Serum Osmolality Calculator

Serum osmolality in brief

Osmolality measures the concentration of all dissolved particles in a kilogram of water. In the blood, it is determined mainly by sodium (with its accompanying anions), glucose and urea. The body keeps serum osmolality within a tight range – about 275–295 mOsm/kg – because changes cause water to move into or out of cells, including brain cells.

The calculator above estimates osmolality with the standard formula in US units:

Calculated osmolality = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8

(sodium in mmol/L, glucose and BUN in mg/dL). In SI units, the formula is simpler:

Calculated osmolality = 2 × Na + glucose + urea (all in mmol/L)

If you also enter a measured osmolality from the laboratory, the calculator shows the osmolal gap – the difference between measured and calculated values. A gap above about 10 mOsm/kg suggests that unmeasured substances, such as alcohol or toxic alcohols, are present.

Worked examples

Osmolal gap example: calculated osmolality 290, measured osmolality 320 mOsm/kg. The gap is 320 − 290 = 30 mOsm/kg, which is raised and needs explanation – most commonly by alcohol (ethanol), but toxic alcohols must be considered.

Step by step in SI units

For sodium 140 mmol/L, glucose 5.0 mmol/L and urea 5.0 mmol/L: 2 × 140 = 280; 280 + 5.0 + 5.0 = 290 mOsm/kg.

Why the formula doubles sodium

Each sodium ion in the blood is accompanied by a negatively charged ion (mostly chloride and bicarbonate). Doubling sodium accounts for both. Glucose and urea do not dissociate, so each molecule counts once. The divisors 18 and 2.8 convert mg/dL to mmol/L for glucose (molecular weight 180) and BUN (28 for the two nitrogen atoms in urea).

How the body controls osmolality

Osmolality is regulated by a feedback system centred in the hypothalamus. Specialised cells called osmoreceptors detect changes as small as 1–2%:

• When osmolality rises, the brain triggers thirst and releases antidiuretic hormone (ADH, or vasopressin) from the pituitary gland. ADH makes the kidneys reabsorb water, producing small volumes of concentrated urine.
• When osmolality falls, thirst is suppressed and ADH release stops, so the kidneys excrete large volumes of dilute urine.

Healthy kidneys can produce urine as dilute as about 50 mOsm/kg or as concentrated as about 1,200 mOsm/kg, which gives the body great flexibility. Problems arise when this system fails – for example, when ADH is released inappropriately (SIADH), when ADH is missing or ineffective (diabetes insipidus), when thirst is impaired (common in older people), or when access to water is limited (infants, people with dementia or disability).

Osmolality and osmolarity

The terms are often used interchangeably, but they differ slightly:

• Osmolality (mOsm/kg of water) is what laboratories measure, using freezing-point depression.
• Osmolarity (mOsm/L of solution) is what formulas technically calculate.

In blood, the difference is small, and the calculated value is conventionally compared directly with measured osmolality.

Effective osmolality (tonicity)

Not all particles affect water movement. Urea crosses cell membranes freely, so it raises osmolality but does not draw water out of cells. Effective osmolality, or tonicity, excludes urea:

Effective osmolality = 2 × Na + glucose (mmol/L) or 2 × Na + glucose ÷ 18 (US units)

Tonicity is what determines whether cells swell or shrink. A person with kidney failure may have a high osmolality because of a high urea, but normal tonicity. In hyperglycaemic emergencies, effective osmolality is the key measure: the 2024 international consensus on hyperglycaemic crises defines hyperosmolar hyperglycaemic state using an effective osmolality above 300 mOsm/kg (or total above 320 mOsm/kg).

Interpreting your result

Low osmolality (below 275 mOsm/kg)

Low osmolality almost always means hypotonic hyponatraemia – too much water relative to sodium. Water moves into cells, which can cause brain swelling if the fall is rapid. Causes include:

• the syndrome of inappropriate antidiuretic hormone (SIADH), caused by lung disease, brain conditions, some cancers, pain, nausea and medicines such as SSRIs, carbamazepine and some chemotherapy drugs
• thiazide diuretics
• heart failure, cirrhosis and nephrotic syndrome
• kidney failure
• excessive water drinking, for example in endurance athletes or people with primary polydipsia
• adrenal insufficiency or severe hypothyroidism

High osmolality (above 295 mOsm/kg)

High osmolality means the blood is concentrated. Causes include:

• dehydration and hypernatraemia, from inadequate water intake or excessive losses
• very high glucose, as in DKA or HHS
• high urea, in kidney failure or after gastrointestinal bleeding
• diabetes insipidus (now also called arginine vasopressin deficiency or resistance), in which the body cannot conserve water
• alcohols and other unmeasured substances (which also raise the osmolal gap)
• mannitol given as a treatment

The osmolal gap

Osmolal gap = measured osmolality − calculated osmolality

A normal gap is usually below about 10 mOsm/kg, although published normal ranges vary and individual values can be negative. A raised gap means that particles not included in the formula are present.

Causes of a raised osmolal gap

Accounting for ethanol

If blood ethanol is known, it can be added to the formula as ethanol (mg/dL) ÷ 3.7 – some sources use 4.6 – or, in SI units, as ethanol in mmol/L. If the gap remains raised after accounting for ethanol, toxic alcohols must be considered.

Why the osmolal gap matters in poisoning

Methanol and ethylene glycol are not toxic themselves; their metabolites cause severe acidosis, blindness (methanol) and kidney failure (ethylene glycol). Early after ingestion, the parent alcohol raises the osmolal gap while the anion gap is still normal. Later, as it is metabolised, the osmolal gap falls and the anion gap rises. A normal osmolal gap therefore does not rule out toxic alcohol poisoning, especially if the ingestion happened many hours earlier. Treatment involves fomepizole or ethanol to block metabolism and, often, dialysis. Suspected toxic alcohol ingestion is a medical emergency.

