Serum osmolality calculator
Estimate your serum osmolality from three values on a standard blood panel: sodium, glucose and BUN (urea). If you also have a measured osmolality from the lab, the calculator shows the osmolal gap. It works in US units (mg/dL) or SI units (mmol/L) and gives the result in mOsm/kg.
Your privacy: this calculator runs entirely in your browser — nothing you type is sent anywhere. It's the same on-device principle Quanome is built on.
Educational reference only — not medical advice. Calculated osmolality is an estimate. An abnormal osmolality or a raised osmolal gap needs a clinician, read with symptoms, medicines and the rest of your results. Reference ranges vary by lab.
## What serum osmolality numbers mean
| Calculated osmolality | Usual reading |
| --- | --- |
| < 275 mOsm/kg | Low: the blood is more dilute than usual (often linked to low sodium) |
| 275–295 mOsm/kg | Typical adult reference range |
| > 295 mOsm/kg | High: the blood is more concentrated than usual (dehydration, very high sodium or glucose, and other causes) |
Labs print their own range, so use the one on your report. For the causes of a high or low value and when to get checked, see [serum osmolality normal range: what high and low mean](/blog/serum-osmolality-normal-range/).
## The formulas
- **Calculated osmolality (US units)** = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8, with sodium in mmol/L and glucose and BUN in mg/dL.
- **Calculated osmolality (SI units)** = 2 × Na + glucose + urea, all in mmol/L.
- **Effective osmolality (tonicity)** = 2 × Na + glucose ÷ 18 (mg/dL) or 2 × Na + glucose (mmol/L). Urea is left out because it crosses cell membranes freely.
- **Osmolal gap** = measured osmolality − calculated osmolality.
The factors come from molecular weights: glucose about 180 (hence ÷ 18 to go from mg/dL to mmol/L), and the nitrogen in urea about 28 (hence ÷ 2.8 for BUN). Sodium is doubled to account for the anions that travel with it.
## Osmolality or osmolarity?
**Osmolality** is the number of dissolved particles per kilogram of water, and it is what a laboratory osmometer measures. **Osmolarity** is the number of particles per litre of solution. Formula results are often called "osmolarity", but in practice the two are treated as nearly interchangeable, and this calculator reports mOsm/kg.
## The osmolal gap
The osmolal gap compares what the lab measured with what the formula predicts from sodium, glucose and urea. A gap under about **10 mOsm/kg** is usually normal. A bigger gap means something that the formula does not count is adding to the measured osmolality. Ethanol is a common reason for a modest gap, acid build-up in severe illness can also raise it, and toxic alcohols such as methanol and ethylene glycol (antifreeze) are less common but serious, which is why a raised gap is taken seriously in an emergency setting. It is a clue for a clinician, not a diagnosis, and it needs a measured osmolality from a lab (a bedside formula cannot produce it). The usual cut-off of about 10 assumes this formula; other formulas and labs differ.
**A normal gap does not rule out poisoning,** especially hours after someone has swallowed a toxic alcohol, because the gap falls as the alcohol is broken down into acids. If poisoning is possible, seek emergency care whatever any number says.
## The rest of your panel
Osmolality is built from numbers you may already have on a metabolic panel:
- **Sodium** — see [what your sodium level means](/blog/what-your-sodium-level-means/), and correct it for high sugar with the [corrected sodium calculator](/tools/corrected-sodium-calculator/).
- **Glucose** — see [what your fasting glucose means](/blog/what-your-fasting-glucose-means/).
- **BUN** — see [what your BUN/creatinine ratio means](/blog/what-your-bun-creatinine-ratio-means/) and the [BUN/creatinine ratio calculator](/tools/bun-creatinine-ratio-calculator/).
- **Electrolyte balance** — the [anion gap calculator](/tools/anion-gap-calculator/) and [what a high or low anion gap means](/blog/anion-gap-high-low/).
## Track your labs over time — privately
A single calculation is a snapshot; the value is in the **trend**. [Quanome](/) pulls your labs, Apple Health, and even your DNA into one private timeline on your device — parsed locally, never uploaded — with an AI coach that reads your results in whichever units you use.
Track your labs over time, privately
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