How does the Osmolarity Calculator work?

Osmolarity = 2×Na + Glucose(mg/dL)/18 + BUN(mg/dL)/2.8 Then: Normal: 275–295 mOsm/kg Then: > 295: hyperosmolar (dehydration, DKA, hyperglycaemia) Then: < 275: hypo-osmolar (SIADH, water overload).

Can you give an example of how to use the Osmolarity Calculator?

Example: Input Na 142, Glucose 108, BUN 14 gives a result of Osmolarity = 284 + 6 + 5 = 295 mOsm/kg (upper normal).

Is the Osmolarity Calculator free to use?

Yes — completely free with no registration, download, or subscription required. All calculations happen instantly in your browser.

How accurate is the Osmolarity Calculator?

Our Osmolarity Calculator uses verified mathematical formulas and is accurate to multiple decimal places. Results are calculated in real-time using the same methods used by professionals.

What formulas does this Osmolarity Calculator use?

This calculator uses standard chemistry formulas as taught in A-Level, AP, and university courses. Results are rounded to appropriate significant figures.

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Osmolarity Calculator

Serum Osmolarity Calculator

Sodium Na⁺ (mEq/L)

Glucose (mg/dL)

BUN (mg/dL)

Calculated: 2×Na + Glucose/18 + BUN/2.8

Calculated serum osmolarity estimates plasma solute concentration using sodium, glucose, and BUN. Normal range: 275–295 mOsm/kg.

Symbol	Name	Unit	Description
Na	Serum sodium	mEq/L (135–145 normal)	—
Glucose	Serum glucose	mg/dL	—
BUN	Blood urea nitrogen	mg/dL	—
Osmolality	Serum osmolality	mOsm/kg	—

Value	Status	Common cause
< 275	Hypo-osmolar	SIADH, water overload
275–295	Normal	—
> 295	Hyperosmolar	Dehydration, DKA

References

›Serum osmolality — StatPearls

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