Osmolal Gap Calculator

Q: Is the osmolal gap diagnostic on its own?

No. A normal gap does not exclude toxic alcohol ingestion (early ethylene glycol or methanol ingestion can still have a normal gap before metabolism), and an elevated gap is not specific to any one cause. Always correlate with the anion gap, clinical history, and confirmatory testing.

Range	Gap (mOsm/kg)
Normal	≤ 10
Elevated — consider unmeasured osmoles	> 10

Category	Examples
Toxic alcohols	Ethanol, methanol, ethylene glycol, isopropyl alcohol
Osmotic agents	Mannitol, sorbitol, glycine (irrigation fluid)
Severe metabolic derangement	Severe hyperlipidemia, severe hyperproteinemia, lactic acidosis, ketoacidosis (contributes a small amount)
Renal failure	Accumulated unmeasured solutes in advanced kidney disease

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Walk-through

How to Use This Calculator

3 steps

1

Enter the measured osmolality

Enter the serum osmolality reported by the lab (run on a freezing-point-depression osmometer, in mOsm/kg). This value cannot be estimated — it must come from an ordered lab test.

2

Enter sodium, glucose, and BUN

Enter sodium (mEq/L), glucose (mg/dL), and BUN (mg/dL) from the same basic metabolic panel. The calculator uses these to derive the calculated osmolality instantly.

3

Read the gap and interpretation

The Osmolal Gap tab shows the gap and whether it is normal or elevated. Switch to the Calculated Osmolality tab to see the formula broken into its three components.

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Reference

Formula & Methodology

1 formula

Osmolal gap

Osmolal Gap = Measured Osmolality − (2 × Na + Glucose/18 + BUN/2.8)

Calculated osmolality uses the standard US-lab-unit formula (sodium in mEq/L; glucose and BUN in mg/dL). The gap is the measured value minus this calculated estimate. Source: Erstad BL, Pharmacotherapy 2003;23(9):1085-6.

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High YMYL

Trust, Methodology & Sources

Reviewed by Calculover Editorial Team · Updated 2026-07-22 · 2 sources

Editorial accountability

Author: Calculover Editorial Team - Clinical content editors

Owner: Calculover Editorial Team - Clinical content editors

Reviewer: Calculover Editorial Team - Clinical content review

Last reviewed: 2026-07-22

Last verified: 2026-07-22

Data effective: 2026-07-22

Methodology

Calculated osmolality uses the standard 2 × Na + Glucose/18 + BUN/2.8 formula (sodium in mEq/L; glucose and BUN in mg/dL), the version most widely cited in US clinical practice and reproduced by MDCalc. The osmolal gap is measured osmolality minus this calculated value; a gap over roughly 10 mOsm/kg is treated as elevated per convention.

Assumption: Measured osmolality was obtained via a lab freezing-point-depression osmometer, not estimated.

Assumption: Sodium, glucose, and BUN were drawn from the same blood draw (or close to it) as the measured osmolality.

Assumption: No correction is applied for co-ingested ethanol, which independently widens the gap and should be assessed separately when suspected.

Limitations & guidance

The normal-gap cutoff (~10 mOsm/kg) is a common convention, not a universal biological constant — some labs report a wider reference range.

A normal gap does not exclude toxic alcohol ingestion, particularly early after exposure before the parent alcohol is metabolized.

This tool does not diagnose poisoning; it is a screening calculation to be interpreted alongside the anion gap, history, and clinical picture.

Professional guidance: This calculator is for educational and clinical decision-support purposes only and is not a substitute for professional medical judgment, diagnosis, or treatment.

Primary sources

MDCalc — Osmolal Gap

Erstad BL. "Osmolality and osmolarity: narrowing the terminology gap." Pharmacotherapy. 2003;23(9):1085-6.

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Glossary

Key Terms Explained

7 terms

Osmolal gap The difference between measured and calculated serum osmolality, expressed in mOsm/kg. A gap greater than roughly 10 mOsm/kg is considered elevated and raises suspicion for unmeasured osmoles.

Measured osmolality Serum osmolality determined directly by a lab osmometer using freezing-point depression, reported in mOsm/kg. This value cannot be estimated from other labs — it must be ordered.

Calculated osmolality An estimate of serum osmolality derived from sodium, glucose, and BUN using the formula 2 × Na + Glucose/18 + BUN/2.8, reported in mOsm/kg.

Toxic alcohol An alcohol other than ethanol that is metabolized into a toxic acid, most notably methanol (to formic acid) and ethylene glycol (to glycolic and oxalic acid), both of which elevate the osmolal gap before metabolism.

Unmeasured osmoles Osmotically active solutes not accounted for in the calculated-osmolality formula — including toxic alcohols and mannitol — that widen the osmolal gap when present in the blood.

Ethylene glycol A toxic alcohol found in antifreeze. Ingestion elevates the osmolal gap early, then the anion gap as it is metabolized into glycolic and oxalic acid, which can cause severe metabolic acidosis and kidney injury.

Methanol A toxic alcohol found in windshield-washer fluid and some solvents. Ingestion elevates the osmolal gap early, then metabolizes into formic acid, which can cause severe metabolic acidosis and optic nerve damage.

