Vitamin D (25-OH) Unit Converter
Vitamin D (25-OH) Unit Converter
Convert total 25-hydroxyvitamin D between ng/mL, nmol/L and µg/L, with the deficiency tiers and an honest account of the contested sufficiency threshold.
Vitamin D (25-OH) converter
Mass ⇄ molarTotal 25-hydroxyvitamin D 28 ng/mL
Formula and conversion factors
ng/mL = nmol/L ÷ 2.49601
µg/L = ng/mL (numerically identical)
- 2.49601
- derived from the molecular mass of 25-hydroxyvitamin D3, 400.64 Da
- µg/L
- numerically identical to ng/mL; one microgram per litre is one nanogram per millilitre
- D3 by convention
- assays report total 25-OH-D — D2 plus D3 — against the D3 factor, although 25-OH-D2 is 412.65 Da, a difference of about 3% applied only to the D2 fraction
- total, not free
- this is total 25-OH-D, most of it bound to vitamin D binding protein; free 25-OH-D is a separate measurement and is not derived from this number
Worked example
Total 25-hydroxyvitamin D 28 ng/mL
28 × 2.49601 = 70 nmol/L
= 28.0 µg/L — ng/mL and µg/L are the same number
Within the Institute of Medicine sufficiency range of 50–125 nmol/L, but below the 75 nmol/L threshold the Endocrine Society uses
Interpretive tiers in both units
| 25-OH-D | ng/mL | nmol/L | What it means |
|---|---|---|---|
| Deficiency | < 12 | < 30 | Risk of rickets in children and osteomalacia in adults; treatment indicated |
| Insufficiency | 12 – 20 | 30 – 50 | Inadequate for bone health in most people |
| Sufficiency (Institute of Medicine) | 20 – 50 | 50 – 125 | Adequate for essentially the whole population |
| Sufficiency (Endocrine Society) | 30 – 50 | 75 – 125 | A higher threshold, argued for patients at risk rather than for population screening |
| Potential harm | > 50 | > 125 | No added benefit demonstrated; adverse effects reported above this |
What the number does and does not tell you
| Question | Answer |
|---|---|
| Is this deficiency? | Below 30 nmol/L, yes, and it should be treated. Between 30 and 50 nmol/L it is insufficiency, and above 50 nmol/L the answer depends on which guideline you follow |
| Does it include D2 and D3? | Yes, a total 25-OH-D reports both. The molar factor is derived from D3, so the small mass difference of D2 introduces about a 3% error applied only to the D2 fraction |
| Is it the active hormone? | No. 1,25-dihydroxyvitamin D is the active metabolite, but it has a short half-life and is often normal or high in deficiency, so it is not the test for vitamin D status |
| Should everyone be tested? | No. Population screening is not recommended; testing is for people with symptoms, malabsorption, chronic kidney disease, osteoporosis or a clinical reason to suspect deficiency |
An honest account of a contested threshold
Total 25-hydroxyvitamin D is reported in ng/mL in the United States and in nmol/L elsewhere, with a factor of 2.49601 between them. A level of 28 ng/mL is 70 nmol/L. The third unit that appears, µg/L, is numerically identical to ng/mL. The factor is derived from the molecular mass of 25-hydroxyvitamin D3, 400.64 daltons, even though the assay reports D2 as well; 25-OH-D2 is 412.65 daltons, so applying the D3 factor introduces an error of about three per cent, and only in the D2 fraction. That convention is universal and worth knowing about rather than worrying about.
The interpretive tiers are reasonably settled at the bottom. Below 30 nmol/L, roughly 12 ng/mL, is deficiency, carrying a real risk of rickets in children and osteomalacia in adults, and it should be treated. Between 30 and 50 nmol/L is insufficiency: inadequate for bone health in most people. At the top, above 125 nmol/L there is no demonstrated additional benefit and evidence of potential harm, with frank toxicity and hypercalcaemia well above that, invariably from supplements rather than from sunlight.
The sufficiency threshold itself is genuinely contested, and pages that present one figure as settled are misleading their readers. The US Institute of Medicine, reviewing the evidence for population requirements, concluded that 50 nmol/L covers the needs of essentially everyone. The Endocrine Society, writing for patients at risk rather than for the population, recommended a threshold of 75 nmol/L. Both positions are defensible and neither has been retired, so a result of 60 nmol/L is sufficient by one standard and insufficient by the other.
Two practical points follow. Assay standardisation has improved since the Vitamin D Standardization Program, but methods still differ, and immunoassays recover 25-OH-D2 less reliably than mass spectrometry does — which matters in anyone taking ergocalciferol. And this test is 25-OH-D, not the active hormone: 1,25-dihydroxyvitamin D has a short half-life and is often normal or even raised in deficiency, so measuring it to assess vitamin D status gives a falsely reassuring answer.
Frequently asked questions
How do I convert vitamin D from ng/mL to nmol/L?
Multiply by 2.49601, a factor derived from the molecular mass of 25-hydroxyvitamin D3, 400.64 Da. A level of 28 ng/mL is 70 nmol/L. To go the other way, divide the nmol/L figure by 2.49601.
What counts as vitamin D deficiency?
Below 30 nmol/L, about 12 ng/mL, is deficiency and carries a real risk of rickets in children and osteomalacia in adults. Between 30 and 50 nmol/L is insufficiency, inadequate for bone health in most people. Above 125 nmol/L there is no added benefit and some evidence of harm.
Is sufficiency 50 or 75 nmol/L?
That is a live disagreement rather than a settled figure. The US Institute of Medicine set 50 nmol/L as sufficient for essentially the whole population, while the Endocrine Society recommended 75 nmol/L for patients at risk. A result of 60 nmol/L is therefore sufficient by one standard and insufficient by the other, and the laboratory’s own report will follow whichever its guideline uses.
Does the conversion factor change for vitamin D2?
Strictly yes, but in practice no. The factor of 2.49601 comes from 25-OH-D3 at 400.64 Da, while 25-OH-D2 is 412.65 Da, a difference of about 3%. Assays report a combined total against the D3 factor by convention, so the small error applies only to the D2 portion of the result.
Should 1,25-dihydroxyvitamin D be measured instead?
No, not for assessing vitamin D status. The active hormone has a short half-life and is often normal or even raised in deficiency because rising PTH stimulates its production, so it gives a falsely reassuring answer. It has a place in suspected hereditary rickets, granulomatous disease and some hypercalcaemias.
Related calculators
References
- Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington DC: National Academies Press; 2011.
- Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911–1930.
- Sempos CT, Heijboer AC, Bikle DD, et al. Vitamin D assays and the definition of hypovitaminosis D: results from the First International Conference on Controversies in Vitamin D. Br J Clin Pharmacol. 2018;84(10):2194–2207.
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.