Molecular Weight Calculator (Molar Mass)

Calculate an approximate molar mass in g/mol from a chemical formula, together with each element’s atom count and mass contribution. Enter a formula directly or select one of the listed compounds or elements.

How to enter a formula

• Use case-sensitive element symbols and ordinary digits: H2O, NaCl or Ca(OH)2. For example, Co is cobalt, whereas CO contains carbon and oxygen.
• Use one level of round parentheses only. Expand nested groups before entering them: use K4FeC6N6 instead of K4(Fe(CN)6). Nested groups are blocked because the legacy calculator can give an incorrect total.
• For a hydrate, put the water term last, separated by a keyboard full stop: CuSO4.5H2O. The 5 applies to five whole H2O units. One hydrate/adduct separator is supported; do not use the typographic middle dot in this input.
• Do not enter ionic charges, isotope labels, square brackets, chemical equations, decimal atom counts or a reaction coefficient before the main formula. Use only the elements available in the element list. The guard limits formulas to 500 characters and individual counts or hydrate multipliers to 1,000,000.

Check the formula. Use listed element symbols, positive whole-number atom counts, one level of round parentheses and at most one full-stop hydrate term. Expand nested parentheses, remove charge/isotope notation, and omit spaces or leading reaction coefficients. For example, use K4FeC6N6 rather than K4(Fe(CN)6).

What the result means

Molar mass, M, is mass per amount of substance, usually expressed in g/mol. This calculator adds the element molar-mass contributions implied by the entered formula: M = Σ n_iM_i. Each percentage in the table is that element’s mass contribution divided by the total, multiplied by 100. Displayed rounding can make the percentages sum to slightly more or less than 100%.

Molecular mass is the mass of one molecule and may be expressed in daltons (Da), also called unified atomic mass units (u). Relative molecular mass, often called molecular weight, is dimensionless. For ordinary chemical calculations, its numerical value is approximately the same as the molar mass expressed in g/mol. For ionic substances such as NaCl, the formula describes a formula unit rather than a discrete molecule.

The mole and Avogadro constant

One mole contains exactly 6.02214076 × 10²³ specified entities, such as molecules, atoms, ions or formula units. The Avogadro constant is N_A = 6.02214076 × 10²³ mol⁻¹. A mole does not generally have a mass of one gram; its mass depends on the substance’s molar mass.

Use n = m/M to convert a mass in grams into moles, and N = nN_A to obtain the number of specified entities. For example, using 78.11 g/mol for benzene, 1.00 g corresponds to approximately 0.0128 mol, or 7.71 × 10²¹ molecules. This describes the amount of benzene; it does not assess water quality or health risk.

Water-treatment examples

When concentration is expressed in g/L, dividing by molar mass in g/mol gives mol/L. Numerically, mg/L divided by g/mol gives mmol/L: for example, 100 mg/L of NaCl corresponds to approximately 1.71 mmol/L of NaCl formula units. Hydrated and anhydrous products have different molar masses, so use the formula and assay of the actual chemical product when preparing a solution.

Scope and limitations

The entered formula determines atom counts; it does not establish molecular structure, charge balance, chemical identity, purity or whether a substance exists. Compounds with the same formula can have different structures. The calculation uses the legacy calculator’s stored element mass values; the displayed precision does not establish the accuracy or currency of those values. For reference work, compare them with current CIAAW data. Standard atomic weights can vary with natural isotopic composition, and some radioactive elements have no standard atomic weight.

This is not an isotope-specific exact-mass or mass-spectrometry calculator. The cellulose preset gives the mass of one C6H10O5 repeat unit, not a polymer’s complete molecular mass or molecular-mass distribution. A formula mass also does not determine membrane rejection: see molecular weight cutoff (MWCO) for that separate topic.

JavaScript is required for the interactive calculation. Check the atom counts and displayed total after changing a formula. Invalid input hides the previous result rather than leaving it visible.

References and related tools

BIPM: SI definition of the mole; CIAAW: abridged standard atomic weights; PubChem: 2-methylpropene (C4H8).

Calculators and converters · Periodic table · Molecular weight cutoff (MWCO)

Lenntech BV cannot be held responsible for errors in the calculation, the program itself or the explanation. For questions or remarks please contact us.