Millimoles count particles; milligrams measure mass. To convert, multiply the millimoles by the molar mass (grams per mole) of the substance — for example glucose is about 180 g/mol.
For the lab
Chemistry essentials.
Lab scale (0.001 g)
Weigh to the milligram.
Periodic table poster
Molar masses at a glance.
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Frequently asked questions
What is the mmol to mg formula?
mg = mmol × molar mass (g/mol). The result is already in milligrams because millimoles × g/mol gives mg.
Where do I find molar mass?
From the periodic table for elements, or the sum of atomic masses for compounds.