Monoisotopic Molecular Weight Calculator

Calculate the exact monoisotopic mass of any compound using the most abundant isotope of each element — built for mass spectrometry and high-resolution analytical chemistry.

Try:
Molecular Weight
0.000 g/mol
Calculation Breakdown
Element Atomic Mass Atoms Total %
Total Molecular Weight
Elements in this compound
In formula
Other elements

Quick Examples

Click any compound to calculate its molecular weight instantly.

Water
H₂O
18.015 g/mol
Carbon Dioxide
CO₂
44.009 g/mol
Sodium Chloride
NaCl
58.443 g/mol
Glucose
C₆H₁₂O₆
180.156 g/mol
Ethanol
C₂H₅OH
46.069 g/mol
Sulfuric Acid
H₂SO₄
98.077 g/mol

How to Use the Monoisotopic Molecular Weight Calculator

To calculate the monoisotopic mass of a compound, enter its chemical formula into the input field and click "Calculate." The calculator uses the mass of the most abundant stable isotope for each element to compute the exact monoisotopic molecular weight.

Follow these 4 steps to find the monoisotopic mass:

  1. Enter the chemical formula using standard notation (e.g., C₆H₁₂O₆ for glucose, C₉H₈O₄ for aspirin). Use uppercase for the first letter of each element and lowercase for the second letter.
  2. Include subscript numbers after each element to indicate atom count. Use parentheses for polyatomic groups. The calculator handles complex formulas including hydrates and salts.
  3. Press "Calculate" or hit Enter. The calculator looks up the monoisotopic mass of each element's most abundant isotope and multiplies by the atom count. All individual contributions are summed for the total.
  4. Read the monoisotopic mass result. Compare this value to your high-resolution mass spectrometry data to verify compound identity and calculate mass accuracy in ppm.

This tool is built for analytical chemists running LC-MS or GC-MS experiments, natural product chemists identifying unknown metabolites, forensic scientists confirming drug compound identities, and anyone who needs exact mass values for mass spectrometry data interpretation.

How the Monoisotopic Molecular Weight Calculator Works

The monoisotopic molecular weight calculator computes exact mass by using the precise nuclear mass of the most abundant naturally occurring isotope for each element, rather than the isotope-averaged atomic weight used in standard molecular weight calculations. These isotopic masses are determined by mass spectrometry and are known to very high precision (6+ decimal places).

The calculation method and data sources:

M₀
Monoisotopic Mass Formula
Exact Mass = Σ(nᵢ × mᵢ), where nᵢ = number of atoms of element i, and mᵢ = monoisotopic mass (most abundant isotope mass) of element i.
vs
Monoisotopic vs Average
Average mass uses weighted means of all isotopes. Monoisotopic mass uses only the single most abundant isotope. For C: average = 12.011 Da; monoisotopic = 12.000000 Da exactly.
ppm
Mass Accuracy Calculation
ppm error = [(observed m/z - calculated m/z) ÷ calculated m/z] × 10⁶. Modern HRMS instruments achieve <1 ppm accuracy, allowing unique molecular formula assignment.
M+
Ion Mass Prediction
For [M+H]⁺: m/z = monoisotopic mass + 1.007276 Da. For [M+Na]⁺: m/z = monoisotopic mass + 22.989218 Da. For [M-H]⁻: m/z = monoisotopic mass - 1.007276 Da.

For caffeine (C₈H₁₀N₄O₂), the monoisotopic mass calculation is: (8 × 12.000000) + (10 × 1.007825) + (4 × 14.003074) + (2 × 15.994915) = 96.000000 + 10.078250 + 56.012296 + 31.989830 = 194.080376 Da. The observed [M+H]⁺ ion in ESI-MS appears at m/z 195.0877, consistent with a protonated caffeine molecule.

Monoisotopic mass calculations are central to molecular formula determination from HRMS data, metabolite identification in metabolomics studies, impurity profiling in pharmaceutical quality control, and de novo structure elucidation of natural products. The precision of the monoisotopic mass value directly enables confident compound identification at sub-ppm mass accuracy.

