Vitamin D ng/mL to nmol/L Converter

Vitamin D in two units: the short answer

To convert vitamin D from ng/mL to nmol/L, multiply by 2.496 (2.5 is close enough for everyday use). To convert nmol/L back to ng/mL, divide by 2.496, or multiply by 0.4. A result of 20 ng/mL is therefore about 50 nmol/L, and a result of 75 nmol/L is about 30 ng/mL. The converter above does the arithmetic in both directions and shows where a value falls against the cut-offs used by the US National Academy of Medicine (formerly the Institute of Medicine) and the NIH Office of Dietary Supplements.

Vitamin D is one of the tests where the unit causes the most confusion. The same person can read "30" on one report and "75" on another and be told both times that the level is fine – or read "50" and not know whether it means borderline-low (50 nmol/L) or the upper edge of the comfortable range (50 ng/mL). This guide explains what is actually measured, how the conversion works, which thresholds different organisations use, and what makes vitamin D results move.

What the vitamin D blood test measures

The standard test measures 25-hydroxyvitamin D, written 25(OH)D. Vitamin D made in the skin from sunlight, or taken in from food and supplements, is first converted in the liver into 25(OH)D. This form circulates for two to three weeks, so its level reflects vitamin D supply from all sources over the previous weeks to months. That makes it the best single marker of vitamin D status.

The kidneys then convert a small amount of 25(OH)D into the active hormone, 1,25-dihydroxyvitamin D (calcitriol). Despite being the active form, 1,25(OH)2D is a poor marker of status: it has a half-life of only hours and is tightly regulated by parathyroid hormone, so it often stays normal or even rises when a person is deficient. It is measured only in specific situations, such as kidney disease, unusual calcium disorders or sarcoidosis. If your report says "1,25" or "calcitriol", the ranges on this page do not apply – that test is reported in pg/mL or pmol/L and has completely different reference values.

Many reports show a total 25(OH)D value, and some also list 25(OH)D3 (cholecalciferol-derived) and 25(OH)D2 (ergocalciferol-derived) separately. The total is the figure used for the thresholds below.

ng/mL and nmol/L explained

ng/mL (nanograms per milliliter) is a mass concentration. It is used by most laboratories in the United States and by some in other countries.

nmol/L (nanomoles per liter) is a molar concentration, the SI unit. It is used in the UK, Ireland, Canada, Australia, New Zealand and most of Europe.

Note that ng/mL is numerically the same as µg/L (micrograms per liter), so a report in µg/L can be read exactly like one in ng/mL.

Why the factor is 2.496

The molar mass of 25-hydroxyvitamin D3 is about 400.6 g/mol. One nanomole of it therefore weighs 400.6 nanograms. Converting 1 ng/mL (1,000 ng per liter) into nanomoles gives 1,000 ÷ 400.6 = 2.496 nmol/L. Laboratories round this factor in different ways – 2.496, 2.5 or 2.4963 – and the difference is never clinically meaningful.

How to convert vitamin D step by step

1. Check the unit printed next to the result on the report.
2. If it is in ng/mL (or µg/L), multiply by 2.496 to get nmol/L.
3. If it is in nmol/L, divide by 2.496 (or multiply by 0.4006) to get ng/mL.
4. Round to whole numbers. Vitamin D assays vary by roughly 10% between methods, so decimals add no useful information.

Worked examples

• 12 ng/mL × 2.496 = 29.95 → 30 nmol/L. This is the deficiency threshold in both systems.
• 20 ng/mL × 2.496 = 49.9 → 50 nmol/L. This is the level the National Academy of Medicine considers sufficient for bone health in almost everyone.
• 30 ng/mL × 2.496 = 74.9 → 75 nmol/L. This is the cut-off some endocrine societies have used as "sufficient".
• 50 ng/mL × 2.496 = 124.8 → 125 nmol/L. Above this the NAM advises caution about possible adverse effects.
• 45 nmol/L ÷ 2.496 = 18.0 → 18 ng/mL.
• 100 nmol/L ÷ 2.496 = 40.1 → 40 ng/mL.

Mental shortcut

Multiply ng/mL by 2.5 or divide nmol/L by 2.5 (that is, multiply by 4 and divide by 10). For 60 nmol/L: 60 × 4 = 240, ÷ 10 = 24 ng/mL. The exact answer is 24.0.

Vitamin D conversion chart

Vitamin D levels: what the numbers mean

The cut-offs above follow the National Academy of Medicine's 2011 report, which the NIH Office of Dietary Supplements uses:

• Below 12 ng/mL (30 nmol/L) – at risk of deficiency, which in children can cause rickets and in adults osteomalacia (soft bones), muscle weakness and bone pain.
• 12–19 ng/mL (30–49 nmol/L) – potentially inadequate for bone and overall health in some people.
• 20–50 ng/mL (50–125 nmol/L) – adequate for bone health for practically all healthy people.
• Above 50 ng/mL (125 nmol/L) – linked to potential adverse effects, particularly at levels well above this.

Why different organisations disagree

You may see other numbers in other sources, and the disagreement is real rather than a unit error:

• The National Academy of Medicine and many European bodies, including the UK's Scientific Advisory Committee on Nutrition, treat 20 ng/mL (50 nmol/L) as adequate for the population. The UK uses 25 nmol/L (10 ng/mL) as the population threshold for deficiency risk.
• The Endocrine Society's 2011 guideline defined deficiency as below 20 ng/mL (50 nmol/L) and recommended aiming for 30 ng/mL (75 nmol/L) or above in at-risk patients. Its updated 2024 guideline no longer endorses a specific target level for the general population and advises against routine testing in healthy people, because large trials did not show benefits from pushing levels higher.

