediately before them, and a subscript outside parentheses multiplies everything inside. For Ca(OH)₂, you have one calcium, two oxygens, and two hydrogens: 40.08 + 2 x (15.999 + 1.008) = 74.09 g/mol. Counting atoms correctly is the key step.