Molecular Weight Calculator

The Molecular Weight Calculator is a simple, fast, and accurate web-based tool that computes the molecular (molar) mass of chemical compounds from their molecular formula. It accepts a broad range of formula formats, including organic, inorganic, ionic, hydrated salts, and compounds with parentheses or dot notation, and returns the mass in grams per mole (g·mol⁻¹), along with a parsed composition breakdown.

This tool is ideal for students, laboratory technicians, chemists, and anyone who needs quick, reliable molecular weights for stoichiometry, solution preparation, reaction planning, or educational purposes. Try the Molecular Weight Calculator now—type a chemical formula into the input field and press Calculate.

Key features

• Instant calculation: Type or paste a molecular formula and receive the result immediately.
• Biopolymer support: Calculates molecular weights for protein, DNA, and RNA sequences, as well as small molecules and salts.
• Flexible input formats: Supports standard formula conventions including parentheses (e.g., Al2(SO4)3), dot notation for hydrates (e.g., CuSO4·5H2O), and biopolymer sequence input modes.
• Breakdown of elements: Displays the elemental composition and the contribution (mass and percentage) of each element to the total molecular weight.
• Standard atomic weights: Uses up-to-date IUPAC-standard atomic weights for calculations to ensure reliable results.
• Clear output units: Results are reported in Daltons (Da), which are numerically equivalent to grams per mole (g·mol⁻¹).

How it works

1. The calculator first determines the input type: molecular formula (e.g., C6H12O6), protein sequence, DNA sequence, or RNA sequence.
2. Each element (for chemical formulas) or residue/nucleotide (for biopolymers) is mapped to its standard molecular or atomic mass.
3. The tool multiplies each mass by its count or frequency and sums all contributions.
4. The final molecular weight is reported in Daltons (Da) or equivalently g·mol⁻¹, along with a detailed composition table.

Table: Atomic Weights of Elements

Supported input examples

• Simple molecules: H2O, CO2, C6H6
• Parentheses and multipliers: Ca(OH)2, Al2(SO4)3
• Hydrates (dot notation): CuSO4·5H2O, CoCl2·6H2O
• Complex organics: C27H46O (cholesterol fragment example)
• Charged species (charge is parsed but does not affect mass): NH4+, SO4^2-
• Fractional stoichiometry for advanced users: Fe0.5O (the tool will warn and treat counts numerically)

Example calculations

• Water (H₂O): H2O → 2 × 1.00794 + 15.999 = 18.015 g·mol⁻¹ (value shown with chosen precision)
• Glucose (C₆H₁₂O₆): C6H12O6 → 180.156 g·mol⁻¹
• Calcium hydroxide (Ca(OH)₂): Ca(OH)2 → 74.093 g·mol⁻¹
• Copper(II) sulfate pentahydrate: CuSO4·5H2O → 249.685 g·mol⁻¹

Input rules and tips

• Use proper element symbols: capital letter followed by optional lowercase (e.g., Fe, Cl, Mg).
• Avoid spaces inside formulas; if present, they will be ignored or prompt a warning.
• Parentheses may be nested (e.g., K4[Fe(CN)6])—the parser supports common nesting patterns, but extremely complex or malformed formulas may trigger an error.
• For hydrates, use a middle dot (keyboard alternative: .) or a space followed by a number: CuSO4·5H2O or CuSO4 5H2O.
• Charges can be included for clarity (e.g., NH4+), but do not change the calculated mass.

Accuracy…

The calculator uses widely accepted, standard atomic weights (IUPAC-recommended), which represent the best single-value estimate for naturally occurring elemental isotopic composition. For elements with significant isotopic variation or when a more precise isotopic composition is required (e.g., enriched isotopes), explicitly specify the isotopic masses in the input if the tool provides an advanced mode. For compounds where isotopic composition matters (research-grade work), verify results against laboratory-grade data or use isotope-specific calculation tools.

The tool reports values in g·mol⁻¹ and rounds the displayed result according to the selected precision while keeping more digits in the calculation backend.

Error handling and common warnings

The calculator includes robust validation and error handling to ensure reliable results. If an unknown element symbol is entered—one that does not correspond to any element in the periodic table (for example, Xy123)—the tool will flag the input and notify the user. In cases of a malformed formula, such as mismatched parentheses or incorrect numeric placement, a clear error message is displayed along with guidance on how to correct the structure. Additionally, when fractional stoichiometry is provided (e.g., Fe0.5O), the calculator issues a warning to confirm that the fractional values are intentional, then proceeds with a numerical calculation based on the given input.

FAQ