Molecular Weight Calculator
Type a chemical formula below, for example H2O, CO2 or Ca(OH)2. The molar mass and the full working appear as you type.
A molecular weight calculator finds the molar mass of a chemical formula: the mass, in grams, of one mole of that substance. Type any formula, such as H2O for water or CO2 for carbon dioxide, and this molecular weight calculator adds up the atomic weights of every atom in the formula and shows the working in a table, so you can see exactly where the number comes from.
How to use this molecular weight calculator
Step 1: Type your chemical formula in the box. Use a capital letter to start each element symbol, and put the number of atoms after the symbol, with no spaces. For water, type H2O, with a capital H and a capital O.
Step 2: Read the result as you type. The result updates live, so there is nothing to submit, although the Calculate button is there if you want it. The result shows the molecular weight in g/mol, to three decimal places.
Step 3: Check the working in the breakdown table. It lists each element, how many atoms of it are in the formula, its atomic weight, and the subtotal for that element. Add the subtotals together and you get the molecular weight.
Worked example: water, H2O
Let us work out the molecular weight of water, H2O, step by step. The formula H2O has two hydrogen atoms and one oxygen atom. The standard atomic weight of hydrogen is 1.008, and the standard atomic weight of oxygen is 15.999. So the calculation is 2 \u00d7 1.008 + 15.999, which is 2.016 + 15.999, which equals 18.015. The molecular weight of water is therefore 18.015 g/mol.
Here is another example with brackets. Calcium hydroxide is written Ca(OH)2. The brackets mean the OH group appears twice: one calcium atom, two oxygen atoms and two hydrogen atoms. Using the calculator’s table, that is 40.078 for calcium, plus 2 \u00d7 15.999 for oxygen, plus 2 \u00d7 1.008 for hydrogen, which is 40.078 + 31.998 + 2.016, giving 74.092 g/mol. A formula like NaCl, common salt, is even simpler: one sodium at 22.990 and one chlorine at 35.45, which add up to 58.440 g/mol. For carbon dioxide, CO2, the calculator gives 44.009 g/mol from the table (12.011 + 2 \u00d7 15.999), which is the same value usually quoted in textbooks as 44.01 g/mol, rounded to two decimal places.
What molecular weight means
Molecular weight is the mass of one mole of a substance, measured in grams per mole, written g/mol. A mole is a counting unit in chemistry: one mole of any substance contains 6.022 \u00d7 10\u00b2\u00b3 particles, which are molecules for a substance like water. So the molecular weight of water, 18.015 g/mol, means that one mole of water, which is 6.022 \u00d7 10\u00b2\u00b3 water molecules, weighs 18.015 grams.
That is why molecular weight matters so much in chemistry. If you know how many grams of a substance you have, you can work out how many moles you have, and from there how many molecules you have. It is the bridge between the mass you can weigh on a balance and the number of particles taking part in a reaction. Lab work, cooking with chemicals, and exam questions all use it the same way.
How to write a formula
Writing a formula correctly is mostly about capital letters and numbers. Every element symbol starts with a capital letter, and only the second letter of a two-letter symbol is lowercase, so water is H2O, not h2o, and table salt is NaCl, with a capital N and a capital C, not nacl. The number after an element tells you how many atoms of that element are in the formula, and if there is no number, there is one atom.
Groups in brackets take a multiplier after the closing bracket. In Ca(OH)2, the number 2 after the closing bracket multiplies everything inside the brackets, so there are two oxygens and two hydrogens. This calculator handles one level of brackets like that. If a bracket is missing its pair, or a symbol is spelled wrong, it will tell you in plain words instead of showing a wrong number.
Frequently asked questions
What is molecular weight?
Molecular weight is the mass of one mole of a substance, in grams per mole (g/mol). It is found by adding up the atomic weights of all the atoms in the chemical formula. For water, H2O, it is 18.015 g/mol.
What is the difference between molecular weight and molar mass?
They are the same number. Molar mass is the mass of one mole of a substance, in g/mol, and molecular weight is the same quantity, calculated from the formula. The two terms are used for the same value.
How do I write a formula with brackets?
Type the group in brackets and put the multiplier after the closing bracket, like Ca(OH)2. The number after the bracket multiplies everything inside it, so Ca(OH)2 has one calcium, two oxygens and two hydrogens.
Why is water 18.015 and not 18?
Because hydrogen is 1.008 and oxygen is 15.999, not whole numbers, and the extra decimals add up. Two hydrogens give 2.016 and one oxygen gives 15.999, which add up to 18.015, not 18.
Is this exact?
The weights are standard atomic weights, and real samples vary very slightly by isotope. For school and everyday lab work, the result from this calculator is the value you should use.