# Osmolar Gap Calculator

Calculate the osmolar gap (osmolal gap, OG) = measured serum osmolality − calculated osmolality. Calculated osmolality uses the conventional formula 2 × Na + Glucose/18 + BUN/2.8 (mg/dL) or the SI formula 2 × Na + Glucose + Urea (mmol/L). A normal OG is approximately −10 to +10 mOsm/kg. An elevated OG indicates unmeasured osmoles — ethanol; toxic alcohols (methanol, ethylene glycol, diethylene glycol, propylene glycol); mannitol, sorbitol, glycerol; severe DKA; sepsis/end-stage renal disease. An optional ethanol level computes the ethanol-corrected gap (OG_corr = OG − EtOH/4.6) to reveal a residual unmeasured osmole such as methanol or ethylene glycol. A normal OG does NOT exclude late-stage toxic-alcohol poisoning (the parent compound is metabolized to acids, raising the anion gap instead). Derived from Kraut 2024, Whittington 2009, and MDCalc. Interpret with anion gap, blood gas, and history. Not medical advice.

> Canonical page: https://elysiatools.com/en/tools/osmolar-gap-calculator

- **Category:** Health

- **Keywords:** osmolar gap, osmolal gap, osm gap, toxic alcohol, methanol, ethylene glycol, measured osmolality, calculated osmolality, ethanol, toxicology, emergency

## Overview

The Osmolar Gap Calculator estimates the osmolar gap by subtracting calculated serum osmolality from measured serum osmolality. It supports conventional mg/dL and SI mmol/L inputs, with an optional ethanol correction to identify a residual unmeasured osmole.

## Inputs

- **Unit System** (select)
- **Measured Osmolality (mOsm/kg)** (number): Measured serum osmolality by freezing-point depression (mOsm/kg).
- **Serum Sodium Na⁺ (mmol/L)** (number): Serum sodium (mmol/L), always in mmol/L regardless of unit system.
- **Glucose** (number): Serum glucose. mg/dL under Conventional units, mmol/L under SI units.
- **BUN / Urea** (number): BUN in mg/dL under Conventional units, or serum urea in mmol/L under SI units.
- **Ethanol (mg/dL, optional)** (number): Serum ethanol in mg/dL. Optional; computes ethanol-corrected OG_corr = OG − EtOH/4.6.
- **Decimal Places** (number)

## When to use

- Evaluate an elevated measured serum osmolality when common solutes do not fully explain the result.
- Screen for unmeasured osmoles such as ethanol or toxic alcohols in a compatible clinical context.
- Adjust the gap for a known ethanol level and assess whether a residual gap remains.

## How it works

- Select Conventional units for glucose and BUN in mg/dL, or SI units for glucose and urea in mmol/L.
- Enter measured osmolality, sodium, glucose, and BUN or urea.
- The calculator computes osmolality as 2 × Na + Glucose/18 + BUN/2.8 in conventional units, or 2 × Na + Glucose + Urea in SI units.
- If ethanol is entered, it calculates the corrected gap using OG_corr = OG − EtOH/4.6 and reports the result in mOsm/kg.

## Use cases

- Emergency evaluation of suspected methanol or ethylene glycol exposure.
- Toxicology review when ethanol may explain part of an elevated osmolar gap.
- Clinical assessment of unexplained hyperosmolality alongside laboratory findings and patient history.

## Frequently asked questions

### What is the osmolar gap?

It is measured serum osmolality minus calculated serum osmolality.

### What is the usual reference range?

A normal osmolar gap is approximately −10 to +10 mOsm/kg.

### Which units does the calculator support?

It supports conventional units with glucose and BUN in mg/dL, and SI units with glucose and urea in mmol/L. Sodium remains in mmol/L.

### Why enter an ethanol level?

An ethanol level allows the calculator to estimate an ethanol-corrected gap and identify a possible residual unmeasured osmole.

### Does a normal osmolar gap exclude toxic-alcohol poisoning?

No. In later poisoning, the parent alcohol may be metabolized while the anion gap rises. Interpret the result with the anion gap, blood gas, and clinical history.

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