Folate ng/mL to nmol/L Converter

Folate in two units: the short answer

To convert serum folate from ng/mL to nmol/L, multiply by 2.266. To convert nmol/L back to ng/mL, divide by 2.266 (or multiply by 0.441). A folate of 4 ng/mL is therefore about 9 nmol/L, and 20 nmol/L is about 8.8 ng/mL. Because ng/mL equals µg/L, a result in µg/L can be read exactly like one in ng/mL. The converter above handles both directions and compares your result with the thresholds the World Health Organization uses.

Folate is usually tested together with vitamin B12, because deficiency of either causes the same type of anaemia. As with B12, thresholds look very different in the two units – "7" may be normal in ng/mL and low in nmol/L – and there are two quite different folate tests, serum and red cell, with very different numbers. This guide explains both tests, the conversion, the thresholds, the causes of deficiency and the role of folic acid in pregnancy.

What folate is

Folate (vitamin B9) is a family of compounds needed to make DNA and RNA and to process amino acids. It is essential for rapidly dividing cells, particularly in the bone marrow, which is why deficiency causes megaloblastic anaemia – large, immature red blood cells – and why it matters so much in early pregnancy, when the baby's neural tube forms.

Folate is the form found naturally in foods such as leafy green vegetables, legumes, citrus fruit and liver. Folic acid is the synthetic form used in supplements and fortified foods; it is more stable and better absorbed. More than 60 countries, including the US and Canada, add folic acid to flour or other staple foods to prevent neural tube defects, and the UK has legislated to fortify non-wholemeal wheat flour.

The body stores only enough folate for roughly three to four months, so deficiency can develop much faster than B12 deficiency.

Serum folate and red cell folate

There are two blood tests, and their numbers are not interchangeable:

• Serum (or plasma) folate reflects recent intake over the past few days to weeks. A single folate-rich meal or supplement can raise it, and a few days of poor intake can lower it. It is the most commonly ordered test.
• Red blood cell (RBC) folate reflects folate incorporated into red cells as they were made, so it represents average status over the previous two to three months. Its values are much higher – typically hundreds of ng/mL or nmol/L – and its thresholds differ.

This converter and its ranges apply to serum folate. The same conversion factor works for red cell folate, but red cell results must be compared with red cell thresholds.

ng/mL and nmol/L explained

• ng/mL (nanograms per milliliter) – common in the United States; identical to µg/L.
• nmol/L (nanomoles per liter) – the SI unit used in many other countries.

Why the factor is 2.266

Folic acid (pteroylglutamic acid) has a molar mass of 441.4 g/mol. A concentration of 1 ng/mL is 1,000 ng per liter, which is 1,000 ÷ 441.4 = 2.266 nmol/L. Conversely, 1 nmol/L = 0.441 ng/mL.

How to convert folate step by step

1. Check the unit and whether the test is serum or red cell folate.
2. For ng/mL (or µg/L) to nmol/L, multiply by 2.266.
3. For nmol/L to ng/mL, divide by 2.266.
4. Round to one decimal place; folate assays have substantial variation.

Worked examples

• 3 ng/mL × 2.266 = 6.8 nmol/L – deficient by WHO criteria.
• 4 ng/mL × 2.266 = 9.1 nmol/L – close to the WHO deficiency cut-off of 10 nmol/L.
• 6 ng/mL × 2.266 = 13.6 nmol/L – the lower end of the adequate range.
• 15 ng/mL × 2.266 = 34 nmol/L.
• 10 nmol/L ÷ 2.266 = 4.4 ng/mL.
• 25 nmol/L ÷ 2.266 = 11.0 ng/mL.
• RBC folate 400 ng/mL × 2.266 = 906 nmol/L – the WHO population threshold for preventing neural tube defects.

Mental shortcut

Double the ng/mL value and add a little over a tenth: for 8 ng/mL, 8 × 2 = 16, plus about 2 = 18 nmol/L (exact: 18.1).

Folate conversion chart

Folate levels: what the numbers mean

WHO thresholds for serum folate

The World Health Organization's thresholds, based on rising homocysteine (a marker of folate shortage at cell level), are:

• Below 4 ng/mL (10 nmol/L) – folate deficiency.
• 4–6 ng/mL (10–13.4 nmol/L) – possible deficiency or low status.
• 6–20 ng/mL (13.5–45 nmol/L) – adequate.
• Above 20 ng/mL (45 nmol/L) – elevated, usually due to supplements or fortification.

Many laboratories use a lower limit of about 3–4 ng/mL (7–10 nmol/L) in their reference ranges. Ranges vary with the method used, and the British Society for Haematology notes that there is no universally agreed cut-off; clinical features and other tests help decide.

WHO thresholds for red cell folate

• Below 100 ng/mL (226 nmol/L) – deficiency, based on the risk of megaloblastic anaemia.
• Above 400 ng/mL (906 nmol/L) – the population-level concentration in women of reproductive age associated with the greatest reduction in neural tube defects.

Causes of folate deficiency

• Poor diet – few fruits and vegetables; common with heavy alcohol use, poverty, restrictive diets and in older adults living alone.
• Alcohol – reduces absorption, increases excretion and is often associated with poor diet.
• Increased requirement – pregnancy, breastfeeding, haemolytic anaemias, severe skin disease, dialysis and some cancers.
• Malabsorption – coeliac disease, tropical sprue, inflammatory bowel disease, surgery on the small intestine.
• Medicines – methotrexate, trimethoprim, sulfasalazine, some anticonvulsants (phenytoin, phenobarbital, primidone) and others interfere with folate absorption or use.
• Cooking – prolonged boiling destroys a large share of food folate.

