Molar mass calculator

Type a formula and get its molar mass, with every step laid out: which elements are in it, how many atoms of each, what each one weighs, and how they add up. The working is the point, not just the answer.

Chemical formula

Write it the way your textbook does. Capital letter for the element, lowercase for the second letter, number after for how many atoms. Brackets work too, like Ca(OH)2.

What molar mass actually means

Molar mass is the mass of one mole of a substance, measured in grams per mole (g/mol). A mole is just a counting word, like "dozen" but far bigger: one mole is 6.022 × 10²³ particles. So molar mass answers a simple question. If you counted out 6.022 × 10²³ molecules of this substance and put them on a balance, what would the balance read?

The useful part is that you never have to count anything. Every element's atomic mass is already measured and published, and those numbers are set up so that adding them together gives you the answer directly in grams per mole. Water is 18.015 g/mol, so one mole of water weighs 18.015 grams. That is roughly a tablespoon.

The one thing students mix up: a subscript multiplies only the atom directly before it, unless there are brackets. In Ca(OH)₂ the 2 applies to everything inside the brackets, so there are two oxygens and two hydrogens, not two hydrogens alone.

How to work it out by hand

The calculator shows these steps for whatever you type, but here is the method itself, using H₂SO₄ as the example.

Step 1. List each element and count its atoms. Read the formula left to right. H₂SO₄ has 2 hydrogen, 1 sulfur, 4 oxygen. The sulfur has no number after it, and no number means one.

Step 2. Look up each atomic mass. From the periodic table: hydrogen 1.008, sulfur 32.06, oxygen 15.999. These are the standard atomic weights published by IUPAC, and they are what your exam formula sheet will use.

Step 3. Multiply each atomic mass by its atom count. Hydrogen: 2 × 1.008 = 2.016. Sulfur: 1 × 32.06 = 32.06. Oxygen: 4 × 15.999 = 63.996.

Step 4. Add the results. 2.016 + 32.06 + 63.996 = 98.072 g/mol. That is the molar mass of sulfuric acid.

Common questions

Is molar mass the same as molecular weight?: They give the same number, but they mean slightly different things. Molecular weight (or relative molecular mass, M_r) is a ratio and has no units. Molar mass has units of g/mol. In school chemistry you can usually treat them as the same value, but write the units on molar mass.
What about the dot in CuSO4.5H2O?: That dot means water of crystallisation, water molecules built into the crystal structure. You add their mass in as normal: five whole water molecules on top of the copper sulfate. This calculator handles the dot notation.
Why do different sources give slightly different atomic masses?: Atomic masses are averages across an element's naturally occurring isotopes, and IUPAC revises the published values as measurements improve. Small differences in the last decimal place are normal. Use whatever your exam board's data sheet gives if it differs from this.
How many significant figures should my answer have?: Match the least precise atomic mass you used, or follow whatever your syllabus specifies. This calculator shows the full unrounded value so you can round it yourself to whatever your marking scheme wants.