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Scenarios

Real-World Examples

3 worked examples

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Routine BMP

Normal gap

Measured osmolality 295 mOsm/kg Sodium 140 mEq/L Glucose 90 mg/dL BUN 14 mg/dL

Calculated osmolality = 2(140) + 90/18 + 14/2.8 = 290 mOsm/kg. Gap = 295 − 290 = 5 mOsm/kg, within the normal range (≤10) — no evidence of unmeasured osmoles.

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Suspected ingestion

Elevated gap

Measured osmolality 320 mOsm/kg Sodium 140 mEq/L Glucose 90 mg/dL BUN 14 mg/dL

Calculated osmolality is unchanged at 290 mOsm/kg, but the gap is 320 − 290 = 30 mOsm/kg — clearly elevated. This pattern raises suspicion for a toxic alcohol ingestion (ethylene glycol, methanol) or another unmeasured osmole and warrants urgent correlation with the anion gap and clinical picture.

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Breakdown check

Calculated osmolality components

Sodium 138 mEq/L Glucose 100 mg/dL BUN 20 mg/dL

2 × 138 = 276, plus 100/18 = 5.6, plus 20/2.8 = 7.1, for a calculated osmolality of about 288.7 mOsm/kg — this is the value that gets subtracted from the measured result to find the gap.

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Deep Dive

Understanding the Osmolal Gap

3 chapters · 3 min read

The osmolal gap compares a lab-measured serum osmolality against a value calculated from routine labs. A wide gap suggests something osmotically active is in the blood that the calculated formula does not account for — most importantly, a toxic alcohol. This tool is built for clinicians and students who need a quick, transparent way to compute and interpret the gap.

How the osmolal gap works

Calculated osmolality estimates what the serum osmolality should be based on sodium, glucose, and BUN — the three solutes that normally make up nearly all of it (2 × Na accounts for sodium and its accompanying anions, glucose/18 converts mg/dL to mOsm/L, and BUN/2.8 does the same for urea). The measured osmolality is obtained separately, directly from the lab via a freezing-point-depression osmometer. Subtracting calculated from measured yields the gap — any osmotically active substance not included in the formula (ethanol, methanol, ethylene glycol, isopropyl alcohol, mannitol) widens it.

Inputs and what they mean

Measured osmolality (mOsm/kg) must come from an ordered lab test — it is the only input this calculator cannot substitute for you. Sodium (mEq/L), glucose (mg/dL), and BUN (mg/dL) come from a routine basic metabolic panel drawn around the same time. Because sodium is doubled in the formula, small errors or delays in the sodium value have the largest effect on the calculated result — draw the BMP and the osmolality sample as close together as possible.

Limits and edge cases

A normal osmolal gap does NOT exclude toxic alcohol ingestion — early after ingestion, before the parent alcohol is metabolized, the gap can still be normal even though a dangerous acid metabolite will accumulate later. Ethanol itself also widens the gap and should be accounted for separately when suspected. The 10 mOsm/kg cutoff is a common convention, not an absolute biological threshold — lab-to-lab and osmometer-to-osmometer variation means some sources cite a wider normal range. Always interpret the gap alongside the anion gap, history, and clinical presentation rather than as a standalone diagnostic test.

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Questions

Frequently Asked Questions

6 questions

What is a normal osmolal gap?+

A normal osmolal gap is roughly 10 mOsm/kg or less, though the exact cutoff varies slightly by lab and osmometer. Borderline values should be interpreted alongside the clinical picture rather than as a hard cutoff.

What causes an elevated osmolal gap?+

An elevated osmolal gap points to unmeasured osmotically active substances in the blood — most commonly the toxic alcohols (ethanol, methanol, ethylene glycol, isopropyl alcohol) or mannitol, and less commonly severe hyperlipidemia or hyperproteinemia.

What is the osmolal gap formula?+

Osmolal gap = measured osmolality minus calculated osmolality, where calculated osmolality = 2 × sodium + glucose/18 + BUN/2.8 (sodium in mEq/L; glucose and BUN in mg/dL).

Does this calculator require a lab value?+

Yes. Measured osmolality must be ordered from the lab and run on an osmometer — it cannot be estimated from other values. This calculator only computes the gap once you enter that measured result.

Is the osmolal gap diagnostic on its own?+

No. A normal gap does not exclude toxic alcohol ingestion, since early ethylene glycol or methanol ingestion can present with a normal gap before metabolism. Always correlate with the anion gap, clinical history, and confirmatory testing.

What units does this calculator use?+

Measured and calculated osmolality are in mOsm/kg, sodium is in mEq/L, and glucose and BUN are in mg/dL — the standard US clinical lab units.

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Labs

Osmolal Gap & Interpretation

Calculated Osmolality Breakdown

Osmolal Gap Reference Ranges

Causes of an Elevated Osmolal Gap

How to Use This Calculator

Enter the measured osmolality

Enter sodium, glucose, and BUN

Read the gap and interpretation

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Routine BMP

Suspected ingestion

Breakdown check

Understanding the Osmolal Gap

How the osmolal gap works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

2 × Sodium	—
Glucose ÷ 18	—
BUN ÷ 2.8	—
Calculated Osmolality	—

Osmolal Gap Calculator

Labs

Osmolal Gap & Interpretation

Calculated Osmolality Breakdown

Osmolal Gap Reference Ranges

Causes of an Elevated Osmolal Gap

How to Use This Calculator

Enter the measured osmolality

Enter sodium, glucose, and BUN

Read the gap and interpretation

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Understanding the Osmolal Gap

How the osmolal gap works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

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