Universal µg/dL to nmol/L Converter
Universal µg/dL to nmol/L Converter
Convert µg/dL, ng/mL, µg/L, µg/mL and mg/dL to nmol/L for any analyte by entering its molecular weight. This is the ladder steroids and fat-soluble vitamins are reported on, and the trap is not the arithmetic — it is that testosterone is printed in ng/dL and vitamin D in ng/mL, a hundredfold apart from each other.
µg/dL to nmol/L, any analyte
Mass + MW → nmol/LCortisol 15 µg/dL, molecular weight 362.46 g/mol
The formula, derived rather than quoted
µg/dL = nmol/L × MW ÷ 10 000
because 1 µg/dL is 10⁻⁵ g/L, and 10⁻⁵ ÷ MW mol/L × 10⁹ = (10⁴ ÷ MW) nmol/L
- the derivation
- one microgram per decilitre is ten micrograms per litre, which is 1 × 10⁻⁵ grams per litre. Divide by grams per mole for moles per litre. Multiply by 10⁹ for nanomoles. The 10⁻⁵ and the 10⁹ leave 10⁴
- × 10 000 ÷ MW
- the whole conversion in one step. Cortisol at 362.46 gives 10 000 ÷ 362.46 = 27.59, which is the factor every cortisol converter prints
- ng/mL = µg/L
- identical, always. One nanogram in a millilitre is one microgram in a litre, and both are a tenth of the µg/dL figure
- ng/dL
- not on the list above, and the reason is worth knowing: ng/dL is a hundredth of µg/dL. Testosterone is reported in ng/dL in the United States, so a testosterone of 500 ng/dL is 5 µg/dL — enter 5, not 500
- which weight
- the molecule the assay measures. For total 25-hydroxyvitamin D that is a convention rather than a fact: assays report D₂ and D₃ together against the D₃ molar mass of 400.64, even though D₂ is 412.65
Worked example
Cortisol 15 µg/dL, molecular weight 362.46 g/mol
15 µg/dL = 1.5 × 10⁻⁴ g/L
1.5 × 10⁻⁴ ÷ 362.46 = 4.138 × 10⁻⁷ mol/L = 413.84 nmol/L
The same thing in one step: 15 × 10 000 ÷ 362.46 = 413.8
That matches the 27.58925 factor the dedicated cortisol converter ships, because 10 000 ÷ 362.46 is 27.589 — and 15 × 27.589 is 413.8
The steroids and vitamins this ladder is for
| Analyte | Molecular weight | Usual reporting unit | µg/dL → nmol/L | Its own page |
|---|---|---|---|---|
| Cortisol (C₂₁H₃₀O₅) | 362.46 | µg/dL or nmol/L | × 27.589 | cortisol converter |
| Testosterone (C₁₉H₂₈O₂) | 288.42 | ng/dL — a hundredth of µg/dL | × 34.672 | testosterone converter |
| 25-hydroxyvitamin D₃ (C₂₇H₄₄O₂) | 400.64 | ng/mL — a tenth of µg/dL | × 24.960 | 25-OH vitamin D converter |
| Retinol, vitamin A (C₂₀H₃₀O) | 286.45 | µg/dL or µmol/L | × 34.910 | retinol converter |
The prefix ladder, in one place
| Unit | In g/L | Relative to µg/dL | Where you meet it |
|---|---|---|---|
| mg/dL | 1 × 10⁻² | × 1 000 | Glucose, cholesterol, urea, calcium |
| µg/mL = mg/L | 1 × 10⁻³ | × 100 | Protein assays, some drug levels |
| µg/dL | 1 × 10⁻⁵ | × 1 | Cortisol, retinol, lead, copper, zinc |
| ng/mL = µg/L | 1 × 10⁻⁶ | ÷ 10 | 25-OH vitamin D, most therapeutic drugs |
| ng/dL | 1 × 10⁻⁸ | ÷ 1 000 | Testosterone, free thyroxine, aldosterone |
| pg/mL = ng/L | 1 × 10⁻⁹ | ÷ 10 000 | Oestradiol, PTH, B12, troponin |
Why the steroid ladder catches people out
The derivation is short enough to do in your head. A microgram per decilitre is ten micrograms per litre, which is 10⁻⁵ grams per litre. Divide grams per litre by grams per mole and you have moles per litre. Multiply by a billion and you have nanomoles per litre. The 10⁻⁵ and the 10⁹ leave 10⁴, so the whole conversion is ten thousand divided by the molecular weight. Cortisol’s 362.46 gives 27.59; retinol’s 286.45 gives 34.91; testosterone’s 288.42 gives 34.67. Those are the published factors, and they are not measurements — they are one division each.
