How this instrument works
Molecular weight (also called molecular mass) is the total mass of one molecule, found by summing the atomic mass of every atom in it. Glucose, C6H12O6, is six carbons (12.011 u each), twelve hydrogens (1.008 u each), and six oxygens (15.999 u each), adding up to a molecular weight of about 180.16 u. The unit here is the dalton (Da) or, equivalently, the unified atomic mass unit (u) — both are defined so that a single carbon-12 atom weighs exactly 12, which is why atom-counting a molecule directly gives you its weight in these units without any extra conversion factor.
This calculator uses element dropdown rows rather than a free-text formula field: pick an element, enter how many atoms of it are in the molecule, and repeat for up to four distinct elements. Each dropdown option already carries that element's standard atomic weight, so the total is just atom count multiplied by atomic weight, summed across every row you fill in — the arithmetic itself is the entire calculation.
Molecular weight and molar mass are, numerically, the same computation — both sum atomic weights across a formula. Where they differ is context and units: molecular weight is the term you'll see attached to a single molecule's size in biochemistry, molecular biology, and pharmacology (proteins, drugs, oligonucleotides), usually in Da or u, while molar mass is the general-chemistry term for the same figure in g/mol used when converting a lab-balance mass into a mole quantity for a reaction.
- Pick the first element in your molecule from the Element 1 dropdown, then enter how many atoms of it appear into Number of atoms of element 1.
- Repeat for Element 2 and Number of atoms of element 2.
- If your molecule has a third or fourth distinct element, fill in Element 3 / Number of atoms of element 3 and Element 4 / Number of atoms of element 4; otherwise leave their atom counts at 0.
- Read Molecular weight (u / Da) below the inputs — it updates instantly as any selection or count changes.
- For a molecule built from more than four distinct elements, total the first four here, then add the remaining elements' atom-count-times-atomic-weight contributions by hand.
Worked example — glucose, C6H12O6
Select Carbon (C) as Element 1 and enter 6 into Number of atoms of element 1, select Hydrogen (H) as Element 2 and enter 12 into Number of atoms of element 2, select Oxygen (O) as Element 3 and enter 6 into Number of atoms of element 3, and leave Element 4's count at 0. Molecular weight (u / Da) reads about 180.156 u: 6 x 12.011 + 12 x 1.008 + 6 x 15.999 = 180.156.
Introductory courses often round this to exactly 180 u using whole-number atomic masses (12, 1, 16) for carbon, hydrogen, and oxygen — the small 0.156 u difference here comes purely from using full-precision IUPAC standard atomic weights instead of those rounded textbook shortcuts, not from any different method. Either figure is close enough for everyday lab dilutions; the full-precision value matters more in contexts like mass spectrometry, where the extra digits are actually resolvable.
Questions
Is molecular weight the same thing as molar mass?
Numerically, essentially yes — both are computed by summing atomic weights across a formula, and for a given compound they come out to the same number. The difference is mainly which field uses which term and unit: molecular weight (in u or Da) is the everyday term in biochemistry and molecular biology for describing how big a single molecule is, while molar mass (in g/mol) is the general-chemistry term for the same quantity, framed around converting a weighed mass into a mole count for a reaction.
What's the difference between a dalton (Da) and a unified atomic mass unit (u)?
They're the same size unit, defined identically as 1/12 the mass of a single carbon-12 atom — dalton is just the more common name in biology and biochemistry, while unified atomic mass unit (or simply 'atomic mass unit') is more common in physics and general chemistry. A molecular weight of 180.156 u and 180.156 Da are the exact same statement, just using different naming conventions from different fields.
Why does this use element dropdowns instead of a typed formula?
Typing an arbitrary chemical formula like C6H12O6 requires parsing subscripts and (for more complex molecules) nested parentheses correctly before any arithmetic can happen — a meaningfully different kind of computation from simply adding numbers. The dropdown-and-count-row layout produces the identical result for any molecule built from up to four elements, with each element's atomic weight already attached to its dropdown option so there's no separate lookup or typo risk.
What if my molecule has more than four distinct elements?
Compute it in two passes: enter the first four elements and their atom counts here to get a subtotal, then by hand add each remaining element's atom count multiplied by its atomic weight to that subtotal. Small molecules — water, sugars, simple organic acids, common drugs like aspirin (C9H8O4) — typically fit in four elements or fewer; larger biomolecules like proteins usually don't.
Does molecular weight change if the same atoms are arranged differently?
No. Molecular weight only depends on which atoms are present and in what quantity, not on how they're bonded or arranged in space. Structural isomers — different molecules built from the identical set of atoms, like glucose and fructose (both C6H12O6) — always have exactly the same molecular weight even though they're chemically and biologically distinct compounds.