Osmolality Calculator

Calculate serum osmolality, osmol gap, and free water deficit.

Sodium Na+ (mEq/L) (mEq/L)

Glucose (mg/dL) (mg/dL)

BUN (mg/dL) (mg/dL)

Measured osmolality (mOsm/kg) (mOsm/kg)

0 = skip osmol gap

Body weight (kg) for free water deficit (kg)

Please check your inputs and try again.

Effective Versus Ineffective Osmoles

Calculated osmolality sums the major solutes, but only some of them actually drive water movement across cell membranes. A solute that crosses membranes freely cannot generate an osmotic gradient.

Calculated = 2[Na⁺] + glucose/18 + BUN/2.8

Solute	Crosses membranes	Osmotically effective?
Sodium	No (actively pumped)	Yes
Glucose	Slowly, insulin-dependent	Yes in most tissues
Urea	Freely	No
Ethanol	Freely	No

This is why urea contributes to measured osmolality but not to tonicity. A uraemic patient has high osmolality without cellular dehydration, because urea equilibrates across membranes and pulls no water. Tonicity, not osmolality, determines cell volume.

The Osmolal Gap

Gap = measured − calculated (normal < 10 mOsm/kg)

An elevated gap means unmeasured solutes are present. In an acutely unwell patient this is a critical finding, since it points toward toxic alcohol ingestion.

Gap	Interpretation
< 10	Normal
10–25	Possible toxic alcohol, or lab variability
> 25	Strongly suggests methanol, ethylene glycol or isopropanol

A crucial caveat: a normal gap does not exclude poisoning. Methanol and ethylene glycol are metabolised to acids over hours, so a late presentation may show a normal osmolal gap with a large anion gap metabolic acidosis instead. Checking both is essential.

Correction Rate in Hypernatraemia

Chronic hypernatraemia must be corrected slowly — no faster than about 10–12 mmol/L per 24 hours. Brain cells generate idiogenic osmoles over days to defend their volume, and rapid correction makes the extracellular fluid relatively hypotonic, drawing water in and causing cerebral oedema.

Worked Examples

Example 1: DI: Na=155, Gluc=90, BUN=15

Calc=310+5+5.4=320

Result: Hypernatremia — FWD=70×0.6×(155/140-1)=4.5L

Need 4.5L free water to correct

Example 2: Toxic alcohol: measured=320, calculated=295

Osmol gap=25 — elevated

Result: Suspect ethanol, methanol, ethylene glycol

Ethanol 100mg/dL adds ~22 mOsm/kg

Example 3: Osmolality without dehydration

Uraemic patient, high BUN, high calculated osmolality

Result: No cellular dehydration

Urea equilibrates across membranes so it exerts no effective osmotic force. Tonicity is normal despite the raised osmolality figure.

Example 4: Late toxic alcohol presentation

Normal osmolal gap, anion gap 28, severe acidosis

Result: Poisoning not excluded

The parent alcohol has been metabolised to acid metabolites. The gap has closed while the acidosis has developed — check both.

Common Mistakes

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Treating osmolality and tonicity as the same

Urea contributes to osmolality but crosses membranes freely, so it is osmotically ineffective. Tonicity determines cell volume; osmolality does not.

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Excluding poisoning because the gap is normal

Methanol and ethylene glycol are metabolised over hours. A late presentation shows a normal osmolal gap with a raised anion gap acidosis instead.

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Correcting hypernatraemia too quickly

Chronic hypernatraemia requires correction no faster than about 10–12 mmol/L per day, or cerebral oedema can result as brain cells retain idiogenic osmoles.

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Forgetting ethanol in gap interpretation

Ethanol raises the osmolal gap substantially and is by far the commonest cause. It should be measured before pursuing toxic alcohols.

Frequently Asked Questions

Effective vs ineffective osmoles?

Effective osmoles (tonicity): create osmotic gradient across cell membrane → cell volume changes. Na+, glucose are effective. BUN is ineffective (freely permeable). Hypertonic saline expands plasma; pure water distributes throughout body water.

Hypernatremia correction rate?

Chronic hypernatremia (>48hr): brain adapts with idiogenic osmoles. Rapid correction → cerebral edema. Limit correction to ≤10 mEq/L Na per 24 hours. Acute hypernatremia can be corrected faster.

What is the difference between osmolality and tonicity?

Osmolality counts all solutes; tonicity counts only those that cannot cross membranes. Urea raises osmolality but not tonicity, so it does not shift water between compartments.

What does an elevated osmolal gap indicate?

Unmeasured solutes. Ethanol is the commonest cause, but a gap above 25 in an acidotic patient strongly suggests methanol or ethylene glycol.

Does a normal gap exclude toxic alcohol poisoning?

No. The parent alcohols are metabolised over hours, so late presentations show a normal osmolal gap with a raised anion gap acidosis instead.

How fast can hypernatraemia be corrected?

No more than about 10–12 mmol/L per 24 hours in chronic cases. Faster correction risks cerebral oedema as brain cells retain accumulated osmoles.