# Serum Osmolality Calculator

URL: https://speedycalculator.com/calculators/health/serum-osmolality/
Last updated: 2026-10-25

> The serum osmolality calculator estimates the concentration of particles in blood from sodium, glucose and urea (BUN). Comparing it with measured osmolality gives the osmolal gap.

## Inputs

- Sodium (mEq/L)
- Glucose (mg/dL)
- BUN (mg/dL)
- Measured osmolality (optional) (mOsm/kg)

## Results

- Calculated osmolality
- Osmolal gap

## Formula

```
Calculated Osm = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8   (glucose and BUN in mg/dL)
```

## Explanation

Sodium and its anions are the main contributors to osmolality, with glucose and urea adding smaller amounts.

### How to use the Serum Osmolality Calculator

- **Sodium**
- **Glucose**
- **BUN**
- **Measured osmolality (optional)**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Double the sodium, add glucose divided by 18 and BUN divided by 2.8.

### Inputs explained

FieldWhat to enterSodiumIn mEq/LGlucoseIn mg/dLBUNIn mg/dLMeasured osmolality (optional)In mOsm/kg
*Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.*

## Worked example

Inputs — Sodium: 140 mEq/L; Glucose: 100 mg/dL; BUN: 14 mg/dL; Measured osmolality (optional): 295 mOsm/kg.

Results — **Calculated osmolality = 291 mOsm/kg**; Osmolal gap = 4 mOsm/kg.

## FAQ

**What is a normal value?**
About 275–295 mOsm/kg.

**What does a high osmolal gap suggest?**
Unmeasured substances such as alcohols may be present.

**What formula does the Serum Osmolality Calculator use?**
Calculated Osm = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8   (glucose and BUN in mg/dL).