Molecular Weight Calculator

The calculator above turns a chemical formula into a molecular weight, breaks it down atom by atom, and draws two charts: how the molecule divides by count of atoms, and how it divides by mass. Those two answers are rarely the same, and the difference between them is worth understanding.

Terms worth pinning down first

Atom is the basic particle of a chemical element, built from a nucleus of protons and neutrons with electrons around it. What makes an element that element is its proton count, which is its atomic number.

Isotope is an atom of the same element with a different number of neutrons. Every atom with 12 protons is magnesium, but magnesium has three stable isotopes, ²⁴Mg, ²⁵Mg and ²⁶Mg, carrying 12, 13 and 14 neutrons.

Mole is the SI unit for amount of substance. One mole is exactly 6.02214076 × 10²³ particles, the Avogadro number, and those particles can be atoms, molecules, ions or anything else you care to count.

Molecule is two or more atoms held together by chemical bonds. Sample is a small amount of material taken from a larger quantity for testing.

Atomic weight

Atomic weight, more properly relative atomic mass, is the ratio of the average mass of a sample of atoms of an element to the atomic mass constant. Because it is a ratio of two masses it is dimensionless, which is the part people miss. It is a weighted average across all the isotopes present in a normal sample, weighted by how abundant each one is.

Atomic mass is a different quantity: the mass of one single atom, measured in daltons.

Hydrogen makes the averaging concrete. Hydrogen-1 accounts for 99.9855% of natural hydrogen and deuterium, hydrogen-2, for 0.0145%, with tritium in negligible traces. Their masses are 1.007825031898 Da and 2.01410177811 Da, so:

1.007825031898 × 99.9855% + 2.01410177811 × 0.0145% ≈ 1.008

That is where the 1.008 in the table comes from. It is not the mass of a hydrogen atom; it is the average of a bucket of them.

Molecular weight

Molecular weight, more properly relative molecular mass, is the same ratio applied to a molecule instead of an atom, and it is dimensionless for the same reason. Since a molecule is a collection of atoms, its molecular weight is the sum of the atomic weights of everything in it. For water, using 1.008 for hydrogen and 15.999 for oxygen:

2 × 1.008 + 15.999 = 18.015

Molar mass, and why this page treats it as the same thing

Molar mass is the mass of one mole of a substance, normally in grams per mole. It is a mass per amount, so unlike molecular weight it genuinely has units.

Numerically the two used to be identical by definition. Since the 2019 redefinition of the SI base units they differ very slightly, by far less than the rounding in any abridged table. So for every practical purpose, including this calculator and any chemistry class, the same number serves both, which is why the answer above is labelled g/mol.

Molecular mass is a third thing again: the mass of one specific molecule, in daltons, counting the actual nuclides in it rather than element averages. The relative molecular mass of water is 18.015, but an individual water molecule can have a molecular mass anywhere from about 18.0106 to 22.0277 Da depending on which isotopes it happens to contain.

In casual use these three run together, and mostly that causes no harm. It matters when precision does, such as in mass spectrometry, where you are weighing individual molecules and the isotope you caught is exactly the point.

Working one out by hand

Three steps: count the atoms of each element, look up each atomic weight, then multiply and add.

Writing the formula so it parses

Chemical formulas are case sensitive and this is not a formatting preference. Co is cobalt; CO is carbon monoxide. One is 58.933 g/mol, the other 28.01, and nothing in the string tells you which was meant except the capital letter.

The calculator accepts the notation you would actually write. Brackets nest, so Al2(SO4)3 and [Co(NH3)6]Cl3 both work. Hydrates take a dot, so CuSO4.5H2O is read as the anhydrous salt plus five waters. Where an element symbol is not recognised, it says so rather than quietly treating it as something else.

Reading the two composition charts

The atomic composition chart counts atoms. The mass composition chart weighs them. For water, hydrogen is 66.67% of the atoms and only 11.19% of the mass, because oxygen is nearly sixteen times heavier per atom.

That gap is the practically useful part. If you want to know how much of a fertiliser bag is nitrogen, or how much of an ore is iron, the mass chart is your answer, since that is what you weigh and what you pay for. The atomic chart tells you about the structure of the molecule instead, which is what matters when balancing an equation.

What the number is for

Molecular weight is the bridge between the two things a lab can actually do: count moles and weigh grams. A reaction equation is written in moles, but a balance reads in grams, and molar mass is what converts one into the other.

That single conversion underpins working out how much reactant to weigh for a given yield, making up a solution to a target molarity, calculating percentage composition, and turning a measured product mass back into a percentage yield. Get the molar mass wrong and every downstream number inherits the error at full size.

Abridged standard atomic weights

The values below are the IUPAC abridged standard atomic weights, and they are what the calculator uses. Abridged means rounded to a fixed number of places with the published uncertainty dropped, which is what makes them usable for ordinary work.

Some elements have no single natural composition, so their entry is the mass number of the most stable or best-known isotope rather than an average. That is why the heavy synthetic elements show round numbers, and why several have no density: not enough has ever been made to weigh.

Reading your result

Check the atom counts in the table before the total. A molecular weight that looks wrong is almost always a miscounted formula rather than a bad atomic weight, and the count column makes that visible at a glance.

Round sensibly. Three decimal places is more than any teaching lab needs, and the abridged weights themselves carry uncertainty in the last digit, so a molar mass quoted to five decimals is claiming precision the input never had.

To turn this into a solution concentration, the Molarity Calculator takes the molecular weight straight from here, and for mass and volume unit changes the Conversion Calculator handles the arithmetic.