Molecular Weight Calculator · Practical guide
Molecular Weight vs. Molar Mass
Same number, different framing — 18.015 Da per molecule equals 18.015 g per mole.
At a glance
The Short Answer
Molecular weight and molar mass are numerically identical; they differ in units and viewpoint. Water's molecular weight is 18.015 daltons per molecule, while its molar mass is 18.015 g/mol. In everyday lab and homework use, the terms are interchangeable.
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Molecular Weight Calculator
Enter your own values, then compare the result with the examples below.
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The Technical Difference
- Molecular weight (more precisely, relative molecular mass) is dimensionless — a ratio compared to one-twelfth the mass of a carbon-12 atom, expressed per molecule in daltons.
- Molar mass is a physical quantity with units of g/mol: the mass of 6.022 × 10^23 particles of the substance.
A Concrete Example
One molecule of water weighs about 18.015 Da, which is 18.015 ÷ (6.022 × 10^23) = 2.99 × 10^−23 g. Scale that single molecule up by Avogadro's number and you get one mole of water weighing 18.015 g — the number never changes, only the scale.
What About Ionic Compounds?
Strictly speaking, ionic compounds like NaCl do not form discrete molecules, so chemists use the term formula mass instead of molecular weight. The calculation is identical: 22.990 + 35.45 = 58.44 g/mol. This calculator handles molecular and formula masses the same way — enter the formula and read the total.
Which Term Should You Use?
When weighing chemicals for a solution, say molar mass (g/mol), because you are converting between grams and moles. When discussing a single molecule in biochemistry — a protein of 50 kDa, for instance — molecular weight in daltons is the convention.
Frequently Asked Questions
Is molecular weight the same as atomic weight?
No. Atomic weight refers to a single element (for example, oxygen at 15.999), while molecular weight sums the atomic weights of all atoms in a molecule.
Why do proteins use daltons instead of g/mol?
Daltons describe mass per molecule, which suits huge biomolecules. A 66 kDa albumin molecule also has a molar mass of 66,000 g/mol — the dalton figure is simply easier to say.
Does molar mass change with isotopes?
Standard molar masses use the natural isotope mix. A sample enriched in a heavy isotope, like carbon-13, would have a slightly higher molar mass than the table value.
Which value do I use in molarity calculations?
Use the molar mass in g/mol. Molarity = grams ÷ molar mass ÷ liters, so a g/mol value plugs in directly.