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Molecular Weight Calculator — formula, SMILES, or draw a structure. Average + monoisotopic mass, free.

Type a molecular formula like C9H8O4, paste a SMILES, or draw a structure, and get the average molecular weight and the monoisotopic mass — both labeled, in g/mol — plus elemental composition. Paste or draw a structure and you also get logP, TPSA, and Lipinski drug-likeness. Runs in your browser. Free, no sign-up.

Try an example:

Or draw / edit a structure to compute its properties:

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Best on a larger screen — drawing is limited on phones. You can still type a formula or paste a structure, and read and copy every result.

This tool runs entirely in your browser. Your structures are never sent to a server.

Worked example: glucose vs aspirin

Glucose (C6H12O6) and aspirin (C9H8O4) both have an average molecular weight of 180.16 g/mol — but their monoisotopic masses are clearly different: 180.0634 for glucose and 180.0423 for aspirin. That gap is exactly why a mass-spec match needs the monoisotopic mass, not the average — and why this tool labels both.

Frequently asked questions

Average vs monoisotopic molecular weight — which do I need?
Use the average molecular weight when you weigh out or dose a compound, or compute molarity — it sums each element’s standard atomic weight, the natural mix of isotopes, which is what a balance measures. Use the monoisotopic mass when you match a peak in high-resolution mass spectrometry — it counts only the single most-abundant isotope of each element (¹²C, ¹H, ¹⁶O), so it lines up with the principal isotopic peak. The two numbers differ, and the difference grows with chlorine, bromine, sulfur, and other heavy elements. Glucose and aspirin both average 180.16 g/mol but their monoisotopic masses, 180.0634 and 180.0423, are clearly different — which is exactly why a mass-spec identification uses the monoisotopic mass.
Is Lipinski’s Rule of Five a pass/fail law?
No — it’s a heuristic for the oral bioavailability of small molecules, not a rule a compound must obey. The Rule of Five flags a molecule when more than one of four properties is out of range: molecular weight over 500, calculated logP over 5, more than five hydrogen-bond donors, or more than ten acceptors. Plenty of approved drugs break one of these, and whole classes (peptides, macrocycles) sit well outside it. This tool names which criterion is violated rather than reporting a bare pass or fail, so you can judge whether the violation matters for your series.
Why does my logP differ from another tool’s?
logP is predicted, not measured, and different tools use different prediction models — so two calculators routinely disagree by a few tenths of a log unit. This tool reports the Wildman-Crippen atom-contribution estimate (the model RDKit uses). Topological polar surface area can differ too: the default here is the Ertl N+O variant, which omits the sulfur and phosphorus contributions that the extended definition includes. A different number from another tool usually means a different model or definition, not a bug — so the value is labeled with the method that produced it.
What can I enter — formula, SMILES, or a drawing?
All three. Type a molecular formula such as C9H8O4 and you get the average molecular weight, the monoisotopic mass, and the elemental composition straight away — no structure needed. Paste a SMILES or InChI, or draw a structure, and you additionally get logP, TPSA, hydrogen-bond donor and acceptor counts, rotatable bonds, and the Lipinski Rule-of-Five verdict, because those depend on how the atoms are connected. The tool detects which kind of input you entered and tells you, and when a short token could be read either as a formula or a structure it lets you switch with one click.
How are salts and hydrates handled?
A multi-component input — a drug plus a counterion, or a hydrate — is detected, and the component count is shown. By default every component is kept and the reported molecular weight is for the whole entered species (the salt form). You can strip the counterion to get the parent molecule’s weight instead; the reported mass is labeled so you always know whether you’re looking at the salt or the parent.
Does this work in my browser / is my structure private?
Everything runs in your browser. Parsing the formula, computing both masses and the composition, laying out the structure, and predicting the properties all happen client-side, so your structures and formulas are never uploaded to a server — safe for proprietary or unpublished compounds.

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