Molar Mass Calculator
The molar mass of any chemical formula, with the mass each element contributes and the percent composition.
Capital letters start an element. Use brackets for groups, Ca(NO3)2, and a dot for hydrates, CuSO4·5H2O.
Molar mass of H₂O
18.015 g/mol
3 atoms per formula unit. The molecular weight is the same number without units.
Composition by element
| Element | Atoms | Atomic mass | Mass | % of total |
|---|---|---|---|---|
| Oxygen | 1 | 15.999 | 15.999 | 88.81% |
| Hydrogen | 2 | 1.008 | 2.016 | 11.19% |
| Total | 3 | 18.015 | 100% |
Each element links to its full data. Percentages are the percent composition by mass — the same numbers a percent composition calculator gives.
Convert grams and moles
- Mass
- 1 g
- Amount
- 0.0555 mol
- Particles
- 3.343 × 10²²
How molar mass is calculated
Molar mass is an addition problem, not a lookup. Every atom in the formula contributes its own standard atomic weight, so the work is: count the atoms, multiply each count by its element’s atomic weight, add the results. The table above shows exactly that sum, which is why it is worth reading rather than skipping to the total — if your hand calculation disagrees, the row that differs tells you where.
The atomic weights used here come from the IUPAC Commission on Isotopic Abundances and Atomic Weights, the same values printed on a modern periodic table. They are averages over the isotopes found in nature, which is why chlorine is 35.45 rather than a whole number: natural chlorine is roughly three quarters chlorine-35 and one quarter chlorine-37.
Writing formulas this calculator understands
- Brackets multiply everything inside them: Ca(NO3)2, Fe2(SO4)3, and nested groups such as Ca(H2PO4)2.
- Hydrates use a dot: CuSO4·5H2O. A full stop or asterisk works too.
- Ionic charges can be written as SO4^2-, SO42- or Ca++. The charge is reported but does not change the mass, since an electron weighs about 1/1836 of a proton.
- Pasted subscripts such as H₂O are converted automatically, so a formula copied out of a textbook or a web page works without retyping.
Molar mass, molecular weight and formula mass
These three terms describe the same arithmetic and differ only in what they claim about the substance. Molecular weight assumes discrete molecules, which is true of water and glucose but not of sodium chloride: NaCl is a lattice of ions, and there is no such thing as an NaCl molecule. For ionic compounds the honest term is formula mass — the mass of one formula unit. Molar mass sidesteps the argument entirely by talking about a mole of whatever the substance is made of, which is why it is the term used in the SI.
Common molar masses
Each of these has its own page with the full working:
- Water18.02
- Sodium chloride58.44
- Glucose180.16
- Sulfuric acid98.07
- Calcium carbonate100.09
- Ammonia17.03
- Sodium hydroxide40.00
- Copper(II) sulfate pentahydrate249.68
- Acetic acid60.05
- Carbon dioxide44.01
- Potassium permanganate158.03
- Sucrose342.30
Browse all 121 compounds · Balance a chemical equation · Periodic table
Related Calculators
Frequently Asked Questions
What is molar mass?
Molar mass is the mass of one mole of a substance, in grams per mole. Numerically it equals the sum of the standard atomic weights of every atom in the formula, so water — two hydrogens at 1.008 and one oxygen at 15.999 — comes to 18.015 g/mol.
What is the difference between molar mass and molecular weight?
They are the same number. Molecular weight (or relative molecular mass) is a ratio against 1/12 the mass of a carbon-12 atom, so it has no units; molar mass is that same value expressed in g/mol. In practice people use the terms interchangeably, and any calculator that gives you one has given you the other. Molecular weight is also slightly wrong for ionic compounds like NaCl, which have no molecules — "formula mass" is the correct term there.
How do you calculate molar mass by hand?
Count the atoms of each element in the formula, multiply each count by that element's atomic weight from the periodic table, and add the results. For Ca(NO₃)₂: one calcium at 40.078, two nitrogens at 14.007, six oxygens at 15.999 — the brackets multiply everything inside them by 2 — giving 164.09 g/mol.
How do I type a hydrate like copper sulfate pentahydrate?
Write it as CuSO4·5H2O. A full stop or an asterisk works too, so CuSO4.5H2O and CuSO4*5H2O both parse. The number in front of H2O multiplies the water, and those waters count towards the molar mass — 249.68 g/mol for the pentahydrate against 159.61 for the anhydrous salt.
Why does capitalisation matter?
Because the case is the symbol. Co is cobalt; CO is carbon monoxide. Writing "co" or "CO" for cobalt gives a different compound with a different mass, so this calculator is case-sensitive and will tell you when a symbol does not exist rather than guessing.
How do I convert grams to moles?
Divide the mass in grams by the molar mass: n = m ÷ M. For 25 g of water, 25 ÷ 18.015 = 1.388 mol. To go the other way, multiply: m = n × M. The converter above does both, and also gives the number of particles by multiplying moles by Avogadro's number, 6.02214076 × 10²³.
Why does the calculator warn about some elements?
Elements with no stable isotope — technetium, promethium, and everything above bismuth — have no standard atomic weight. The periodic table shows the mass number of their longest-lived isotope instead, usually in brackets. Any molar mass involving them is conventional rather than measured, so it is flagged.