Molecular Weight Calculator
Enter the molecular formula using element symbols (case-sensitive). Use parentheses for groups and a dot (.) for hydrates.
Element symbols are case-sensitive: Co (Cobalt) ≠ CO (Carbon + Oxygen). Always use the correct capitalization for each element.
Enter a Chemical Formula
Type a chemical formula like H2O or C6H12O6 to instantly see the molecular weight, elemental composition breakdown, mass percentages, and moles-to-grams conversion.
Calculate molar mass, elemental composition, and unit conversions for any chemical formula
The Molecular Weight Calculator is an essential chemistry tool for students, researchers, pharmacists, and laboratory professionals. By entering any chemical formula — from simple compounds like water (H2O) to complex hydrated salts like copper sulfate pentahydrate (CuSO4·5H2O) — you instantly receive the molecular weight in grams per mole, a full elemental composition breakdown, mass percentages for each element, and the Hill formula notation. Whether you are preparing solutions in a biochemistry lab, formulating pharmaceuticals, teaching general chemistry, or simply checking your textbook answers, this calculator eliminates manual arithmetic errors and saves valuable time. Molecular weight, also called molar mass, is one of the most fundamental properties of any chemical substance. It is defined as the sum of the atomic weights of all atoms in a molecule, with each atomic weight weighted by the number of times that atom appears. For example, water (H2O) has a molecular weight of 18.015 g/mol: two hydrogen atoms each contribute 1.008 g/mol, and one oxygen atom contributes 15.999 g/mol. This value is crucial because it links the macroscopic world (grams you can weigh on a balance) to the microscopic world (individual molecules and atoms), via Avogadro's number (6.022 × 10²³ molecules per mole). Our calculator uses the most current IUPAC 2021 standard atomic weights for all 118 elements in the periodic table — from hydrogen (1.008) to oganesson (294). The parsing engine handles the full complexity of real-world chemical formulas: nested and sequential parentheses (e.g., Al2(SO4)3, Ca3(PO4)2, [Co(NH3)6]Cl3), square bracket notation, hydrate dot notation for crystallized water (CuSO4·5H2O or CuSO4.5H2O), and common organic chemistry functional group shorthands (Me for CH3, Et for C2H5, Ph for C6H5, Bu for C4H9). Beyond simple molecular weight, this tool provides a complete elemental composition analysis. For each element present in your formula, you see the atom count, individual atomic weight, contribution to total mass (in g/mol), mass percentage, and atomic percentage. The visual donut chart and horizontal bar chart make it immediately clear which elements dominate the composition by mass — an essential insight for tasks such as stoichiometry calculations, elemental analysis verification, and formula confirmation. The integrated moles-to-grams and grams-to-moles converter removes the need for a separate tool. Simply enter either a mass in grams or a number of moles alongside your formula, and the calculator instantly reports the corresponding value. This bi-directional conversion is indispensable in quantitative chemistry: if you need to prepare 250 mg of caffeine (C8H10N4O2, MW = 194.19 g/mol) for a cell culture experiment, you can immediately confirm you need 1.287 mmol. For advanced users, the Hill formula notation output provides the canonical ordering of elements (carbon first, hydrogen second, then remaining elements alphabetically) as defined by Edwin A. Hill in 1900 — still the standard in chemical databases, patent literature, and most chemical information systems. The monoisotopic mass — calculated using the most abundant isotope of each element rather than the naturally weighted average — is essential for mass spectrometry, where instruments measure exact molecular masses rather than average values. The multi-compound comparison mode lets you enter two formulas side by side, instantly visualizing their molecular weight difference and ratio. This is useful for comparing salt forms (e.g., comparing a free base drug molecule to its hydrochloride salt), checking isomers, or understanding structural relationships between molecules. All calculations run entirely in your browser — no data is sent to any server, no account is required, and the tool works offline. Results can be copied to clipboard, shared via the Web Share API, exported as a formatted CSV spreadsheet, or printed for lab notebooks and reports.
Understanding Molecular Weight
What Is Molecular Weight?
