Lab Chemistry

RMM / RFM Calculator

Relative molecular (or formula) mass from a chemical formula using IUPAC 2021 atomic weights, including salts, hydrates and solvates. Approve it, then tick the box to import it into the dosing and formulation calculators.

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Scientifically reviewed by:D Yap, MBiochem (Oxon), DIC, PhD (Lond)

Clinical Medicine Research Scientist·LinkedIn Profile ↗·ORCID: 0000-0002-5370-4592 ↗

⚠️ Research Reference Notice: For reference and research education only. Reviewer is a clinical medicine research scientist and not a practicing physician or veterinarian, and does not provide clinical certification or medical advice. Experimental drug doses and clinical measurements must be verified against institutional protocols, primary manufacturer labeling, and physician orders.

1. Compound

Compound name

Lot / CoA no. (optional)

Formula of the material as supplied (vial)

Separate salt, hydrate or solvate parts with · . or * — e.g. C22H24N4O·CH4O3S, C8H10N4O2·0.5H2O. Brackets and parentheses are supported.

Active moiety formula (free base / acid)

Leave blank to use the first part of the supplied formula. Enter it explicitly for sodium/potassium salts, where the metal replaces a hydrogen.

2. Result

Supplied form (RMM / RFM)579.983 g/mol

Active moiety543.525 g/mol

Active fraction (RMM ratio)0.9371 (93.71%)

Salt factor (supplied ÷ active)1.0671×

Hill formula: C27H30ClNO11

MW stated on vial / CoA (optional cross-check)

Elemental breakdown of supplied form
Element	Atoms	Atomic wt	Subtotal	Mass %
H Hydrogen	30	1.008	30.240	5.21
C Carbon	27	12.011	324.297	55.91
N Nitrogen	1	14.007	14.007	2.42
O Oxygen	11	15.999	175.989	30.34
Cl Chlorine	1	35.45	35.450	6.11

I have checked this formula, salt/solvate form and RMM against the vial label or Certificate of Analysis and approve it as correct.

Make this compound available to import in the Animal Dosing and Formulation calculators (approve it first)

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Atomic weights: IUPAC CIAAW Standard Atomic Weights 2021 (Prohaska et al., Pure Appl Chem 2022;94:573-600). Elements with an atomic-weight interval use IUPAC conventional values, so results can differ from a CoA in the second decimal place.

RMM, RFM and why the salt form matters

Relative molecular mass (RMM) applies to discrete molecules; relative formula mass (RFM) is the same sum of atomic weights applied to the formula unit of an ionic or salt compound. Both are numerically equal to the molar mass in g/mol.

Many drugs are supplied as salts or hydrates (for example doxorubicin hydrochloride, imatinib mesylate). If a published dose refers to the free base, weigh more of the salt: mass of salt = mass of free base × (RMM_salt ÷ RMM_{free base}). If the dose refers to the material as supplied, no correction is needed. Check which basis your protocol or reference uses.

Atomic weights: Prohaska T, et al. Standard atomic weights of the elements 2021 (IUPAC Technical Report). Pure Appl Chem. 2022;94(5):573–600. doi:10.1515/pac-2019-0603. Batch-specific MW on a Certificate of Analysis may include residual water or solvent and takes precedence.