Health & Fitness
Serum Osmolality Calculator
Estimate calculated serum osmolality from sodium, glucose, and BUN.
Preparing your result…
What this calculator does
Calculates Serum Osmolality Calculator using the formula verified against the cited source.
What your result means
An estimate from the inputs you enter, within the scope stated below — not advice.
Formula
Calculated osmolality ≈ 2 × Na + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8; SI: 2 × Na + glucose + urea (all mmol/L)
The step-by-step working for your own numbers appears with the result above and in the PDF report.
Frequently asked questions
Symbols and assumptions
- Inputs
- The values entered in the fields above
- • Conventional calculated serum osmolality≈2×Na mmol/L+glucose mg/dL/18+BUN mg/dL/2.8, reported approximately mOsm/kg. If all are mmol/L and urea rather than BUN is used: 2Na+glucose+urea.
Sources and review
Reviewed 2026-10-18 · Next review 2027-04-18
- • Distinguish calculated from measured osmolality and effective tonicity, which excludes urea. Do not enter urea as BUN or mix SI units. Iowa’s current lab uses a different glucose coefficient for its osmolar-gap calculation; identify this conventional formula/version and do not silently mix reference conventions.
Important limitation
This is an educational estimate based only on the values and formula shown. Verify current rates, rules, units, and professional standards before relying on the result. It is not personalized financial or investment advice.
Quick reference
| Scenario | Sodium (mmol/L) | Glucose | BUN (mg/dL) or urea (mmol/L) | Calculated serum osmolality |
|---|---|---|---|---|
| Worked example | 140 | 100 | 14 | 290.5556 mOsm/kg |
| Half the amounts | 100 | 50 | 7 | 205.2778 mOsm/kg |
| Double the amounts | 200 | 200 | 28 | 421.1111 mOsm/kg |
Need the full picture? Get the detailed PDF report with charts, tables, formulas and working steps.
Reviewed 2026-10-18 · Next review 2027-04-18
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