Molecular weight (MW), also called molar mass, is the mass of one mole of a chemical substance expressed in grams per mole (g/mol). It is calculated by summing the atomic weights of all atoms in the molecular formula, where each element's atomic weight is multiplied by its atom count. For example, glucose (C6H12O6) has a molecular weight of 180.156 g/mol: 6 carbons × 12.011 + 12 hydrogens × 1.008 + 6 oxygens × 15.999. The terms 'molecular weight' and 'molar mass' are used interchangeably in modern chemistry, though strictly speaking molecular weight is the dimensionless ratio of the average molecular mass to 1/12 the mass of carbon-12, while molar mass carries units of g/mol. For practical calculations — solution preparation, stoichiometry, and dosing — the distinction is immaterial.
How Is Molecular Weight Calculated?
The formula is: MW = Σ(Ni × AWi), where Ni is the number of atoms of element i and AWi is the standard atomic weight of element i in g/mol. Standard atomic weights are internationally agreed values published by IUPAC that reflect the naturally occurring isotopic composition of each element on Earth. Our calculator uses the IUPAC 2021 standard atomic weights. The parser handles complex formulas by applying a recursive descent algorithm: it processes parenthetical groups and their trailing multipliers recursively, expands hydrate dot notation (e.g., CuSO4·5H2O), recognizes 1–2 character element symbols with case sensitivity (distinguishing Co from CO), and resolves functional group shorthands like Me (CH3) and Ph (C6H5). The monoisotopic mass uses the most abundant isotope mass for each element, which is important for mass spectrometry applications.
Why Does Molecular Weight Matter?
Molecular weight is the cornerstone of quantitative chemistry. In solution preparation, you cannot make a 1 M solution without knowing the molecular weight — it determines how many grams to weigh per liter. In pharmaceutical sciences, drug dosing is based on moles (which requires MW) because pharmacokinetics is fundamentally molecular. In stoichiometry, reaction yield calculations and reagent amounts all depend on molar ratios, which require MW. In biochemistry, protein and nucleic acid concentrations are often expressed in molarity, requiring the molecular weight of the macromolecule. In mass spectrometry, the molecular weight is the primary identification criterion for compounds. Quality control laboratories verify molecular weights to confirm compound identity and purity. Even everyday applications like checking a product label or interpreting an MSDS sheet often involve molecular weight.
Limitations and Important Notes
The average molar mass calculated here is appropriate for bulk chemistry (solution preparation, gravimetric analysis, stoichiometry). However, it is not the same as the exact monoisotopic mass used in high-resolution mass spectrometry. The calculator also computes average masses using terrestrial isotopic abundances — samples with non-natural isotopic compositions (deuterium-labeled compounds, radioactive isotopes) will deviate. For real-world compounds, the actual measured molecular weight of a batch may differ slightly due to purity, salt forms, solvates, or hydration states — pharmaceutical suppliers always specify which form (free base, HCl salt, hydrate) their molecular weight refers to. Finally, this calculator determines formula weight from the empirical/molecular formula but cannot validate whether the formula itself is chemically valid or stable.
How to Use the Molecular Weight Calculator
Enter Your Chemical Formula
Type the molecular formula in the input field using standard element symbols. The parser is case-sensitive (Na is sodium, but NA would be read as N + A). You can use parentheses for groups like Al2(SO4)3, square brackets like [Co(NH3)6]Cl3, and a period or middle dot for hydrates like CuSO4.5H2O. Functional group shorthands Me, Et, Ph, and Bu are also supported.
Select a Preset or Unit (Optional)
If you need a common compound, select it from the preset dropdown — it will automatically populate the formula field. You can also choose your preferred display unit: g/mol (standard), kg/mol, Da (daltons), or lb/mol. The molecular weight is immediately displayed in your chosen unit.
Add Mass or Moles for Conversion (Optional)
If you need to convert between grams and moles, enter either a mass in grams or a quantity in moles in the conversion fields below the formula. The calculator will instantly show you the corresponding value using your compound's molecular weight. This is useful for preparing solutions or calculating reaction yields.
