About

In clinical chemistry and endocrinology, laboratory results are often reported in different units depending on the country or the specific assay used. While mass concentrations like micrograms per liter (mcg/L or μg/L) measure the weight of the substance, molar concentrations like nanomoles per liter (nmol/L) measure the number of molecules. Converting between these requires the specific Molecular Weight (MW) of the analyte.

This conversion is critical for normalizing data in research studies or interpreting patient results against international reference ranges. For example, Vitamin D is commonly reported in ng/mL or mcg/L in the United States, but in nmol/L in the UK and Australia. An error in this conversion can lead to misdiagnosis of deficiency or toxicity. This tool automates the process with a built-in database of standard molecular weights for common hormones, vitamins, and proteins.

Formulas

The conversion relies on the relationship between mass, molar mass, and volume. To switch from mass concentration to molar concentration, we divide by the molecular weight and adjust for the unit scale.

C_nmol/L = C_mcg/LMW × 1000

Where:

C_nmol/L is the concentration in nanomoles per liter.
C_mcg/L is the concentration in micrograms per liter.
MW is the Molecular Weight in grams per mole (g/mol).
1000 is the conversion factor derived from 10⁹ (nano)10⁶ (micro).

Reference Data

Analyte	Molecular Weight (g/mol)	100 mcg/L converts to
Vitamin D (25-OH)	400.64	249.6 nmol/L
Cortisol	362.46	275.9 nmol/L
Thyroxine (T4)	776.87	128.7 nmol/L
Triiodothyronine (T3)	650.97	153.6 nmol/L
Ferritin	474,000 (approx)	0.21 nmol/L
Testosterone	288.42	346.7 nmol/L
Progesterone	314.47	318.0 nmol/L
Estradiol (E2)	272.38	367.1 nmol/L
Folic Acid	441.40	226.5 nmol/L
Vitamin B12	1355.37	73.8 nmol/L

Frequently Asked Questions

Mass units (mcg) measure how heavy a substance is, while molar units (nmol) measure the count of molecules. A heavy molecule like Ferritin requires more mass to make up one mole than a light molecule like Cortisol. Without the specific Molecular Weight, it is impossible to convert mass to count.

Yes. Micrograms per liter (mcg/L) is numerically equivalent to nanograms per milliliter (ng/mL). You can use this tool for values reported in ng/mL without any adjustment.

You can usually find the Molecular Weight (MW) on the reagent insert of the laboratory assay or in chemical reference databases like PubChem. Enter this value manually into the "Molecular Weight" field if your substance is not pre-listed.

Yes. The mathematical relationship holds true regardless of the matrix (serum, plasma, urine, CSF), provided the concentration is reported in mass per volume.