Common Compound Molecular Weights

Reference table of frequently searched chemical compounds.

Chemical Formula Compound Name Molecular Weight
H₂O Water 18.015 g/mol
CO₂ Carbon Dioxide 44.009 g/mol
NaCl Sodium Chloride 58.443 g/mol
NH₃ Ammonia 17.031 g/mol
CH₄ Methane 16.043 g/mol
H₂SO₄ Sulfuric Acid 98.077 g/mol
CaCO₃ Calcium Carbonate 100.086 g/mol
C₆H₁₂O₆ Glucose 180.156 g/mol
C₂H₅OH Ethanol 46.069 g/mol
HCl Hydrochloric Acid 36.461 g/mol
NaOH Sodium Hydroxide 39.997 g/mol
Ca(OH)₂ Calcium Hydroxide 74.092 g/mol
HNO₃ Nitric Acid 63.012 g/mol
KMnO₄ Potassium Permanganate 158.032 g/mol
CuSO₄ Copper(II) Sulfate 159.607 g/mol

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Monoisotopic Molecular Weight Calculator FAQs and Common Questions

Answers to common questions about monoisotopic mass, exact mass calculations, isotope selection, and mass spectrometry data interpretation.

What is monoisotopic molecular weight?
Monoisotopic molecular weight (also called exact mass) is the molecular weight calculated using the mass of the most abundant naturally occurring stable isotope of each element. For carbon, this is ¹²C (12.000000 Da); for hydrogen, ¹H (1.007825 Da); for nitrogen, ¹⁴N (14.003074 Da); for oxygen, ¹⁶O (15.994915 Da); and for sulfur, ³²S (31.972071 Da). This value differs from the average molecular weight, which uses isotope-abundance-weighted averages.
What is the difference between monoisotopic mass and average molecular weight?
Monoisotopic mass uses the single most abundant isotope of each element, producing a precise value that corresponds to the lightest isotope peak (M+0) in a mass spectrum. Average molecular weight uses the weighted mean of all isotopes, matching the centroid of the isotope envelope. For small molecules (<1,000 Da), mass spectrometers typically resolve the monoisotopic peak. For larger molecules, the average mass better approximates the observed peak center.
When should I use monoisotopic mass instead of average mass?
Use monoisotopic mass when working with high-resolution mass spectrometry (HRMS) instruments such as Orbitrap, FTICR, or TOF analyzers that can resolve individual isotope peaks. Use average mass for low-resolution instruments, larger molecules (>2,000 Da) where the monoisotopic peak is no longer the tallest peak in the isotope cluster, or when preparing solutions where average mass gives more practical results.
How is monoisotopic mass used in mass spectrometry?
In mass spectrometry, the monoisotopic mass predicts the exact m/z position of the lightest isotope peak. Analysts compare observed m/z values against calculated monoisotopic masses to identify compounds, confirm molecular formulas, and measure mass accuracy in parts per million (ppm). Mass accuracy = [(observed - calculated) ÷ calculated] × 10⁶ ppm. High-resolution instruments routinely achieve <5 ppm accuracy.
What isotopes does the monoisotopic mass calculator use for common elements?
The calculator uses the most abundant stable isotope of each element: ¹H (1.007825 Da), ¹²C (12.000000 Da), ¹⁴N (14.003074 Da), ¹⁶O (15.994915 Da), ¹⁹F (18.998403 Da), ²³Na (22.989769 Da), ³¹P (30.973762 Da), ³²S (31.972071 Da), ³⁵Cl (34.968853 Da), and ⁷⁹Br (78.918338 Da). For monoisotopic elements (like fluorine, sodium, phosphorus), the monoisotopic and average masses are identical.
How much does monoisotopic mass differ from average mass for typical organic compounds?
For small organic molecules (C, H, N, O only), the difference between monoisotopic and average mass is typically 0.01–0.1 Da. For a molecule like glucose (C₆H₁₂O₆): monoisotopic mass = 180.0634 Da; average mass = 180.156 Da; difference = 0.093 Da. The gap widens as molecular size increases or when heavier elements with widely spread isotope distributions (like Cl, Br, or S) are present.