—
180.156
g/mol
180.06342
Da
24
1
—
180.156
g/mol
180.06342
Da
24
1
The Molecular Formula from Structure Calculator assembles a complete molecular formula and computes key molecular properties from individual atom counts. Simply enter the number of each element present in your organic compound — carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, and halogens — and this tool generates the Hill-system molecular formula, average molecular weight, monoisotopic exact mass, and degree of unsaturation. This calculator is essential for converting structural drawings into formula notation, verifying proposed structures against mass spectrometry data, and preparing molecular descriptors for chemical database submission. The distinction between average molecular weight (using standard atomic weights) and exact monoisotopic mass (using the most abundant isotope masses) is critical for high-resolution mass spectrometry identification, where mass accuracy below 5 ppm can confirm elemental composition.
The molecular formula is assembled following the Hill system: carbon first, hydrogen second, then all remaining elements in alphabetical order. This standardized ordering ensures consistency across chemical databases.
The average molecular weight uses IUPAC standard atomic weights, which are weighted averages over all natural isotopes:
$$MW = \sum n_i \times A_i^{avg}$$
For example, chlorine has $$A_{Cl}^{avg} = 35.453$$ because natural chlorine is ~75.8% $$^{35}Cl$$ and ~24.2% $$^{37}Cl$$.
The monoisotopic exact mass uses the mass of the most abundant isotope of each element:
$$M_{exact} = \sum n_i \times m_i^{mono}$$
Key monoisotopic masses: $$^{12}C = 12.00000$$, $$^{1}H = 1.00783$$, $$^{16}O = 15.99491$$, $$^{14}N = 14.00307$$. This mass matches the monoisotopic peak (M+) observed in high-resolution mass spectrometry.
The degree of unsaturation is computed as:
$$DBE = \frac{2C + 2 + N + P - H - X}{2}$$
The molecular formula in Hill system format is the standard representation used in all major chemical databases (PubChem, ChemSpider, SciFinder). The average molecular weight is used for solution preparation, gravimetric analysis, and any calculation involving macroscopic quantities. The exact monoisotopic mass is used for high-resolution mass spectrometry identification — compare this value to the observed m/z to verify the elemental composition (typically within 5 ppm accuracy). The total atom count provides a quick measure of molecular size. The degree of unsaturation indicates rings plus pi bonds, helping verify structural consistency — if your drawn structure has 3 rings and 2 double bonds, the DBE should be 5.
Inputs
Results
Glucose has MW 180.06 g/mol and DBE = 1, which corresponds to the carbonyl group in the open-chain aldehyde form.
Inputs
Results
Penicillin G has 9 degrees of unsaturation: the benzene ring (4), the beta-lactam ring (1), the thiazolidine ring (1), the amide carbonyl (1), and the carboxylic acid carbonyl (1), plus one more from the lactam C=O.
The Hill system orders elements with carbon first, hydrogen second, then all remaining elements in alphabetical order. For organic compounds, this always starts with C and H. For inorganic compounds (no carbon), all elements are listed alphabetically. This is the standard format used by CAS, PubChem, and most chemical databases.
Molecular weight uses average atomic weights (weighted by natural isotope abundances), while exact mass uses the mass of the most abundant isotope of each element. For example, Cl has MW = 35.453 but exact mass = 34.969. Molecular weight is for bulk calculations; exact mass is for mass spectrometry identification.
By definition, the atomic mass unit (u or Da) is defined as exactly 1/12 of the mass of carbon-12. Therefore, the monoisotopic mass of carbon-12 is exactly 12.00000 Da. All other atomic masses are measured relative to this standard.
Monoisotopic mass matches the M+ peak in high-resolution mass spectrometry, where the instrument resolves individual isotopic peaks. By comparing the measured monoisotopic mass to calculated values, chemists can determine the elemental composition of unknown compounds with ppm-level accuracy.
Modern high-resolution mass spectrometers (Orbitrap, FTICR) achieve accuracy below 1-5 ppm. At this precision, the exact mass often uniquely identifies the molecular formula for compounds under ~500 Da. For larger molecules, additional constraints (isotope pattern, fragmentation) are needed.
Oxygen (valence 2) and sulfur (valence 2) can be inserted into a carbon chain without changing the hydrogen count. Replacing a C-C bond with C-O-C maintains the same number of hydrogens. Therefore, these divalent atoms do not affect the calculation.
This calculator uses natural isotope distributions for average molecular weight and the most abundant isotope for exact mass. For specifically labeled compounds (e.g., deuterium D = 2H, or 13C), you would need to manually adjust the masses for the labeled atoms.
The calculator accepts up to 200 carbons and proportional amounts of other elements, covering most small molecules and many medium-sized natural products. For proteins and polymers with thousands of atoms, specialized bioinformatics tools are more appropriate.
Count every atom in your drawn structure including implicit hydrogens. Enter the counts and compare the generated formula and DBE to your expectations. If the DBE matches your count of rings plus multiple bonds, and the formula matches, your structure is internally consistent.
This calculator is designed for organic compounds. Metal coordination compounds have variable valences and bonding that the DBE formula cannot handle. For organometallic compounds, only the organic ligand portions can be reliably analyzed with this tool.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
How helpful was this calculator?
Be the first to rate!
Degree of Unsaturation Calculator
Organic Chemistry Calculators
Double Bond Equivalent Calculator
Organic Chemistry Calculators
Index of Hydrogen Deficiency Calculator
Organic Chemistry Calculators
Chemical Name Calculator
Organic Chemistry Calculators
IUPAC Name Generator
Organic Chemistry Calculators
SMILES to Structure Converter
Organic Chemistry Calculators
