What does nmol/L mean on a blood test, and how do you convert it?

The short answer

Evidence tier: 1 — factors come from published SI conversion tables (MSD Manuals, Labcorp, Mayo Clinic Laboratories) and the peer-reviewed IFCC–NGSP master equation for HbA1c; worked examples were calculated from them.

nmol/L means nanomoles per litre: the number of molecules of a substance in a litre of blood, counted in billionths of a mole. It is an SI (molar) unit, used by labs in the UK, Europe and most of the world, while US labs usually report the same tests by weight, such as ng/dL or ng/mL. The two are linked by a fixed factor for each marker, so total testosterone of 300 ng/dL is about 10.4 nmol/L. The number changes; the result does not.

If you are tracking bloodwork while using peptides, TRT or a GLP-1, see labs before and during peptides for which tests matter. This page is about reading the units.

What does nmol/L mean?

Evidence tier: 1 — the mole and the nano-, pico- and milli- prefixes are fixed definitions of the International System of Units; nothing here is interpretation.

A mole is a fixed count of molecules (about 6 × 10²³). A nanomole is one billionth of that. So a result in nmol/L tells you how many molecules of the hormone or nutrient are in each litre of serum. pmol/L (picomoles per litre) is the same idea, a thousand times smaller, and is used for markers present in tiny amounts, such as estradiol or free testosterone. mmol/L (millimoles) is used for abundant ones, such as glucose and cholesterol.

Mass units measure weight instead: ng/dL is nanograms per decilitre, ng/mL is nanograms per millilitre, and pg/mL is picograms per millilitre.

Each prefix step (milli, nano, pico) is a factor of 1,000, which is where the most common reading error comes from. A free testosterone result of 347 pmol/L is the same as 0.347 nmol/L. Read the unit on the report before comparing a number with anything else, because mixing up pmol/L and nmol/L shifts a result a thousandfold.

How do you convert between mass units and nmol/L?

Evidence tier: 1 — standard SI arithmetic from each molecule's molecular weight; the results match the published conversion tables.

The bridge is the molecule's molecular weight. Testosterone weighs about 288.4 g per mole. One ng/dL equals 10 ng per litre, and 10 ÷ 288.4 = 0.0347 nmol/L. That is why every SI table lists 0.0347 as the testosterone factor.

The rule for any marker:

• US unit → SI unit: multiply by the factor.
• SI unit → US unit: divide by the factor (or multiply by its inverse).

Two more worked examples show the same method with different molecules:

• Vitamin D (25-OH D3, about 400.6 g/mol). 1 ng/mL equals 1,000 ng per litre, and 1,000 ÷ 400.6 = 2.496 nmol/L. So 30 ng/mL × 2.496 = 74.9, which labs round to 75 nmol/L.
• Cortisol (about 362.5 g/mol). 1 µg/dL equals 10,000 ng per litre, and 10,000 ÷ 362.5 = 27.59 nmol/L. So 10 µg/dL is about 276 nmol/L.

Going the other way works the same. A UK result of 15 nmol/L total testosterone, divided by 0.0347, is about 432 ng/dL.

Because the factor depends on the molecule, there is no single "nmol/L to ng/mL" number. Using the testosterone factor on vitamin D, for example, gives a wrong answer.

Why do US labs use ng/dL while UK and EU labs use nmol/L?

Evidence tier: 5 — editorial context; no single source governs every lab's choice.

Most countries adopted the International System of Units (SI) for clinical chemistry, which expresses amounts in moles. US laboratories largely kept conventional mass units, and many US guidelines, textbooks and reference ranges are written in them. Some labs report both. The practical result is that people comparing a US result with a UK or European one, or reading research from another country, need to convert before comparing.

What are the conversion factors for common lab tests?

Evidence tier: 1 — factors from MSD Manuals' SI conversion table, cross-checked against Labcorp and Mayo Clinic Laboratories. Small differences in the last digit between sources come from rounding.

A few notes on individual rows:

• Free testosterone is sometimes reported in ng/dL in the US. Mayo Clinic Laboratories lists ng/dL × 34.672 = pmol/L, which is the same factor shifted by a factor of ten, because 1 ng/dL equals 10 pg/mL.
• Vitamin D: 2.496 applies to total 25-OH vitamin D and to D3. Mayo lists a slightly lower factor (2.423) for 25-OH D2 alone.
• Vitamin B12: MSD lists 0.7373 and Labcorp lists 738 for ng/mL (equivalent to 0.738 for pg/mL). Either gives the same answer to the nearest whole number at typical levels.
• Ferritin: ng/mL and µg/L are the same quantity, so no conversion is needed. Some tables also give a molar factor (× 2.247 for pmol/L), but UK and EU labs normally report ferritin in µg/L.
• IGF-1: some European labs report µg/L, which equals ng/mL, so the 0.131 factor applies only when the result is in nmol/L.

What do typical values look like in both units?

Evidence tier: 1 — the arithmetic uses the factors in the table above; the example values are illustrations, not reference ranges.

How do you convert HbA1c from % to mmol/mol?

Evidence tier: 1 — the master equation comes from a peer-reviewed method-comparison study by the IFCC working group and is monitored in published intercomparison studies.

