Phosphate mg/dL to mmol/L Converter

Phosphate in two units: the short answer

To convert serum phosphate (phosphorus) from mg/dL to mmol/L, multiply by 0.3229 (or divide by 3.097). To convert mmol/L back to mg/dL, multiply by 3.097. A phosphate of 3.5 mg/dL is therefore 1.13 mmol/L, and 1.45 mmol/L is 4.5 mg/dL. For quick estimates, divide mg/dL by 3 or multiply mmol/L by 3. The converter above handles both directions and shows the typical adult range.

Phosphate is a test where the name itself causes confusion: US reports usually say "phosphorus", while UK and European reports say "phosphate". They refer to the same measurement. The conversion factor is based on the phosphorus atom, not the whole phosphate ion, which is why it differs from what a chemistry textbook might suggest. This guide explains the conversion, normal ranges by age, the role of phosphate in kidney disease and nutrition, and the causes and effects of high and low levels.

What phosphate does

Phosphorus is the second most abundant mineral in the body after calcium. About 85% is in bones and teeth as calcium phosphate (hydroxyapatite), about 14% is inside cells, and only about 1% is in the extracellular fluid, including the blood. Phosphate:

• forms the mineral structure of bone together with calcium
• is part of ATP, the energy currency of every cell
• is part of DNA, RNA and cell membranes (phospholipids)
• helps buffer the blood's acidity
• is involved in oxygen delivery through 2,3-diphosphoglycerate in red blood cells

Phosphate balance is regulated by the kidneys, under the control of parathyroid hormone (PTH), which increases phosphate excretion; fibroblast growth factor 23 (FGF23), a bone-derived hormone that also increases excretion; and active vitamin D (calcitriol), which increases gut absorption.

Phosphorus or phosphate? mg/dL and mmol/L

In the blood, phosphate exists mainly as the ions HPO₄²⁻ and H₂PO₄⁻. Laboratories measure the phosphorus contained in these ions:

• US reports typically say "phosphorus" or "inorganic phosphorus" and use mg/dL, counting only the weight of the phosphorus atoms.
• UK, European and Australian reports say "phosphate" and use mmol/L. One mole of phosphate contains one mole of phosphorus, so the molar value is the same whichever name is used.

Why the factor is 0.3229

Phosphorus has an atomic mass of 30.97 g/mol. A concentration of 1 mg/dL is 10 mg per liter, which is 10 ÷ 30.97 = 0.3229 mmol/L. Conversely, 1 mmol/L = 3.097 mg/dL. If you used the mass of the entire phosphate ion (about 95 g/mol), you would get the wrong answer; laboratories do not report phosphate that way.

How to convert phosphate step by step

1. Check the unit; "phosphorus" in mg/dL and "phosphate" in mmol/L describe the same test.
2. For mg/dL to mmol/L, multiply by 0.3229.
3. For mmol/L to mg/dL, multiply by 3.097.
4. Round mmol/L to two decimal places and mg/dL to one decimal place.

Worked examples

• 1.0 mg/dL × 0.3229 = 0.32 mmol/L – severe hypophosphataemia.
• 2.5 mg/dL × 0.3229 = 0.81 mmol/L – the lower end of a typical adult range.
• 3.5 mg/dL × 0.3229 = 1.13 mmol/L.
• 4.5 mg/dL × 0.3229 = 1.45 mmol/L – the upper end of a typical adult range.
• 5.5 mg/dL × 0.3229 = 1.78 mmol/L – high, as often seen in advanced kidney disease.
• 0.65 mmol/L × 3.097 = 2.0 mg/dL.
• 1.30 mmol/L × 3.097 = 4.0 mg/dL.
• 2.0 mmol/L × 3.097 = 6.2 mg/dL.

Phosphate conversion chart

Normal phosphate ranges

A typical adult range is about 2.5–4.5 mg/dL (0.81–1.45 mmol/L). Many UK laboratories use about 0.80–1.50 mmol/L (2.5–4.6 mg/dL).

