Molecular Weight Calculator
Build a chemical formula from its elements and calculate molecular weight from standard atomic weights.
Molecule composition
Your molecule
Element contribution breakdown
| Element | Symbol | Atoms | Atomic weight (u) | Contribution (u) | Share |
|---|
How to use the molecular weight calculator
What this calculator does
This calculator adds the atomic-weight contribution of every element in a molecular formula. It reports the molecular weight in unified atomic mass units, symbol u, and shows the numerically equivalent molar mass in grams per mole. The result is suitable for routine chemistry calculations, checking formulas, preparing stoichiometric work, and comparing compounds. It does not identify an unknown substance, determine a molecule's structure, account for hydration unless you enter the water atoms explicitly, or replace isotope-specific mass calculations.
When to use it
Use it when checking a formula before a laboratory calculation, converting a chemical formula into a molar mass for solution preparation, comparing the masses of related compounds, or teaching how each element contributes to a molecule's total. Standard atomic weights can vary slightly by source and natural isotopic composition; the values here follow commonly used conventional values consistent with the IUPAC Commission on Isotopic Abundances and Atomic Weights.
How to calculate
- The calculator opens with glucose, C₆H₁₂O₆, as a complete demonstration. Its results and a validated Excel workbook are ready immediately.
- For each row, choose an Element and enter its Number of atoms. Use Add element for another unique element and the × button to remove an unneeded row.
- Read the live molecular weight, formula, totals, and contribution table. The order of rows controls the displayed formula order, so enter elements in the conventional order you need.
- Select Download Excel to export the current inputs and computed results. Reset clears the demonstration and all rows; after Reset, the download is disabled until you enter a complete valid composition again.
Input guide
Element is required and must be selected from the periodic-table list. Each element may appear only once, because repeated symbols would make the formula harder to audit. A realistic entry is Carbon (C). Changing an element changes its atomic weight and therefore its contribution. A common mistake is choosing a similarly named or neighboring element without checking the symbol.
Number of atoms is a required whole number from 1 to 999. Enter plain digits, such as 6; decimals, negative values, scientific notation, symbols, and grouped numbers are rejected. Increasing the count raises the total by exactly one atomic weight for each added atom. Do not enter a coefficient for multiple molecules here: this field describes one formula unit.
Add element creates another composition row, up to 20 rows. The remove button deletes only its own row. These controls change the set of elements, not the arithmetic rules. Avoid splitting one element across two rows; duplicate symbols are rejected.
Output guide
Molecular weight is the sum of each atomic weight multiplied by its atom count, displayed in u to three decimal places. Chemical formula joins the symbols and uses subscripts for counts greater than one. Total atoms is the sum of all atom counts, while Distinct elements counts composition rows. Heaviest contribution identifies the element responsible for the largest share of the total. Equivalent molar mass has the same numerical value in g/mol because the dalton and molar-mass constant are linked by definition; see the BIPM SI Brochure for the SI framework.
The breakdown table lists each element, symbol, atom count, atomic weight, subtotal contribution, and percentage share. A zero result is not valid because every row requires at least one atom. High or low values simply reflect the chosen elements and counts; they are exact consequences of the stored conventional weights, not a recommendation or experimental measurement.
Worked example
For glucose, enter Carbon with 6 atoms, Hydrogen with 12 atoms, and Oxygen with 6 atoms. The calculator applies 6 × 12.011 + 12 × 1.008 + 6 × 15.999. The three contributions are 72.066 u, 12.096 u, and 95.994 u. Their sum is 180.156 u, so the equivalent molar mass is 180.156 g/mol. Oxygen contributes the largest share, about 53.28%, and the formula contains 24 atoms in total.
How the formula works
The calculation is additive. This means every row can be checked independently, then summed. The contribution table is especially useful for spotting a wrong atom count: if one subtotal looks unexpectedly large, verify that row before using the result in a downstream stoichiometric calculation.
Molecular weight, relative molecular mass, and molar mass
In everyday chemistry, “molecular weight” is often used for a value expressed in u or Da, while molar mass is expressed in g/mol. Relative molecular mass is technically dimensionless because it compares a molecular mass with one-twelfth of the mass of carbon-12. The numerical values are usually the same at the displayed precision, but the units and physical meanings differ. The IUPAC Gold Book definition of relative molecular mass provides the formal terminology.
Limits and good practice
Natural materials may have isotope distributions that shift a measured mass from a conventional average. Mass spectrometry often uses monoisotopic or exact isotope masses instead of standard atomic weights. Hydrates, salts, polymers, and coordination compounds also require careful formula entry. Add every atom represented in the chemical formula, and treat variable-length polymers as a separate modeling problem rather than forcing an uncertain repeat count into a single formula.