So a level of 25 ng/mL (62 nmol/L) is "adequate" under the NAM framework and was "insufficient" under the older Endocrine Society definition. When reading a report, check which reference the laboratory is using, and discuss with your clinician which target applies to you – people with osteoporosis, malabsorption or certain medications may have a different goal.

When vitamin D is too high

Vitamin D toxicity is uncommon and almost always caused by high-dose supplements, not sunlight or food. Problems are generally seen at levels well above 150 ng/mL (375 nmol/L), where excess calcium absorption leads to hypercalcemia: nausea, vomiting, thirst, frequent urination, confusion and, in severe cases, kidney damage and abnormal heart rhythms. The NAM's tolerable upper intake level for adults is 4,000 IU (100 µg) per day from all sources. Very large intermittent doses – for example a single annual dose of 500,000 IU – have been associated with more falls and fractures in older adults, which is one reason daily or weekly dosing is preferred.

Supplement units: IU and micrograms

The test result unit (ng/mL or nmol/L) should not be confused with the dose unit on a supplement label. Doses are given in international units (IU) or micrograms (µg or mcg):

• 1 µg of vitamin D = 40 IU
• 400 IU = 10 µg; 1,000 IU = 25 µg; 2,000 IU = 50 µg; 4,000 IU = 100 µg

As a very rough rule, each additional 100 IU per day raises the blood level by about 0.7–1 ng/mL (around 2 nmol/L) in an adult after a few months, but the response varies widely with starting level, body weight and absorption. A person starting at a low level rises more per unit dose than one already near the adequate range.

Recommended intake by age

The NAM's recommended dietary allowances, designed to keep almost everyone at 20 ng/mL (50 nmol/L) or above with minimal sun exposure, are:

The UK advises 10 µg (400 IU) daily for everyone over one year during autumn and winter, and all year round for people who get little sun on their skin.

What affects vitamin D results

• Season and latitude. Levels are typically highest in late summer and lowest in late winter. North of about 37° latitude (or south of 37°S), the sun is too weak in winter for meaningful skin production. A level measured in September and one measured in March in the same person can differ by 10 ng/mL (25 nmol/L) or more.
• Skin pigmentation. Darker skin needs more sun exposure to make the same amount of vitamin D.
• Age. Older skin makes vitamin D less efficiently.
• Body weight. Vitamin D is stored in fat tissue, and people with obesity tend to have lower blood levels and a smaller rise from a given dose.
• Clothing and sunscreen, and time spent indoors.
• Malabsorption – coeliac disease, inflammatory bowel disease, cystic fibrosis, pancreatic insufficiency and weight-loss surgery all reduce absorption.
• Medicines – some anticonvulsants (phenytoin, carbamazepine, phenobarbital), rifampicin, glucocorticoids and some HIV medicines speed up vitamin D breakdown. Orlistat and cholestyramine reduce absorption.
• Liver and kidney disease, which affect the conversion steps.
• The assay itself. Immunoassays and liquid chromatography–tandem mass spectrometry (LC-MS/MS) can disagree, especially for vitamin D2. Standardisation programmes have narrowed the gap, but switching laboratories can still shift a result by several ng/mL.

How long to wait before retesting

Because 25(OH)D has a half-life of two to three weeks, it takes about three months on a steady dose for the level to reach a new plateau. Testing sooner usually underestimates where the level will end up. Testing in the same season each year makes comparisons fairer.

Vitamin D in infants, children and pregnancy

Breast milk contains little vitamin D, so health authorities in the US, UK and many other countries recommend a daily supplement of 400 IU (10 µg) for breastfed infants from shortly after birth. Formula-fed infants taking about a liter of fortified formula a day usually get enough from formula alone. In children, the same 20 ng/mL (50 nmol/L) threshold is generally used for adequacy, and nutritional rickets is most often seen when levels fall below about 12 ng/mL (30 nmol/L) together with low calcium intake.

During pregnancy, the recommended intake does not change (600 IU in the US, 400 IU in the UK). Routine testing of all pregnant women is not recommended by most guidelines, but women at higher risk – for example those with darker skin, limited sun exposure or obesity – may be offered testing or advised to take a supplement.

Who should consider testing

Major guidelines do not recommend routine vitamin D screening in healthy adults. Testing is generally reserved for people with symptoms or conditions that suggest deficiency or where vitamin D matters directly:

• osteoporosis, low-trauma fractures or unexplained bone pain
• malabsorption conditions or previous weight-loss surgery
• chronic kidney or liver disease
• long-term use of medicines that affect vitamin D metabolism
• high calcium or parathyroid disorders
• very limited sun exposure combined with symptoms

Comparing results across countries

Suppose a person living in the UK was told their level of 42 nmol/L was "insufficient", then moved to the US and was told a level of 24 ng/mL was "normal". Converted, 42 nmol/L is 16.8 ng/mL and 24 ng/mL is 60 nmol/L, so the level really did rise – by about 40% – and both labels are consistent with the thresholds in use. Converting all results into one unit before comparing is the simplest way to avoid confusion.

When to see a doctor

See a doctor if your level is below 12 ng/mL (30 nmol/L), especially with bone pain, muscle weakness or a history of fractures, as treatment doses may be higher than standard supplements. Also seek advice if your level is above 100 ng/mL (250 nmol/L), or if you take high-dose vitamin D and develop nausea, excessive thirst, frequent urination or confusion, which can be signs of high calcium. Do not start high-dose supplements on your own to correct a low result without guidance, particularly if you have kidney disease, kidney stones or sarcoidosis.