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Functional Group Mass Calculator

Reviewed by the CalculatorHub team, edited by James Graham. IUPAC 2021 atomic weights. Last verified 15 June 2026.

The mass contribution of each functional group equals the sum of the IUPAC standard atomic weights of its constituent atoms. This calculator lets you enter the number of common functional groups present in a molecule and sums their mass contributions. It is useful for verifying partial structures, calculating mass changes during derivatization, or checking protecting group additions and removals. All atomic weights are from the 2021 IUPAC Standard Atomic Weights (Commission on Isotopic Abundances and Atomic Weights).

MW contribution: 17.01 g/mol each
MW contribution: 15.02 g/mol each
MW contribution: 45.02 g/mol each
MW contribution: 16.02 g/mol each
MW contribution: 46.01 g/mol each
32.03
2

Group contribution formula

MW contribution = sum of (count x group mass) for all groups

Each group mass is the sum of IUPAC 2021 standard atomic weights of its atoms: OH = O + H = 15.999 + 1.008 = 17.007; CH3 = C + 3H = 12.011 + 3(1.008) = 15.035; COOH = C + 2O + H = 12.011 + 2(15.999) + 1.008 = 45.017; NH2 = N + 2H = 14.007 + 2(1.008) = 16.023; NO2 = N + 2O = 14.007 + 2(15.999) = 46.005.

Common functional group masses (IUPAC 2021)

  • -CH3 (methyl): 15.04 g/mol.
  • -OH (hydroxyl): 17.01 g/mol.
  • -NH2 (amino): 16.02 g/mol.
  • -COOH (carboxyl): 45.02 g/mol.
  • -NO2 (nitro): 46.01 g/mol.
  • -Cl (chloro): 35.45 g/mol.
  • -Br (bromo): 79.90 g/mol.
  • -SH (thiol): 33.07 g/mol.

Functional group mass: frequently asked questions

What is group contribution in molecular weight calculation?

Group contribution methods estimate a molecule's molecular weight (or other properties) by summing the atomic masses of all atoms. Each functional group contributes the sum of its constituent atomic masses. For example, a hydroxyl group (OH) contributes 1.008 (H) + 15.999 (O) = 17.007 g/mol.

Why calculate functional group mass contributions separately?

When tracking how a functional group transformation changes molecular weight (e.g., converting OH to OTs, or replacing COOH with COCl), it helps to know the exact mass change per substitution. This calculator shows the net MW of common groups.

What are the most common functional group masses?

Hydroxyl (OH): 17.01. Methyl (CH3): 15.02. Carbonyl (C=O): 28.01. Carboxyl (COOH): 45.02. Amino (NH2): 16.02. Nitro (NO2): 46.01. Halides: F 19.00, Cl 35.45, Br 79.90, I 126.90.

How is this different from calculating the full molecular weight?

A full molecular weight calculator sums all atoms from a molecular formula. The functional group mass calculator shows how specific groups contribute and is useful for verifying partial structures, checking derivatization mass shifts, or planning protecting group strategies.

Are group masses based on IUPAC atomic weights?

Yes. All atomic masses in this calculator use the 2021 IUPAC recommended standard atomic weights: C = 12.011, H = 1.008, O = 15.999, N = 14.007, F = 18.998, Cl = 35.45, Br = 79.904, I = 126.904, S = 32.06, P = 30.974.

Official sources

Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.