What is molarity and why does it matter?

Molarity is how many moles of a compound sit in a litre of solution. Assays and receptor-binding comparisons are defined per molecule, so molar concentration, not mass, is the meaningful figure when comparing compounds.

How do I convert mg/mL to mM?

Divide the concentration in mg/mL by the molecular weight in g/mol and multiply by 1000. At 1 mg/mL, a peptide of 1419.5 g/mol is 0.704 mM (704 µM).

Can I use a peptide that is not listed?

Yes. Choose the custom option and enter any molecular weight in g/mol to run the same conversion.

Why do two peptides at the same mg/mL have different molarity?

Because molarity depends on molecular weight. A heavier molecule packs fewer moles into the same mass, so at an identical mg/mL a heavier peptide is a lower molar concentration.

Molarity and Molecular Weight Calculator

Molarity is how many moles of a compound sit in a litre of solution, so it depends on the compound's molecular weight. The same mg/mL concentration is a different molar concentration for every peptide. Pick a peptide to load its verified molecular weight, or enter your own, then convert between mg/mL and mM or µM.

Concentration to molarity

Peptide

Concentration (mg/mL)

Millimolar (mM): -
Micromolar (µM): -

Molarity to concentration

Enter a target molar concentration to get the matching mg/mL for the selected molecular weight.

Target concentration (mM)

Concentration (mg/mL): -

Molecular weight reference

The molecular weights below are free-base (parent) values, verified against PubChem and authoritative chemical databases. Research vials are often the acetate salt, which adds counter-ion mass.

Peptide	Molecular formula	Molecular weight	CAS
AOD-9604	C78H123N23O23S2	1815.1 g/mol	221231-10-3
BPC-157	C62H98N16O22	1419.5 g/mol	137525-51-0
Cagrilintide	C194H312N54O59S2	4409.01 g/mol	1415456-99-3
CJC-1295 (No DAC)	C152H252N44O42	3367.9 g/mol	-
DSIP	C35H48N10O15	848.8 g/mol	62568-57-4
Epitalon	C14H22N4O9	390.35 g/mol	307297-39-8
GHK-Cu	C14H22CuN6O4	401.91 g/mol	89030-95-5
IGF-1 LR3	-	~9111 g/molapprox.	143045-27-6
Ipamorelin	C38H49N9O5	711.86 g/mol	170851-70-4
KPV	C16H30N4O4	342.4 g/mol	67727-97-3
LL-37	C205H340N60O53	4493.26 g/mol	154947-66-7
Melanotan-2	C50H69N15O9	1024.18 g/mol	121062-08-6
MOTS-c	C101H152N28O22S2	2174.60 g/mol	1627580-64-6
NAD+	C21H27N7O14P2	663.43 g/mol	53-84-9
Retatrutide	C221H342N46O68	4731.3 g/mol	2381089-83-2
Selank	C33H57N11O9	751.9 g/mol	129954-34-3
Semax	C37H51N9O10S	813.92 g/mol	80714-61-0
Sermorelin	C149H246N44O42S	3357.93 g/mol	86168-78-7
SS-31	C32H49N9O5	639.79 g/mol	736992-21-5
TB-500	C212H350N56O78S	4963.4 g/mol	77591-33-4
Tesamorelin	C221H366N72O67S	5135.86 g/mol	218949-48-5
Thymosin Alpha-1	C129H215N33O55	3108.3 g/mol	62304-98-7
Tirzepatide	C225H348N48O68	4813.5 g/mol	2023788-19-2

Approximate: recombinant protein with no single database-corroborated mass.

How this calculator works

The formula

Molar concentration in millimolar equals the concentration in mg/mL divided by the molecular weight in g/mol, multiplied by 1000. At 1 mg/mL, a peptide with a molecular weight of 1419.5 g/mol is 1 divided by 1419.5 times 1000, which is 0.704 mM (704 µM). A heavier peptide at the same mg/mL is a lower molar concentration.

Worked example

Retatrutide has a molecular weight of 4731.3 g/mol. At 5 mg/mL its molar concentration is 5 divided by 4731.3 times 1000, which is 1.057 mM (1057 µM). The same 5 mg/mL of BPC-157 (1419.5 g/mol) is 3.52 mM, because the smaller molecule packs more moles into the same mass.

Why molarity matters for a reagent

Assays, receptor-binding work and comparisons across compounds are defined by molar concentration, not by mass, because activity happens per molecule. Two peptides at the same mg/mL sit at very different molar concentrations whenever their molecular weights differ, and converting to mM or µM puts every compound on the same footing. This tool performs an objective chemical calculation on the numbers you enter; it is a reagent-preparation aid, not medical guidance.

Molarity and molecular weight: frequently asked questions

Open source

This calculator is free and open source under the MIT license. Read the code, open an issue, or embed it on your own site.

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