Molar Mass Calculator

2026-10-06

Type a chemical formula such as Ca(OH)2 or CuSO4·5H2O to get its molar mass in g/mol, the mass percent of every element and the Hill formula. Then convert between grams, moles and number of particles for the same substance.

How to use the molar mass calculator

1. Type a chemical formula in the box or tap one of the example buttons. Capital letters matter: Co is cobalt, CO is carbon monoxide.
2. Group atoms with round, square or curly brackets, for example Ca(OH)2 or K4[Fe(CN)6]. Write a hydrate with a middle dot, a full stop or an asterisk before the water: CuSO4·5H2O.
3. Read the molar mass in g/mol, the formula with proper subscripts, the Hill formula and the total number of atoms.
4. Check the table and the bar for the mass each element contributes and its mass percent.
5. To convert, type a mass in grams or an amount in moles. The other value and the number of particles update at once.

What is molar mass?

Molar mass is the mass of one mole of a substance, written in grams per mole (g/mol). A mole is an amount of substance containing exactly 6.02214076 × 10²³ elementary entities, such as atoms, molecules or formula units. That number is the Avogadro constant, N_A. Since 2019 its value has been fixed by definition instead of being measured. One mole of water molecules has a mass of 18.015 g.

Written in g/mol, the molar mass is numerically equal to the relative molecular mass or relative formula mass of the substance, which has no unit. Water has a relative molecular mass of 18.015 and a molar mass of 18.015 g/mol.

How it works

The calculator reads the formula, counts every atom and adds the standard atomic weight of each atom:

M = Σ (ni × Ai)

Here M is the molar mass in g/mol, ni is the number of atoms of element i in the formula and Ai is the standard atomic weight of that element in g/mol. The sum runs over all elements in the formula. The arithmetic uses whole numbers, so no floating-point noise appears; the result is rounded to three decimals.

The percent composition and the conversions use the same numbers:

mass % of i = ni × Ai / M × 100
n = m / M, m = n × M, N = n × NA

In these formulas m is a mass in grams, n an amount in moles and N a number of particles. Each mass percent is rounded to two decimals, so the displayed values can add up to 99.99 % or 100.01 %. The table tells you when that happens.

How brackets and hydrates are counted

A number right after a closing bracket multiplies every atom inside it, and brackets can be nested. In Al2(SO4)3 the group SO4 occurs three times, which gives 3 S and 12 O. In K4[Fe(CN)6] the group CN is counted six times inside the square brackets. In a hydrate such as CuSO4·5H2O the dot separates two parts, and the number in front of the second part multiplies all of its atoms: 5 H2O means 10 H and 5 O. The atoms of all parts are added together.

The Hill formula lists carbon first and hydrogen second, then all other elements in alphabetical order. When there is no carbon, every element is sorted alphabetically. Ethanol C2H5OH becomes C2H6O and sulfuric acid H2SO4 becomes H2O4S.

Examples

Example 1: water, H2O. 2 × 1.008 + 15.999 = 18.015 g/mol. Hydrogen contributes 2.016 g/mol (11.19 %) and oxygen 15.999 g/mol (88.81 %). The molecule has 3 atoms.

Example 2: calcium hydroxide, Ca(OH)2. 40.078 + 2 × (15.999 + 1.008) = 40.078 + 34.014 = 74.092 g/mol. The atoms are Ca 1, O 2 and H 2, five in total. By mass the compound is 54.09 % calcium, 43.19 % oxygen and 2.72 % hydrogen.

Example 3: copper sulfate pentahydrate, CuSO4·5H2O. 63.546 + 32.06 + 4 × 15.999 + 5 × 18.015 = 249.677 g/mol. The atom count is Cu 1, S 1, O 9 and H 10, which makes 21 atoms. Copper makes up 25.45 % of the mass.

Example 4: grams, moles and molecules. For 36.03 g of water, n = 36.03 / 18.015 = 2.000 mol and N = 2.000 × 6.02214076 × 1023 = 1.204 × 1024 molecules. In the other direction, 0.5 mol of NaCl (58.44 g/mol) has a mass of 0.5 × 58.44 = 29.22 g.

Molar mass, molecular weight and formula mass

The three terms are often used as if they were the same, but they differ slightly. Molar mass has the unit g/mol and applies to any substance. Molecular weight, or relative molecular mass, is a pure number and applies to substances made of discrete molecules, such as water or carbon dioxide. Formula mass, or relative formula mass, is also a pure number and is used for ionic compounds and other lattices, such as NaCl or SiO2, which do not form separate molecules. For a given substance all three have the same numerical value, so this page gives one number and calls it the molar mass.

Why atomic weights differ in the last digit

Most elements are mixtures of isotopes, which are atoms with the same number of protons but different masses. A standard atomic weight is an average of the isotope masses weighted by their natural abundance. Chlorine is roughly 76 % chlorine-35 and 24 % chlorine-37, so its average is 35.45 even though no single chlorine atom has that mass.

Abundances vary slightly with the source of the sample. For hydrogen, carbon, oxygen, sulfur, chlorine and a few other elements IUPAC publishes an interval and a conventional single value. Tables also round differently: a textbook may give 22.99 for sodium, another 22.98977. Results can differ in the third decimal, so use the periodic table your course uses.

Elements without stable isotopes, such as technetium, promethium and polonium, and all heavier elements except thorium, protactinium and uranium, have no standard atomic weight. The table shows the mass number of the longest-lived known isotope in square brackets with a dagger. Results that include them are approximate.

Common mistakes

• Wrong capitals. CO is carbon monoxide (28.010 g/mol), while Co is cobalt (58.933 g/mol). A formula typed entirely in lowercase is rejected.
• Forgetting the multiplier. In Mg(NO3)2 the 2 applies to N and O together: 2 N and 6 O.
• Leaving out the water of hydration. CuSO4 and CuSO4·5H2O are different substances with different molar masses.

Frequently asked questions

What is the difference between molar mass and molecular weight?

Molar mass is expressed in g/mol, molecular weight is a dimensionless relative mass. For the same substance they have the same number, for example 18.015 for water.

Why do Co and CO give different results?

Element symbols are case sensitive. A capital letter followed by a lowercase letter is one element (Co, cobalt). Two capitals are two elements (C and O, carbon monoxide).

Can I enter ions and charges?

No. Enter the neutral formula, for example NaCl or Al2(SO4)3.

How do I find the number of molecules in a sample?

Divide the mass by the molar mass to get moles, then multiply by the Avogadro constant. The conversion card does both steps. For ionic compounds the result counts formula units.

Is my data private?

Yes. The calculation runs in your browser and nothing is uploaded. Your last formula and values stay in your own browser storage.

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