How do I calculate serum osmolality?

You need sodium, glucose and urea from a test. The calculator uses the formula 2 × sodium + glucose + urea (in mmol / L) and compares the result to the normal 275–295 mOsm/kg.

What is a normal plasma osmolality?

Usually 275–295 mOsm/kg. Below 275 is hypo-osmolality (often low sodium/excess water), above 295 is hyperosmolality (dehydration, high glucose or sodium).

What is the osmolar gap?

It is the difference between measured osmolality (by an osmometer) and the calculated value. A large gap may indicate osmotically active substances — ethanol, methanol, ethylene glycol. A doctor interprets it.

Why is sodium multiplied by 2?

Sodium is the main plasma cation, balanced by anions (chloride, bicarbonate). The factor 2 approximately accounts for these anions’ contribution to osmolality.

Is high osmolality dangerous?

Hyperosmolality occurs with dehydration and diabetes decompensation and can be dangerous, especially when rising fast. A doctor assesses and treats it from the whole picture.

Serum osmolality calculator (plasma)

Plasma osmolality reflects the concentration of dissolved particles in the blood. It is estimated from sodium, glucose and urea. Enter these three values from your test (in mmol / L) — the calculator computes osmolality with the common formula and compares it to the normal range of 275–295 mOsm/kg.

Calculate serum osmolality

Enter sodium, glucose and urea — the osmolality appears instantly.

Plasma osmolality: normal and abnormal

A guide to calculated plasma osmolality. Deviations are assessed together with the clinical picture and other tests — this is not a diagnosis.

Osmolality, mOsm/kg	Assessment
below 275	Low (hypo-osmolality)
275–295	Normal
above 295	High (hyperosmolality)

What plasma osmolality is

Osmolality is the concentration of all dissolved particles in blood plasma. It governs how water is distributed between blood, cells and tissues, and the body holds it tightly within 275–295 mOsm/kg.

The main contributors are sodium (with its anions), glucose and urea — so osmolality can be estimated from these three values.

The calculation formula

The calculator uses the common formula: osmolality = 2 × sodium + glucose + urea (all in mmol / L). The factor 2 on sodium accounts for the accompanying anions (mainly chloride and bicarbonate).

This is the calculated osmolality. A lab can measure it directly (with an osmometer); the difference between measured and calculated is the osmolar gap, which matters in poisonings.

When osmolality is high or low

High — with dehydration, high sodium, marked hyperglycemia (e.g. diabetes decompensation) or high urea. It is dangerous, especially when it rises quickly.

Low — with excess water or low sodium (hyponatremia), e.g. in SIADH or fluid overload. Both require a doctor’s assessment.

The osmolar gap

If measured osmolality is markedly higher than calculated (a large osmolar gap), it may indicate extra osmotically active substances — e.g. ethanol, methanol or ethylene glycol in poisonings.

The calculator computes the calculated part; measured osmolality comes from the lab, and the gap is interpreted by a doctor.

A guide, not a diagnosis

Calculated osmolality helps you understand the water-salt balance but does not replace clinical assessment. With worrying symptoms (confusion, intense thirst, edema) seek medical help.

Frequently asked questions

You need sodium, glucose and urea from a test. The calculator uses the formula 2 × sodium + glucose + urea (in mmol / L) and compares the result to the normal 275–295 mOsm/kg.
Usually 275–295 mOsm/kg. Below 275 is hypo-osmolality (often low sodium/excess water), above 295 is hyperosmolality (dehydration, high glucose or sodium).
It is the difference between measured osmolality (by an osmometer) and the calculated value. A large gap may indicate osmotically active substances — ethanol, methanol, ethylene glycol. A doctor interprets it.
Sodium is the main plasma cation, balanced by anions (chloride, bicarbonate). The factor 2 approximately accounts for these anions’ contribution to osmolality.
Hyperosmolality occurs with dehydration and diabetes decompensation and can be dangerous, especially when rising fast. A doctor assesses and treats it from the whole picture.

Sodium, glucose, urea — values from your test

Upload your report — AI reads electrolytes and metabolism together, links the values and explains what to do.

Decode my lab results

This calculator is for reference and information only and is not a diagnosis. Water-salt disturbances are assessed and treated by a doctor.