Symptoms of folate deficiency

Symptoms mainly arise from anaemia: tiredness, weakness, breathlessness, pale skin, palpitations and headaches. A sore tongue, mouth ulcers and changes in taste are also common. Unlike B12 deficiency, folate deficiency does not usually cause nerve damage, although it has been linked to mood changes. Because the anaemia looks the same in both conditions, B12 should always be tested when folate deficiency is suspected.

Folic acid in pregnancy

Taking folic acid before conception and in early pregnancy reduces the risk of neural tube defects such as spina bifida and anencephaly by up to about 70%. Common recommendations are:

The neural tube closes around 28 days after conception, often before a person knows they are pregnant, which is why supplementation is recommended before trying to conceive. Blood folate testing is not required to take these supplements.

Folate and vitamin B12 together

Folate and B12 work together in the same metabolic pathway, so deficiency of either can cause the same blood picture. Two practical rules follow:

1. Always check B12 when treating folate deficiency. High doses of folic acid can correct the anaemia of B12 deficiency while allowing nerve damage to progress.
2. Low B12 can make folate appear normal or high in serum, because folate becomes trapped in an unusable form (the "methyl-folate trap").

Homocysteine rises in both deficiencies, while methylmalonic acid (MMA) rises only in B12 deficiency, which helps tell them apart.

Reading B12 and folate results together

Because the two tests are usually ordered together, it helps to know the common combinations and what they suggest. The table uses serum folate and total B12 in both units.

These are general patterns only; a clinician will interpret them with the full blood count, symptoms and medical history.

Treatment and follow-up

Folate deficiency is usually treated with folic acid 5 mg daily for about four months, according to UK guidance, alongside treating the cause. In the US, doses of 1–5 mg daily are commonly used. People with ongoing risk – for example chronic haemolytic anaemia or long-term methotrexate – may need long-term lower-dose supplementation. Blood counts typically begin to improve within a week or two, and the anaemia usually resolves within about two months. Repeat serum folate testing is seldom needed during treatment, because levels rise quickly once supplements are started; response is judged by blood counts and symptoms.

Food sources of folate

Good dietary sources include:

• dark green leafy vegetables such as spinach, kale and spring greens
• legumes – lentils, chickpeas, black-eyed peas and beans
• asparagus, Brussels sprouts and broccoli
• citrus fruits and juices
• fortified breakfast cereals and, where fortification applies, bread and flour products
• liver (high in folate, but not recommended in pregnancy because of its vitamin A content)

Steaming or microwaving vegetables, rather than boiling them for long periods, preserves more folate. The US recommended dietary allowance for adults is 400 µg dietary folate equivalents (DFE) per day, rising to 600 µg DFE in pregnancy.

Folate, homocysteine and the MTHFR gene

Folate is needed to convert homocysteine into methionine, so low folate raises homocysteine. A common variant of the MTHFR gene (C677T) reduces the activity of one enzyme in this pathway; people with two copies tend to have slightly higher homocysteine, particularly when folate intake is low. MTHFR testing is widely offered commercially, but major bodies – including the American College of Medical Genetics and Genomics – advise against routine MTHFR testing, because the variant is very common and has not been shown to change outcomes when folate intake is adequate. Standard folic acid supplements are effective for people with the variant, and special "methylfolate" products are not required for neural tube defect prevention.

Comparing folate results across countries

Because laboratories use different assays and different units, an apparent change in folate may simply reflect a change of laboratory. For example, a serum folate of 12 nmol/L in one country and 6.5 ng/mL in another look very different, but 6.5 ng/mL is 14.7 nmol/L – a modest change at most. Serum folate also fluctuates with recent meals, so small differences between tests are rarely meaningful. A persistently low value on repeat testing is more significant than any single result.

High folate

High serum folate is usually due to supplements and fortified foods, and folate itself is not toxic. The US tolerable upper intake level of 1,000 µg per day of folic acid exists mainly because high intakes can mask B12 deficiency. Very high serum folate in someone not taking supplements can occasionally point to B12 deficiency, as explained above.

What affects folate results

• Recent meals and supplements raise serum folate quickly; ideally test fasting, or at least avoid supplements on the morning of the test.
• Haemolysis – red cells contain much more folate than serum, so a haemolysed sample gives a falsely high serum result.
• Alcohol lowers folate.
• Medicines such as methotrexate and some anticonvulsants lower folate.
• Laboratory method – different immunoassays can differ by 20% or more, especially at low levels.
• Recent blood transfusion affects red cell folate.

Who should consider a folate test

Testing is usually reserved for people with features that suggest deficiency or with risk factors, including:

• anaemia with large red cells (raised MCV) on a full blood count
• unexplained tiredness with a sore tongue or mouth ulcers
• heavy alcohol use or a very limited diet
• coeliac disease, inflammatory bowel disease or previous bowel surgery
• long-term methotrexate, anticonvulsants or other folate-affecting medicines
• haemolytic anaemias, such as sickle cell disease

When to see a doctor

See a doctor if your serum folate is below 4 ng/mL (10 nmol/L), particularly with tiredness, breathlessness, a sore tongue or a low haemoglobin, or if you are pregnant or planning pregnancy and have risk factors for deficiency. A doctor will check B12 at the same time and look for an underlying cause. Seek advice before taking high-dose folic acid if you have not had your B12 measured, especially if you have tingling, numbness or balance problems.