Children

Children have considerably higher phosphate than adults because growing bones need it. Typical upper limits are about 6.5–7.0 mg/dL (2.1–2.3 mmol/L) in infants and around 5.5–6.0 mg/dL (1.8–1.9 mmol/L) in young children, falling to adult values after puberty. A level that would be high in an adult may be normal in a child, and a level "normal" for an adult may be low in a child, which matters for conditions like rickets. Always use age-specific ranges.

Daily variation

Phosphate follows a daily rhythm, typically lowest in the late morning and highest in the late afternoon and night, with differences of up to about 0.6–1.0 mg/dL (0.2–0.3 mmol/L). Meals, especially carbohydrate, lower phosphate temporarily as glucose and phosphate move into cells. For accurate comparison, fasting morning samples are often preferred.

Hyperphosphataemia: high phosphate

Causes

• Chronic kidney disease – by far the most common cause. As kidney function falls below an eGFR of about 30, the kidneys cannot excrete enough phosphate, and levels rise despite high PTH and FGF23.
• Acute kidney injury.
• Tumour lysis syndrome and rhabdomyolysis – release of phosphate from large numbers of damaged cells.
• Hypoparathyroidism – reduced PTH means less phosphate excretion.
• Excess intake – phosphate-containing enemas or bowel preparations (which can cause acute phosphate nephropathy), excessive vitamin D.
• Diabetic ketoacidosis before treatment (phosphate shifts out of cells, although total body stores are low).
• Pseudohyperphosphataemia – haemolysis, high paraproteins, very high lipids or bilirubin can interfere with some assays and cause falsely high results.

Effects

High phosphate itself causes few direct symptoms, but it binds calcium, which can lower blood calcium and cause tingling or cramps. In chronic kidney disease, persistently high phosphate contributes to vascular calcification, bone disease and higher cardiovascular risk – part of CKD–mineral and bone disorder (CKD-MBD).

Management in kidney disease

KDIGO guidance recommends lowering elevated phosphate towards the normal range in people with CKD, using a combination of diet, phosphate binders taken with meals, and adequate dialysis in those on dialysis. Dietary advice often focuses on reducing phosphate additives in processed foods, cola drinks and fast foods, which are absorbed almost completely, compared with about 40–60% from natural plant and animal sources. Phosphate binders include calcium-based binders (calcium carbonate, calcium acetate), sevelamer, lanthanum and iron-based binders; KDIGO suggests restricting calcium-based binders in adults.

Hypophosphataemia: low phosphate

Low phosphate is graded as:

• Mild: about 2.0–2.5 mg/dL (0.65–0.81 mmol/L)
• Moderate: about 1.0–2.0 mg/dL (0.32–0.65 mmol/L)
• Severe: below about 1.0 mg/dL (0.32 mmol/L)

Causes

• Refeeding syndrome – a potentially dangerous shift of phosphate into cells when nutrition is restarted after starvation, alcohol misuse or prolonged illness. NICE guidance on nutrition support identifies people at high risk and recommends monitoring phosphate, potassium and magnesium daily during early refeeding.
• Alcohol use disorder.
• Treatment of diabetic ketoacidosis – insulin drives phosphate into cells.
• Respiratory alkalosis – for example from hyperventilation or sepsis.
• Vitamin D deficiency and primary hyperparathyroidism – increased urinary loss.
• Medicines – phosphate binders, aluminium- or magnesium-containing antacids, some intravenous iron preparations (especially ferric carboxymaltose, which raises FGF23), tenofovir and others.
• Inherited disorders – such as X-linked hypophosphataemia, a cause of rickets.

Effects

Severe hypophosphataemia reduces ATP production and can cause muscle weakness (including the breathing muscles), rhabdomyolysis, heart failure, confusion, seizures and haemolysis. Long-term low phosphate causes soft bones (osteomalacia in adults, rickets in children) and bone pain.

Refeeding syndrome in more detail

Refeeding syndrome deserves special mention because low phosphate is its hallmark and because it is preventable. During starvation, the body shifts to burning fat and protein, and total body phosphate, potassium and magnesium are depleted even if blood levels look normal. When food – especially carbohydrate – is reintroduced, insulin rises and drives phosphate, potassium and magnesium into cells to support new energy production. Blood phosphate can then fall sharply within two to four days, sometimes below 1.0 mg/dL (0.32 mmol/L).