HbA1c is the exception. It is not a concentration, so there is no simple multiplier. US labs report it as a percentage (the NGSP/DCCT scale); the UK and much of Europe use mmol/mol (the IFCC scale).

The two scales measure the same thing against different reference methods. The IFCC method is more specific, so it gives lower numbers. A method-comparison study across reference laboratories in the US, Japan and Sweden found a stable, linear link between them: NGSP % = 0.915 × IFCC % + 2.15 (Hoelzel et al., 2004). Later intercomparison studies found no drift in that relationship for the NGSP scale (Weykamp et al., 2008). With IFCC values now expressed in mmol/mol rather than percent, the NGSP gives the master equation as:

NGSP (%) = 0.09148 × IFCC (mmol/mol) + 2.152

Rearranged, IFCC (mmol/mol) = (NGSP % − 2.152) ÷ 0.09148.

Values are rounded to whole numbers, as labs report them. The NGSP's own table lists 7.0% as 53 mmol/mol, which matches.

Do reference ranges convert the same way?

Evidence tier: 2 — the testosterone example is a CDC-calibrated four-cohort study and the vitamin D example compares two published guidelines; individual lab ranges are assay-specific.

The units convert exactly, but reference ranges do not transfer between labs. Each laboratory sets its own ranges based on its assay, method and population, and ranges differ by age, sex and, for some hormones, time of day or menstrual cycle phase. Testosterone and cortisol are usually drawn in the morning for this reason, and estradiol results depend on whether a sensitive (mass spectrometry) assay was used.

Testosterone shows how much the assay matters. When researchers recalibrated results from four US and European cohorts against a CDC reference method, a large share of the differences between cohorts disappeared, because much of it came from the assays. The harmonised range for healthy non-obese men aged 19 to 39 was 264 to 916 ng/dL (Travison et al., 2017). Converted with the 0.0347 factor, that is about 9.2 to 31.8 nmol/L. A lab whose assay is not calibrated to that reference may print a different range, and that difference survives unit conversion.

Vitamin D shows the other problem: the cut-offs themselves differ. The Institute of Medicine's 2011 report said a 25-OH vitamin D level of at least 20 ng/mL (50 nmol/L) meets the needs of at least 97.5% of the population (Ross et al., 2011). The 2024 Endocrine Society guideline found no clear evidence defining an optimal target level for disease prevention (Demay et al., 2024). Converting units cannot settle which threshold a lab report uses.

When comparing results over time, the most useful habit is to compare like with like: same lab, same assay, same time of day, and the range printed on that report. Interpreting where a result sits, and whether it calls for any change, is a conversation for the physician who ordered the test.

Limitations

• Rounding. Sources differ in the last digit of some factors (for example 27.59 vs 27.6 for cortisol). The differences do not change a clinical reading but may shift a decimal place.
• Assay differences matter more than unit differences. Two labs can give different results for the same sample even in the same unit, especially for estradiol, free testosterone and vitamin D.
• Free testosterone can be measured directly or calculated from total testosterone, SHBG and albumin; the unit conversion is the same, but the methods are not interchangeable.
• This page converts numbers; it does not interpret them. Nothing here is a diagnosis or a treatment threshold.

The bottom line

nmol/L is a count of molecules per litre; ng/dL and ng/mL are weights per volume. Every marker has its own fixed conversion factor, derived from its molecular weight, so a result means the same thing in either unit once converted correctly. HbA1c is the one common test that needs an equation rather than a multiplier.

Use the factor for the specific marker, compare results only against the reference range printed by the lab that ran the test, and take questions about what a result means to the physician who ordered it.

Related on this site

• Labs before and during peptides
• Longevity biomarkers
• TRT and peptides: how they fit
• Protecting fertility on TRT
• Age-related GH decline
• Longevity pillar

References

• MSD Manuals. Conventional units to SI units: clinical chemistry conversion calculator (accessed 3 November 2026).
• Labcorp. SI unit conversion table (accessed 3 November 2026).
• Mayo Clinic Laboratories. SI unit conversion (accessed 3 November 2026).
• NGSP. Harmonizing hemoglobin A1c testing: IFCC–NGSP master equation (accessed 3 November 2026).
• Hoelzel W, Weykamp C, Jeppsson JO, et al. 2004. IFCC reference system for measurement of hemoglobin A1c in human blood and the national standardization schemes in the United States, Japan, and Sweden: a method-comparison study. Clin Chem. 50(1):166-174. PMID 14709644 · doi:10.1373/clinchem.2003.024802
• Weykamp C, John WG, Mosca A, et al. 2008. The IFCC reference measurement system for HbA1c: a 6-year progress report. Clin Chem. 54(2):240-248. PMID 18223132
• Travison TG, Vesper HW, Orwoll E, et al. 2017. Harmonized reference ranges for circulating testosterone levels in men of four cohort studies in the United States and Europe. J Clin Endocrinol Metab. 102(4):1161-1173. PMID 28324103
• Ross AC, Manson JE, Abrams SA, et al. 2011. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know. J Clin Endocrinol Metab. 96(1):53-58. PMID 21118827
• Demay MB, Pittas AG, Bikle DD, et al. 2024. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 109(8):1907-1947. PMID 38828931