What goes wrong on this ladder is almost never the molecular weight. It is the unit. Steroid and vitamin results are reported across five different mass prefixes depending on the analyte and the country, and the prefixes are close enough in appearance to be interchanged without anything looking odd. Cortisol comes in µg/dL. Testosterone comes in ng/dL, which is a hundred times smaller, so a normal male testosterone of 500 ng/dL is 5 µg/dL — and typing 500 into a µg/dL field gives a hundredfold overestimate that still reads as a plausible number of nanomoles. 25-hydroxyvitamin D comes in ng/mL, ten times smaller than µg/dL, so a sufficient 30 ng/mL is 3 µg/dL. Oestradiol and parathyroid hormone come in pg/mL, ten thousand times smaller. None of these is a hard conversion; all of them are easy to get wrong by a clean power of ten, and a clean power of ten is exactly the kind of error that survives a sanity-check.
Two conventions on this ladder are worth naming because they are conventions rather than facts. Total 25-hydroxyvitamin D assays report vitamin D₂ and D₃ together, and they do it against the D₃ molar mass of 400.64 even though D₂ is 412.65 — so the molar figure on a report from a patient taking ergocalciferol carries a small systematic assumption. And free hormone fractions are a different measurement, not a different unit: a free testosterone in pmol/L has no fixed relationship to a total testosterone in nmol/L, because the fraction that is free depends on how much binding protein is present. No molecular weight will bridge that gap.
As with every generic tool on this site, the dedicated page is the better one where it exists. It opens in the unit that analyte is actually reported in, which removes the commonest error on this page before it can happen, and it carries the reference interval, the timing requirements and the assay caveats. A cortisol of 414 nmol/L means nothing without knowing whether it was drawn at nine in the morning or at midnight. This tool is for the analyte with no page of its own — and for checking that a factor somebody has quoted you is the one the molecular weight actually implies.
Frequently asked questions
How do I convert µg/dL to nmol/L?
Multiply by 10,000 and divide by the molecular weight. A cortisol of 15 µg/dL with a molar mass of 362.46 is 15 × 10,000 ÷ 362.46 = 414 nmol/L. To go back, multiply the nmol/L figure by the molecular weight and divide by 10,000.
Why is the factor 10,000 and not 10?
Because the target unit is nanomoles rather than millimoles. One µg/dL is 10⁻⁵ g/L; dividing by the molar mass gives mol/L, and scaling to nanomoles multiplies by 10⁹. The two exponents leave 10⁴. The mg/dL to mmol/L version of the same derivation leaves 10, which is why that factor looks so different.
Is ng/dL the same as µg/dL?
No — ng/dL is a hundred times smaller. This catches people out with testosterone, which United States laboratories report in ng/dL: a total testosterone of 500 ng/dL is 5 µg/dL, or about 17 nmol/L. Entering 500 as µg/dL gives 1,734 nmol/L, which is a hundredfold wrong.
Which molecular weight should I use for vitamin D?
400.64, the molar mass of 25-hydroxyvitamin D₃. Assays report D₂ and D₃ together as total 25-hydroxyvitamin D and convert both against the D₃ figure by convention, even though D₂ is 412.65 — a small systematic assumption that matters only in patients taking ergocalciferol.
Can I use this for a free hormone level?
You can convert its units, but be clear that free and total are different measurements rather than different units. The proportion of a hormone that is unbound depends on how much binding protein is present, so no molecular weight relates a free concentration to a total one.
Related calculators
References
- Bureau International des Poids et Mesures. The International System of Units (SI Brochure). 9th ed. — SI prefixes and amount-of-substance concentration.
- Commission on Isotopic Abundances and Atomic Weights (IUPAC). Standard atomic weights — every molar mass on this page was recomputed from its molecular formula.
- National Center for Biotechnology Information. PubChem Compound Summaries: cortisol CID 5754, testosterone CID 6013, 25-hydroxyvitamin D₃ CID 5283731, retinol CID 445354.
- Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018 — steroid hormone measurement and units.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.