---
title: Serum Osmolality Calculator (Calculated Osmolality & Osmolal Gap)
type: tool
url: "https://www.test-well.com/tools/serum-osmolality-calculator"
date_published: 2026-10-12
free: true
---

# Serum Osmolality Calculator (Calculated Osmolality & Osmolal Gap)

> 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.

## Key facts

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

## For agents

See https://www.test-well.com/developers for the agent endpoints.

## 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.

## Related

- [sodium](https://www.test-well.com/biomarkers/sodium)
- [glucose](https://www.test-well.com/biomarkers/glucose)
- [bun](https://www.test-well.com/biomarkers/bun)
- [potassium](https://www.test-well.com/biomarkers/potassium)
- [chloride](https://www.test-well.com/biomarkers/chloride)
- [basic metabolic panel](https://www.test-well.com/tests/basic-metabolic-panel)
- [comprehensive metabolic panel](https://www.test-well.com/tests/comprehensive-metabolic-panel)
- [electrolyte panel](https://www.test-well.com/tests/electrolyte-panel)
- [renal panel](https://www.test-well.com/tests/renal-panel)

---

Canonical page: https://www.test-well.com/tools/serum-osmolality-calculator
Machine-readable catalog: https://www.test-well.com/api/catalog.json · MCP server: https://www.test-well.com/mcp · Overview for agents: https://www.test-well.com/llms.txt

Prices are cash-pay, in USD, and include physician authorization; every order adds one flat $6 lab processing fee. Educational content is not a diagnosis — discuss results with a clinician.