What is the normal serum osmolality range?

Normal serum osmolality is 275–295 mOsm/kg. Values above 295 indicate hyperosmolality; values below 275 indicate hypoosmolality.

What is the osmolal gap and when is it clinically significant?

The osmolal gap is the difference between measured and calculated osmolality. A gap greater than 10 mOsm/kg suggests the presence of unmeasured osmoles such as ethanol, methanol, ethylene glycol, or mannitol.

Why is BUN divided by 2.8 and glucose by 18?

These divisors convert mg/dL to mmol/L. BUN molecular weight is 28 g/mol (÷10 for unit conversion → ÷2.8); glucose molecular weight is 180 g/mol (÷10 → ÷18).

What is effective osmolality (tonicity)?

Effective osmolality excludes BUN because urea freely crosses cell membranes and does not affect cell volume. It is calculated as 2×Na + Glucose/18. Normal range is 270–290 mOsm/kg.

Can this tool replace a lab measurement?

No. This is a calculated estimate for educational and screening purposes only. Always obtain measured osmolality from a certified laboratory for clinical decision-making.

Serum Osmolality Calculator

Calculate serum osmolality from sodium, BUN, and glucose to identify hyper- or hypoosmolar conditions.

Input Values

Enter lab values to calculate serum osmolality.

Serum Sodium (Na) mEq/L

Normal: 136–145 mEq/L

Blood Urea Nitrogen (BUN) mg/dL

Normal: 7–20 mg/dL

Serum Glucose mg/dL

Normal fasting: 70–100 mg/dL

Measured Osmolality mOsm/kg — optional

Enter to compute the osmolal gap.

Results

Enter values and press Calculate.

Calculated Osmolality

—

mOsm/kg

Component	Formula	Value
2 × Sodium	2 × Na	—
BUN contribution	BUN / 2.8	—
Glucose contribution	Glucose / 18	—

Effective Osmolality (Tonicity)

2×Na + Glucose/18 — excludes BUN

—

Reference Ranges

Hypoosmolality < 275 mOsm/kg

Normal 275–295 mOsm/kg

Hyperosmolality > 295 mOsm/kg

Severe hyperosmolality > 320 mOsm/kg

Osmolal gap (normal) < 10 mOsm/kg

For educational and screening purposes only. Clinical decisions require measured laboratory values and physician evaluation.

Summary

Serum osmolality measures the concentration of dissolved particles in blood plasma, expressed in mOsm/kg. The standard formula — 2×Na + BUN/2.8 + Glucose/18 — uses sodium, blood urea nitrogen, and glucose to estimate total osmolality. Comparing the calculated value to a measured lab result yields the osmolal gap, which can signal the presence of unmeasured osmoles such as ethanol or methanol.

How it works

Enter the serum sodium (Na) in mEq/L.
Enter the blood urea nitrogen (BUN) in mg/dL.
Enter the serum glucose in mg/dL.
The calculator applies 2×Na + BUN/2.8 + Glucose/18.
Optionally enter a measured osmolality to compute the osmolal gap.
Review the result against the normal range (275–295 mOsm/kg).

Use cases

Identify hyperosmolar hyperglycemic state (HHS) in diabetic patients.
Evaluate the osmolal gap to detect toxic alcohol ingestion.
Screen for SIADH or other causes of hyponatremia.
Monitor fluid and electrolyte status in critically ill patients.
Assess hydration status in perioperative care.
Support diagnosis of diabetes insipidus.
Calculate effective osmolality (tonicity) excluding BUN.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu