How this instrument works
Molar mass is the mass of exactly one mole (6.02214076 x 10^23 formula units) of a substance, expressed in grams per mole. It's built by adding up the standard atomic weight of every element in the compound's formula, each multiplied by how many atoms of that element appear — water, H2O, is two hydrogens (1.008 g/mol each) plus one oxygen (15.999 g/mol), for a molar mass of 18.015 g/mol. This is the number a chemist reaches for constantly, because it's the bridge between mass (what a balance actually reads) and moles (what a balanced chemical equation actually counts).
This calculator uses element dropdown rows rather than a free-text formula box: pick an element from the list, enter how many atoms of it appear in the formula, and repeat for up to four distinct elements. Each dropdown option already carries that element's IUPAC standard atomic weight, so once you've selected the right elements and counts, the total is just those atomic weights multiplied by their atom counts and summed — no formula parsing involved on either side.
Four element rows covers the overwhelming majority of everyday compounds — water, salts, common acids and bases, simple organic molecules — but a compound built from five or more distinct elements (many pharmaceuticals and dyes, for instance) won't fit in four rows here. For those, compute the molar mass in two passes: total the atomic weights for the first four elements, note that subtotal, then add the remaining elements by hand.
- Pick the first element in your compound 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 compound 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 Molar mass, M (g/mol) below the inputs — it updates instantly as any selection or count changes.
- Each dropdown lists the element's symbol, name, and atomic weight directly (for example 'O — Oxygen (15.999 g/mol)'), so you can double-check you picked the right one at a glance.
Worked example — water, H2O
Select Hydrogen (H) as Element 1 and enter 2 into Number of atoms of element 1 — water has two hydrogen atoms per molecule. Select Oxygen (O) as Element 2 and enter 1 into Number of atoms of element 2. Leave Element 3 and Element 4's atom counts at 0. Molar mass, M reads 18.015 g/mol: 2 x 1.008 (hydrogen) + 1 x 15.999 (oxygen) = 18.015.
That 18.015 g/mol figure is exactly why 18 grams of water is the go-to 'about one mole' benchmark in intro chemistry — weigh out 18.015 g of water on a balance and you're holding almost exactly one mole, 6.02214076 x 10^23 individual water molecules, ready to react in any stoichiometric calculation that needs mole quantities rather than mass.
Questions
Why does this use dropdowns instead of typing a formula like 'H2O'?
Because parsing an arbitrary typed chemical formula (handling subscripts, nested parentheses like Ca(OH)2, and every possible element symbol) is a different, more involved kind of calculation than adding up numbers. The dropdown-and-count-row approach gets the same result for any compound built from up to four elements, with the atomic weight already attached to each element option so there's no separate lookup step or typo risk from misspelling a formula.
What's the difference between molar mass and molecular weight?
For almost every practical purpose, nothing — they're numerically identical, both computed by summing atomic weights across a formula. The distinction is really about framing and units: molar mass is the term general and analytical chemistry uses, expressed in g/mol, for converting between a balance reading and a mole quantity in a reaction. Molecular weight is more common in biochemistry and molecular biology, often expressed in daltons (Da) or unified atomic mass units (u), describing the mass of a single molecule like a protein or drug compound.
What if my compound has more than four different elements?
Run this calculator twice: enter the first four elements and their counts to get a subtotal, then repeat the same arithmetic by hand for the remaining elements (atom count x atomic weight, summed) and add that to the subtotal. Most everyday compounds — water, salts, common acids and organic solvents — fit within four elements, but larger organic molecules and many drugs won't.
Why do the atomic weights have so many decimal places?
Because most elements occur naturally as a mix of isotopes, and the standard atomic weight is a weighted average across that natural isotopic abundance — chlorine's 35.45 g/mol, for instance, reflects a roughly 76%/24% natural mix of chlorine-35 and chlorine-37. These are the same IUPAC Standard Atomic Weight values (2021 table) published by the Commission on Isotopic Abundances and Atomic Weights, which is why they carry more precision than the rounded whole numbers used in introductory courses.
Does molar mass depend on how the atoms are arranged?
No — molar mass only depends on which elements are present and how many atoms of each, not on their arrangement or bonding pattern. This means two different compounds with the same molecular formula (isomers, like glucose and fructose, both C6H12O6) have exactly the same molar mass despite being structurally distinct molecules with different chemical and physical properties.