# Serum Osmolality Calculator (Calculated)

Calculate serum osmolality from routinely measured solutes. Conventional formula Osm = 2 × Na + Glucose/18 + BUN/2.8 (Na in mmol/L, glucose & BUN in mg/dL), or the SI-unit form Osm = 2 × Na + Glucose + Urea (all mmol/L). An optional ethanol term (EtOH/4.6) is available when a toxic-alcohol ingestion is being considered. Normal range ≈ 275–295 mOsm/kg. Values < 275 suggest hypo-osmolality (SIADH, primary polydipsia, adrenal insufficiency, hypothyroidism); values > 295 suggest hyperosmolality (hyperglycemia/HHS, hypernatremia, uremia, mannitol, toxic alcohols — the latter also raise the osmolar gap, so measure osmolality and compute the gap). Derived from Dorwart 1975, Rasouli 2014, and MDCalc. Not medical advice.

> Canonical page: https://elysiatools.com/en/tools/serum-osmolality-calculator

- **Category:** Health

- **Keywords:** serum osmolality, calculated osmolality, osmolality formula, tonicity, SIADH, hyperosmolality, hypo-osmolality, sodium, glucose, BUN, electrolytes, nephrology, toxicology

## Overview

The Serum Osmolality Calculator estimates serum osmolality from sodium, glucose, and BUN or urea values. Choose conventional or SI units to calculate an osmolar result in mOsm/kg and compare it with the approximate 275–295 mOsm/kg reference range.

## Inputs

- **Unit System** (select)
- **Serum Sodium Na⁺ (mmol/L)** (number): Serum sodium (mmol/L), always in mmol/L regardless of unit system. Physiologic range ~120–160.
- **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; adds EtOH/4.6 to the calculated osmolality (conventional units).
- **Decimal Places** (number)

## When to use

- Estimate serum osmolality from routinely measured sodium, glucose, and BUN or urea values.
- Interpret whether a calculated result is within the approximate normal range or suggests hypo-osmolality or hyperosmolality.
- Include an optional ethanol term in conventional units when toxic-alcohol ingestion is being considered.

## How it works

- Select Conventional units for glucose and BUN in mg/dL, or SI units for glucose and urea in mmol/L.
- Enter serum sodium in mmol/L, then enter the matching glucose and BUN or urea value.
- Optionally enter ethanol in mg/dL when using the conventional formula.
- The calculator returns calculated osmolality in mOsm/kg, the category, and the formula used.

## Use cases

- Clinical education on serum osmolality formulas and unit conversions.
- Review of sodium, glucose, and BUN or urea results in nephrology and electrolyte assessments.
- Initial estimation of hyperosmolality or hypo-osmolality when reviewing laboratory data.

## Frequently asked questions

### What inputs does the calculator require?

It requires sodium, glucose, and BUN or urea. Sodium is entered in mmol/L.

### Which formulas are used?

Conventional units use 2 × Na + Glucose/18 + BUN/2.8. SI units use 2 × Na + Glucose + Urea.

### What is the approximate normal range?

The approximate normal range is 275–295 mOsm/kg.

### Can ethanol be included?

Yes. Enter ethanol in mg/dL under conventional units to add the EtOH/4.6 term.

### Is this a measured serum osmolality result?

No. This tool calculates an estimate from entered solute values and is not medical advice.

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