Molecular Weight Calculator
Calculate the molecular weight (molar mass) of any chemical compound by selecting elements and atom counts. Free online chemistry calculator with elemental breakdowns, formula display, and interactive charts.
About This Calculator
The Molecular Weight Calculator is a free chemistry tool that computes the molecular weight (molecular mass) and molar mass of any chemical compound. It is designed for students, researchers, chemists, pharmacists, and anyone who needs to quickly determine the mass of a molecule without referencing a periodic table.
The calculator works by summing the atomic masses of all atoms in a molecule based on the chemical formula you build. Each element has a standard atomic weight defined by IUPAC (International Union of Pure and Applied Chemistry). The formula for molecular weight is: MW = Σ(element atomic mass × atom count). For example, glucose (C₆H₁₂O₆) has a molecular weight of (6 × 12.011) + (12 × 1.008) + (6 × 15.999) = 180.156 u.
Results are shown in unified atomic mass units (u), also known as daltons (Da). The calculator also displays the molar mass in grams per mole (g/mol), which is numerically identical to the molecular weight. This dual display helps bridge the gap between molecular-scale and laboratory-scale measurements.
Common use cases include determining formula weights for stoichiometry calculations, preparing solutions in chemistry labs, calculating drug dosages in pharmaceutical applications, and verifying the composition of chemical compounds. You can quickly load common molecules like water, glucose, ethanol, carbon dioxide, ammonia, and more using the Quick Select buttons.
The calculator supports all 118 known elements from Hydrogen to Oganesson, with the ability to build molecules with up to 20 different elements and any number of atoms per element (up to 1000). Results are visualized through bar charts showing element mass contributions and pie charts showing the mass distribution, making it easy to understand the composition of any molecule at a glance.
Frequently Asked Questions
How do I calculate molecular weight using this calculator?
Select elements from the dropdown menu and enter the number of atoms for each element in your molecule. You can add multiple elements, choose from common molecules like water or glucose, and click Calculate to get the molecular weight in atomic mass units (u) and grams per mole (g/mol) with a detailed elemental breakdown.
What is the difference between molecular weight and molar mass?
Molecular weight (or molecular mass) is the mass of a single molecule measured in atomic mass units (u) or daltons (Da). Molar mass is the mass of one mole (6.022 × 10²³ molecules) of a substance, expressed in grams per mole (g/mol). Numerically, they have the same value — for example, water has a molecular weight of 18.015 u and a molar mass of 18.015 g/mol.
How is molecular weight calculated?
Molecular weight is calculated by summing the atomic masses of each element multiplied by the number of atoms of that element in the molecule. For example, water (H₂O) has molecular weight = (2 × 1.008) + (1 × 15.999) = 18.015 u. Atomic masses are obtained from the periodic table based on IUPAC standard values.
What units does the molecular weight calculator use?
The calculator displays molecular weight in unified atomic mass units (u), which are equivalent to daltons (Da). It also shows the equivalent molar mass in grams per mole (g/mol). Both values are numerically identical — for example, 18.015 u = 18.015 g/mol.
Can I calculate the molecular weight of any chemical compound?
Yes, you can calculate the molecular weight of any compound using our calculator. It includes all 118 known elements with their IUPAC standard atomic masses. Simply select the elements present in your molecule, enter the atom counts, and the calculator will compute the total molecular weight and show the elemental mass distribution.
How accurate are the atomic mass values used?
The calculator uses IUPAC standard atomic weights, which are the internationally accepted values for each element. The atomic masses are accurate to 3-6 decimal places, and the final molecular weight is computed to 3 decimal places. For most practical purposes in chemistry, biology, and medicine, this provides sufficient accuracy.
What is Avogadro's number and how does it relate to molar mass?
Avogadro's number (6.022 × 10²³) is the number of molecules in one mole of any substance. If you know the molecular weight in u, the molar mass in g/mol is the same number. For example, if a molecule has a molecular weight of 180 u (like glucose), then one mole (6.022 × 10²³ molecules) has a mass of 180 grams.
Can I share my molecular weight calculation results?
Yes, the calculator saves your element selections and atom counts in the URL. You can copy the URL from your browser's address bar to share your exact calculation with others. When someone opens the link, the calculator will automatically load your molecule and display the results.