People at high risk include those who have eaten little or nothing for more than five to ten days, people with a very low body mass index, recent unintentional weight loss, alcohol use disorder, anorexia nervosa, or those receiving certain chemotherapy. NICE recommends starting nutrition slowly in high-risk people, giving thiamine and other vitamins before and during refeeding, and monitoring phosphate, potassium and magnesium daily during the first days, correcting deficits as they appear.

Intravenous iron and low phosphate

Some intravenous iron preparations, notably ferric carboxymaltose, increase the active form of FGF23, which makes the kidneys excrete phosphate. In randomised trials, ferric carboxymaltose caused low phosphate in a large proportion of patients – far more often than iron derisomaltose (iron isomaltoside). The fall is usually temporary but can persist for weeks to months, and repeated doses have been linked to osteomalacia and fractures. Regulators have added warnings to product information, and many clinicians check phosphate in people receiving repeated infusions or who have symptoms such as bone pain or weakness.

Phosphate in chronic kidney disease by stage

Phosphate typically stays normal until kidney function is quite reduced, because PTH and FGF23 rise to increase excretion. As a general pattern:

In people on dialysis, phosphate is usually checked monthly. Older targets for dialysis patients were often quoted as 3.5–5.5 mg/dL (1.13–1.78 mmol/L); current KDIGO guidance instead suggests lowering elevated phosphate towards the normal range without a fixed numerical target.

The calcium–phosphate product

The calcium–phosphate product (calcium × phosphate) was historically used to estimate the risk of calcium phosphate deposits in tissues, with a threshold of about 55 mg²/dL² (about 4.4 mmol²/L²). Note that the number depends on units: multiplying mg/dL values gives a very different number from multiplying mmol/L values. KDIGO no longer recommends using the product, preferring to assess calcium and phosphate separately, but it still appears in some reports and older references.

Phosphate intake

The US recommended dietary allowance for phosphorus is 700 mg per day for adults and 1,250 mg per day for ages 9–18. Most people in Western diets consume well above this, partly because of phosphate additives in processed foods. Deficiency from diet alone is rare outside starvation, alcohol misuse or certain medical conditions. For people with advanced kidney disease, dietitians often recommend limiting processed foods and choosing fresh foods, as well as checking ingredient lists for words containing "phos".

What affects phosphate results

• Meals – carbohydrate lowers phosphate temporarily; a fasting sample is more reliable.
• Time of day – lowest in the late morning, higher later in the day.
• Haemolysis – red cells release phosphate, causing falsely high results.
• Delayed processing – phosphate can leak from cells if the sample is left unseparated.
• Medicines – antacids, phosphate binders, intravenous iron, diuretics, bisphosphonates and others.
• Intravenous glucose or insulin shift phosphate into cells.

Comparing phosphate results across laboratories

Because phosphate varies with meals and time of day, differences between two results are often due to sampling conditions rather than a real change. For example, a person with kidney disease might have 5.0 mg/dL from a non-fasting afternoon sample in one clinic and 1.40 mmol/L from a fasting morning sample at another. Converted, these are 1.61 mmol/L and 4.3 mg/dL. Part of the difference is probably timing. Clinicians tracking phosphate in kidney disease therefore prefer consistent sampling conditions – for example, before a dialysis session at the start of the week – and look at trends over several months.

Phosphate and vitamin D

Active vitamin D increases phosphate absorption from the gut as well as calcium absorption. Vitamin D deficiency can therefore cause low phosphate, especially as PTH rises in response and increases phosphate losses in urine. Conversely, treatment with active vitamin D analogues (calcitriol, alfacalcidol, paricalcitol) in kidney disease can raise phosphate, which is one reason phosphate is monitored when these medicines are used.

When to see a doctor

See a doctor if your phosphate is outside the reference range on repeat testing, if you have chronic kidney disease and your phosphate is rising, or if you have bone pain, muscle weakness or unexplained fractures. Seek urgent care for severe muscle weakness, breathing difficulty or confusion, particularly in someone who is being re-fed after a period of poor nutrition or recovering from diabetic ketoacidosis.