Free tool

Serum Osmolality Calculator (Calculated Osmolality & Osmolal Gap)

Q: What is the difference between calculated and measured osmolality?

Measured osmolality comes from a laboratory osmometer (freezing-point depression) and counts every dissolved particle. The calculated value estimates it from the three solutes that normally account for almost all of it — sodium with its anions, glucose and urea. In health the two agree within about 10 mOsm/kg.

Q: What does a high osmolal gap mean?

A measured osmolality more than about 10 mOsm/kg above the calculated one means something unmeasured is dissolved in the blood: ethanol most commonly, and dangerously methanol, ethylene glycol or isopropanol. Mannitol, glycerol, high lipids and paraproteins also raise it. Entering a measured ethanol level removes its share of the gap.

Q: Which formula does this calculator use?

The common US bedside form: 2 × sodium + glucose/18 + BUN/2.8, with ethanol/3.7 added when a level is entered. Dozens of variants exist — some add potassium, some use different divisors — and they differ by a few mOsm/kg; the one your laboratory uses to report a gap is the one to compare against.

The short answer

Enter sodium, glucose and BUN from a metabolic panel and the calculator returns the calculated serum osmolality — twice the sodium plus glucose divided by 18 plus BUN divided by 2.8 — with an optional ethanol term. Normal is roughly 275–295 mOsm/kg. Subtracting this value from a laboratory-measured osmolality gives the osmolal gap, which is what clinicians use when an unmeasured substance is suspected.

Inputs: Sodium · glucose · BUN (· ethanol)
Normal: ≈ 275–295 mOsm/kg
Osmolal gap: Measured − calculated, normally < 10
Cost: Free

Sodium

mEq/L

mEq/L = mmol/L · Sodium range

Glucose

mg/dL

Glucose range

BUN (urea nitrogen)

mg/dL

BUN range

Ethanol (optional)

mg/dL

Leave blank unless a blood alcohol level was measured

Enter your values to calculate.

Calculated osmolality = 2 × Na⁺ + glucose [mg/dL] ÷ 18 + BUN [mg/dL] ÷ 2.8 (+ ethanol [mg/dL] ÷ 3.7 when measured)

Educational — a derived index, not a diagnosis. Nothing you enter leaves your browser.

How the Calculated osmolality is calculated

Calculated osmolality = 2 × Na⁺ + glucose [mg/dL] ÷ 18 + BUN [mg/dL] ÷ 2.8 (+ ethanol [mg/dL] ÷ 3.7 when measured)

Worked example

Sodium (mEq/L)	Glucose (mg/dL)	BUN (urea nitrogen) (mg/dL)	Ethanol (optional) (mg/dL)	Calculated osmolality (mOsm/kg)
140	100	14	—	291

Sodium term: 280 · Glucose term: 5.6 · Urea term: 5.0 · Osmolal gap: measured osmolality − this value (normal < 10)

Falls in: Within the usual range.

How to read the result

Calculated osmolality	Reads as	What it usually means
< 275	Low	A calculated osmolality below about 275 mOsm/kg means the body is holding water in excess of solute — almost always a low sodium (hyponatremia) from medications, heart, liver or kidney disease, SIADH or excess water intake. The sodium value and its causes are what need attention.
275 – 295	Within the usual range	Roughly 275–295 mOsm/kg is the normal range for serum osmolality; the calculated value tracks the measured one within about 10 mOsm/kg in health. A normal calculated value with a measured value well above it (a high osmolal gap) points to an unmeasured osmole such as methanol or ethylene glycol.
≥ 295	Elevated	Above 295 mOsm/kg reflects dehydration or a high sodium, a very high glucose (uncontrolled diabetes) or a high urea (kidney failure, GI bleeding) — the three terms show which. Marked elevation with confusion or very high glucose is an emergency pattern.

Cut points reviewed by Youmna DiStefano, MD on 2026-10-12. Sources below.

Limits

This is the calculated (effective-solute) osmolality, not a measurement; laboratories measure osmolality by freezing-point depression, and the difference between the two — the osmolal gap — is the clinically useful number when a toxic alcohol is suspected. Many formula variants exist (some add potassium or use different glucose and urea divisors); this one is the common US bedside form. Mannitol, glycerol, lipids and paraproteins raise the measured value without appearing here.

Sources

Rasouli M. Basic concepts and practical equations on osmolality: Biochemical approach. Clin Biochem 2016;49:936–941. doi.org/10.1016/j.clinbiochem.2016.06.001
Fazekas AS, et al. Evaluation of 36 formulas for calculating plasma osmolality. Intensive Care Med 2013;39:302–308. doi.org/10.1007/s00134-012-2691-0
Purssell RA, et al. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med 2001;38:653–659. doi.org/10.1067/mem.2001.119455
Kraut JA, Kurtz I. Toxic Alcohol Ingestions: Clinical Features, Diagnosis, and Management. Clin J Am Soc Nephrol 2008;3:208–225. doi.org/10.2215/CJN.03220807

Frequently asked questions

What is the difference between calculated and measured osmolality?

Measured osmolality comes from a laboratory osmometer (freezing-point depression) and counts every dissolved particle. The calculated value estimates it from the three solutes that normally account for almost all of it — sodium with its anions, glucose and urea. In health the two agree within about 10 mOsm/kg.

What does a high osmolal gap mean?

A measured osmolality more than about 10 mOsm/kg above the calculated one means something unmeasured is dissolved in the blood: ethanol most commonly, and dangerously methanol, ethylene glycol or isopropanol. Mannitol, glycerol, high lipids and paraproteins also raise it. Entering a measured ethanol level removes its share of the gap.

Which formula does this calculator use?

The common US bedside form: 2 × sodium + glucose/18 + BUN/2.8, with ethanol/3.7 added when a level is entered. Dozens of variants exist — some add potassium, some use different divisors — and they differ by a few mOsm/kg; the one your laboratory uses to report a gap is the one to compare against.

Which biomarkers does this tool cover?

Which tests give you these numbers?

Physician-authorized, drawn at Quest, Labcorp or BioReference. Results in 1–3 business days for most tests.