Leptin Unit Converter

Convert leptin between ng/mL, µg/L, pmol/L and nmol/L — and read why a leptin without a body mass index beside it has no reference interval, and why high leptin in obesity means resistance rather than excess.

Last updated 16 Sep 2026

Leptin converter

Mass ⇄ molar

Leptin

ng/mL and µg/L are the same number. Record the body mass index with the result — leptin is proportional to fat mass, and laboratories publish their intervals for a stated BMI.

748.8pmol/LExample

Leptin 12 ng/mL

The units, and why no single interval is printed

ng/mL = µg/L — the same number
pmol/L = ng/mL × 62.3986
nmol/L = ng/mL × 0.0623986
derived from a molecular weight of 16,026 Da for the 146-residue mature protein

ng/mL = µg/L: a nanogram per millilitre and a microgram per litre are the same concentration, and every published leptin interval is in one or the other
× 62.3986: the molar factor to pmol/L, from a mature protein mass of 16,026 daltons. A leptin of 12 ng/mL is 748.8 pmol/L. Research papers use molar units more often than clinical reports do
why no reference interval is printed: because leptin is produced by adipose tissue in proportion to fat mass, so the expected value depends on how much fat the patient has. The laboratories say so themselves: Labcorp’s specialty test 500716 publishes its interval for a body mass index of 22 — 0.7–5.3 ng/mL in men and 3.3–18.3 ng/mL in women — and instructs the requester to contact the laboratory for other BMI ranges
the sex difference is large and real: at the same body mass index women have roughly three to four times the leptin of men, because of a greater proportion of subcutaneous fat, which expresses more leptin than visceral fat, and an oestrogen effect. Any interval must be sex-specific as well as BMI-specific
what the number is actually for: two narrow questions. Is this severe early-onset obesity caused by congenital leptin deficiency, in which leptin is very low or undetectable and metreleptin is transformative? And is there lipodystrophy, in which leptin is low because there is little adipose tissue to make it? Common obesity is a state of leptin resistance with high levels, and measuring leptin does not usefully inform its management

Worked example

Leptin 12 ng/mL
12 ng/mL = 12.0 µg/L — the same number in the other mass convention
12 × 62.3986 = 748.8 pmol/L, and 12 × 0.0623986 = 0.749 nmol/L
Now try to interpret it. At a body mass index of 22, Labcorp quotes 0.7–5.3 ng/mL for men and 3.3–18.3 ng/mL for women, so the same 12 ng/mL is more than twice the upper limit in a man and unremarkable in a woman
In a patient with a body mass index of 40, 12 ng/mL would be distinctly low for the fat mass present, and the laboratory would quote a different and much higher interval
One number, three readings, decided by sex and body mass index. That is why this page prints no band: without a BMI, a leptin has nothing to be compared against

Leptin intervals are conditional on body mass index and sex

This is the single most useful thing on the page. A laboratory that publishes its interval only for a stated body mass index, and tells requesters to ask for the rest, is telling you that a single leptin range does not exist. Note also the size of the sex difference — women have roughly three to four times the leptin of men at the same BMI — and that these are population intervals from different assays and populations, not interchangeable numbers.
Source	Group	Interval (ng/mL)
Labcorp specialty test 500716	Men at BMI 22	0.7–5.3
Labcorp specialty test 500716	Women at BMI 22	3.3–18.3
Labcorp specialty test 500716	Any other BMI	Not published — the laboratory asks requesters to contact it for other BMI reference ranges
Cheng et al. 2022, Chinese population study	Men, BMI 20 to under 25	0.42–12.32
Cheng et al. 2022, Chinese population study	Women, BMI 20 to under 25	4.11–38.09
Cheng et al. 2022	Higher BMI bands	Substantially higher in both sexes; intervals rise with BMI throughout

What leptin is actually measured for

The distinction in the fourth row is the page’s main value. Leptin signals energy sufficiency to the hypothalamus, and the overwhelming majority of obesity involves high leptin with an impaired response to it. Only the rare deficiency states — congenital leptin deficiency and the lipodystrophies — produce a low leptin that replacement therapy can correct, and those are the situations in which measuring it changes what happens next.
Question	Expected leptin	What follows
Congenital leptin deficiency — severe obesity from the first months of life, relentless hyperphagia	Very low or undetectable despite marked obesity	Genetic confirmation and leptin replacement with metreleptin, which is transformative in this rare disorder
Congenital leptin receptor deficiency	High — often above what the fat mass alone would explain	A leptin measurement does not exclude a monogenic cause; receptor and MC4R defects present with high or appropriate leptin
Lipodystrophy, generalised or partial	Low, because there is little adipose tissue to produce it	Metreleptin is licensed for generalised lipodystrophy in the United States and, in Europe, for leptin deficiency in congenital leptin deficiency and lipodystrophy
Common obesity	High, in proportion to fat mass	This is leptin resistance, not leptin deficiency. Measuring leptin does not usefully guide management and a high value is the expected finding
Hypothalamic amenorrhoea, anorexia nervosa, low energy availability	Low, tracking the loss of fat mass	A marker of energy deficit rather than a diagnostic test; interpreted with the clinical picture

A fat-mass measurement, and the resistance that makes it unhelpful in obesity

Leptin is a 146-residue hormone secreted by adipocytes in proportion to fat mass, and it signals energy sufficiency to the hypothalamus: high leptin suppresses appetite and permits energy-expensive functions such as reproduction, while falling leptin drives hunger and conserves energy. Because production scales with adipose tissue, plasma leptin correlates strongly with percentage body fat. That single fact dominates the interpretation of any result, and it is why this page prints no reference interval. A leptin concentration without a body mass index beside it is close to meaningless — and the laboratories agree: Labcorp’s specialty test publishes its interval for a body mass index of 22, namely 0.7 to 5.3 ng/mL in men and 3.3 to 18.3 ng/mL in women, and instructs requesters to contact the laboratory for other BMI ranges. Note the sex difference in those figures. At the same body mass index women have roughly three to four times the leptin of men, because subcutaneous fat expresses more leptin than visceral fat and oestrogen raises it further.

The clinical use of the measurement is narrow, and knowing why is the point of the page. Congenital leptin deficiency is a rare autosomal recessive disorder in which severe obesity begins in the first months of life with relentless hyperphagia, and in which leptin is very low or undetectable despite that obesity. It matters out of all proportion to its frequency because it is treatable: recombinant leptin — metreleptin — reverses the hyperphagia and the weight gain. Lipodystrophy is the other deficiency state, low leptin arising from an absence of adipose tissue rather than a failure of the gene, and metreleptin is licensed for generalised lipodystrophy in the United States and, in Europe, for leptin deficiency in both congenital leptin deficiency and lipodystrophy. Low leptin also accompanies anorexia nervosa, low energy availability and hypothalamic amenorrhoea, where it is a marker of energy deficit rather than a diagnosis.

Everything else about leptin runs the other way, and this is the distinction most worth taking from the page. Common obesity is not a state of leptin deficiency; it is a state of leptin resistance. Leptin is high, in proportion to the fat mass producing it, and the hypothalamus does not respond to the signal. A raised leptin in a patient with obesity is therefore the expected finding and not an abnormality to be chased, and giving exogenous leptin to such a patient produces little weight loss because the receptor pathway is already saturated. Nor does a high leptin exclude a monogenic cause of obesity: leptin receptor deficiency and MC4R mutations present with leptin that is high or appropriate for fat mass. The measurement identifies the deficiency states and very little else.

The arithmetic is simple. Nanograms per millilitre and micrograms per litre are the same number, and every clinical interval is published in one of them. For the molar units, multiply ng/mL by 62.3986 for pmol/L or by 0.0623986 for nmol/L, from a mature protein mass of 16,026 daltons — figures that appear more often in research papers than on reports. What no factor can supply is the body mass index the result has to be read against.

Frequently asked questions

What is a normal leptin level?

There is no single answer, which is why this page prints no interval. Leptin is produced in proportion to fat mass, so the expected value depends on body mass index and sex. Labcorp’s specialty test quotes 0.7–5.3 ng/mL for men and 3.3–18.3 ng/mL for women at a body mass index of 22, and asks requesters to contact the laboratory for other BMI ranges.

How do I convert leptin from ng/mL to pmol/L?

Multiply by 62.3986, from a molecular weight of 16,026 daltons for the 146-residue mature protein. A leptin of 12 ng/mL is 748.8 pmol/L, or 0.749 nmol/L. Nanograms per millilitre and micrograms per litre are the same number and are what clinical reports use.

Does a high leptin mean too much leptin is causing the obesity?

No — it means leptin resistance. Leptin rises with fat mass, so almost everyone with obesity has a high level, and the problem is an impaired hypothalamic response to that signal rather than a shortage of hormone. A raised leptin in obesity is the expected finding, and giving more leptin achieves little.

When is measuring leptin actually useful?

In two narrow situations. Congenital leptin deficiency — severe obesity from the first months of life with relentless hyperphagia and a very low or undetectable leptin — because it responds dramatically to metreleptin. And lipodystrophy, where leptin is low because there is little adipose tissue to make it. Outside those, the result rarely changes management.

Why do women have higher leptin than men at the same BMI?

Because of body fat distribution and hormonal effects. Subcutaneous adipose tissue expresses more leptin than visceral fat and women carry proportionally more of it, and oestrogen raises leptin further while androgens lower it. The difference is roughly three- to fourfold, which is why any reference interval has to be sex-specific as well as BMI-specific.

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References

Labcorp Specialty Testing. Test 500716: Leptin — enzyme-linked immunoassay; reference interval at BMI 22, males 0.7–5.3 ng/mL and females 3.3–18.3 ng/mL; contact the laboratory for other BMI reference ranges. Accessed 2026.
Considine RV, Sinha MK, Heiman ML, et al. Serum immunoreactive-leptin concentrations in normal-weight and obese humans. N Engl J Med. 1996;334(5):292–295.
Montague CT, Farooqi IS, Whitehead JP, et al. Congenital leptin deficiency is associated with severe early-onset obesity in humans. Nature. 1997;387(6636):903–908.
Farooqi IS, Jebb SA, Langmack G, et al. Effects of recombinant leptin therapy in a child with congenital leptin deficiency. N Engl J Med. 1999;341(12):879–884.
Cheng J, Luo Y, Li Y, et al. Sex- and body mass index-specific reference intervals for serum leptin: a population based study in China. Nutr Metab (Lond). 2022;19:54.
Brown RJ, Araujo-Vilar D, Cheung PT, et al. The diagnosis and management of lipodystrophy syndromes: a multi-society practice guideline. J Clin Endocrinol Metab. 2016;101(12):4500–4511.

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.