Osmolality in hyperglycaemic emergencies

In hyperosmolar hyperglycaemic state (HHS), glucose often exceeds 600 mg/dL (33.3 mmol/L), and severe water loss through the urine pushes osmolality far above normal. Total osmolality above 320 mOsm/kg is typical, and values above about 330–340 are associated with drowsiness and coma. Treatment aims to lower osmolality gradually: the UK Joint British Diabetes Societies guideline recommends a fall of about 3–8 mOsm/kg per hour, because a faster fall can cause brain swelling.

In diabetic ketoacidosis, osmolality is often raised but less extreme. Monitoring effective osmolality and corrected sodium together helps clinicians manage fluid replacement safely. Our corrected sodium calculator covers the sodium side of this calculation.

Exercise-associated hyponatraemia

Endurance athletes – marathon runners, triathletes and hikers – can develop dangerously low sodium and osmolality by drinking more water or sports drink than they lose in sweat, especially when exercise stimulates ADH release. Symptoms include nausea, headache, confusion and, in severe cases, seizures. International consensus statements recommend drinking according to thirst rather than to a fixed schedule. Anyone who becomes confused or vomits after prolonged exercise needs urgent medical assessment, and should not be given large amounts of plain water.

Diabetes insipidus and the water deprivation test

When a person passes large volumes of dilute urine, clinicians compare serum and urine osmolality. In diabetes insipidus, urine stays dilute (often below 300 mOsm/kg) even as serum osmolality rises. A water deprivation test, done under supervision, measures how urine osmolality responds to withholding fluids and then to a dose of desmopressin. Newer approaches measure copeptin, a stable marker of vasopressin release, after stimulation with hypertonic saline or arginine. In 2022, an international group recommended renaming central diabetes insipidus as arginine vasopressin deficiency (AVP-D) and nephrogenic diabetes insipidus as AVP resistance (AVP-R), to avoid confusion with diabetes mellitus.

Reference ranges

Variation by age, sex and pregnancy

Men and women have the same osmolality range. Older adults tend to have slightly higher osmolality and are more vulnerable to dehydration because thirst sensation and kidney concentrating ability decline. In pregnancy, the body resets osmolality about 8–10 mOsm/kg lower from early in the first trimester, so a value of around 270–280 mOsm/kg can be normal. Newborns have a range similar to adults.

Osmolality as a hydration measure in older adults

In older people, especially in care homes and hospitals, low-intake dehydration – not drinking enough – is common and often missed, because traditional signs such as dry mouth, skin turgor and urine colour are unreliable. The European Society for Clinical Nutrition and Metabolism (ESPEN) guideline on clinical nutrition and hydration in geriatrics recommends directly measured serum osmolality as the reference test, with values above 300 mOsm/kg indicating low-intake dehydration. Where measured osmolality is not available, a calculated osmolarity of 1.86 × (Na + K) + 1.15 × glucose + urea + 14 (all in mmol/L), with a cut-off of 295 mOsm/L, has been proposed as a screening alternative.

Urine osmolality

Urine osmolality reflects how concentrated the urine is and how well the kidneys are responding to antidiuretic hormone (vasopressin). It is often measured alongside serum osmolality:

• In hyponatraemia, a urine osmolality below 100 mOsm/kg suggests excess water intake; above 100 suggests that vasopressin is active, as in SIADH.
• In polyuria (passing large volumes of urine), a low urine osmolality with a high-normal or high serum osmolality suggests diabetes insipidus.

A worked example: low sodium in an older adult

A 78-year-old woman who recently started a thiazide diuretic for blood pressure feels unsteady and nauseated. Her blood tests show sodium 124 mmol/L, glucose 5.5 mmol/L (99 mg/dL) and urea 4.0 mmol/L (BUN 11 mg/dL).

• Calculated osmolality: 2 × 124 + 5.5 + 4.0 = 257.5 mOsm/kg – low, consistent with hypotonic hyponatraemia.
• Measured osmolality: 259 mOsm/kg, confirming the result and giving a normal osmolal gap.
• Urine osmolality: 410 mOsm/kg, showing that ADH is active and the kidneys are retaining water.
• Urine sodium: 45 mmol/L, consistent with a diuretic effect or SIADH.

Her clinician stops the thiazide, checks thyroid and adrenal function, and monitors sodium closely, raising it gradually – guidelines advise limiting the rise to about 10 mmol/L in the first 24 hours to avoid osmotic demyelination. This illustrates how serum and urine osmolality guide the investigation of hyponatraemia step by step.

Factors that affect osmolality results

• Hydration status – the main driver.
• Sodium – contributes about 90% of normal osmolality.
• Glucose and urea – significant when very high.
• Alcohol – raises measured osmolality.
• Sample handling – delays and evaporation can change results.
• Measurement method – freezing-point osmometers are standard; vapour-pressure osmometers miss volatile substances such as alcohols and should not be used for the osmolal gap.

When to see a doctor

Discuss any osmolality result outside the reference range with your clinician, who will interpret it with sodium, glucose, kidney function and your clinical situation.

Seek emergency care for:

• suspected ingestion of antifreeze, methanol, windscreen-washer fluid, home-distilled spirits or other toxic substances – even if you feel well
• confusion, drowsiness, seizures or severe headache with abnormal sodium or osmolality
• extreme thirst and passing large volumes of urine, particularly with high blood sugar