Review Composition and Export Results
The results card shows the molecular weight, Hill formula notation, monoisotopic mass, an elemental composition chart, and a detailed table with mass % and atomic % for each element. Use the Copy, Share, Print, or Export CSV buttons to save or share your results. Enable the comparison mode to evaluate two formulas side by side.
Frequently Asked Questions
What is the difference between molecular weight and molar mass?
Technically, molecular weight is a dimensionless ratio — the ratio of the average mass of a molecule to 1/12 the mass of a carbon-12 atom. Molar mass, on the other hand, has units of grams per mole (g/mol). However, their numerical values are identical: water has a molecular weight of 18.015 (dimensionless) and a molar mass of 18.015 g/mol. In everyday chemistry practice, the two terms are used interchangeably, and this calculator reports the value in g/mol (molar mass convention) which is the most useful form for laboratory calculations like solution preparation and stoichiometry.
What is the difference between average molar mass and monoisotopic mass?
Average molar mass uses the weighted average of all naturally occurring isotopes of each element — for example, carbon's standard atomic weight of 12.011 reflects the natural mix of carbon-12 (98.9%) and carbon-13 (1.1%). Monoisotopic mass uses only the most abundant isotope of each element — for carbon this is exactly 12.000 Da. The difference matters in mass spectrometry: high-resolution instruments measure exact masses and report monoisotopic masses, while bulk chemical calculations use average molar mass. For small molecules (up to ~1000 Da), the monoisotopic peak is typically the most abundant in the mass spectrum.
Why does element capitalization matter in chemical formulas?
Chemical element symbols are case-sensitive by convention: the first letter is always uppercase and the second (if present) is always lowercase. This distinction is essential for unambiguous parsing. For example, 'CO' is parsed as carbon (C) plus oxygen (O) — carbon monoxide — with molecular weight 28.010 g/mol, while 'Co' is the single element cobalt with atomic weight 58.933 g/mol. Similarly, 'Ca' (calcium, 40.078) differs from 'CA' which would be parsed as carbon + argon. The calculator enforces IUPAC case conventions and will report an error for unrecognized symbol combinations.
How do I enter hydrated compounds (water of crystallization)?
Hydrated compounds are entered using a dot or middle dot before the water molecules. For example, copper sulfate pentahydrate can be written as CuSO4.5H2O or CuSO4·5H2O. The dot acts as a multiplication sign for the following group. The calculator correctly computes the molecular weight as the sum of the anhydrous salt and the hydration water: CuSO4 (159.610 g/mol) + 5 × H2O (5 × 18.015 = 90.075 g/mol) = 249.685 g/mol. You can also write it fully without dots as CuSO4H10O5, though the dot notation is the standard chemical convention.
What are functional group shorthands and how do I use them?
Functional group shorthands are abbreviations commonly used in organic chemistry to represent frequently occurring structural units. This calculator supports: Me (methyl, CH3), Et (ethyl, C2H5), Ph (phenyl, C6H5), and Bu (butyl, C4H9). For example, you can enter PhOH for phenol (C6H5OH), EtOH for ethanol (C2H5OH), or PhCOOH for benzoic acid (C6H5COOH). These shorthands are expanded automatically before molecular weight calculation, and the expanded formula is used for the elemental composition breakdown. Deuterium (D, mass 2.014) is also supported as a single-character element symbol.
How accurate are the atomic weights used in this calculator?
This calculator uses the IUPAC 2021 standard atomic weights, which are the most current internationally agreed values reflecting the best available knowledge of natural isotopic abundances. For most elements, the values are accurate to 5 or more significant figures — sufficient for virtually all laboratory and academic purposes. Some elements (Tc, Pm, and elements 104–118) are radioactive with no stable isotopes, so their atomic weights are approximations based on the most stable isotope. For ultra-precise work in metrology or isotope science, always consult the primary IUPAC publication and use certified reference materials with stated uncertainty values.