Serum Osmolality Calculator

Serum Osmolality Calculator for Endocrinology. The displayed result is a calculated estimate, conventionally labeled mOsm/kg, not an osmometer measurement. Total osmolality includes urea and is not the same as effective osmolality or tonicity. A high calculated value does not identify an unmeasured toxic alcohol.

For shared guidance on choosing a tool and documenting results, read the clinical calculator workflow.

How this calculator works

This component estimates total serum osmolality as 2 x sodium + glucose + urea + ethanol/4.6, with sodium, glucose and urea in mmol/L and ethanol in mg/dL. In US mode it converts glucose by 18.0182 and BUN by 2.8. The mg/dL nitrogen input is BUN, not urea mass. There is no measured-osmolality field and no calculated osmolal-gap output.

When to use this calculator

Use as an approximate solute calculation alongside laboratory data. It is not a measured osmolality or a stand-alone poisoning screen. For an osmolal gap, obtain measured osmolality and use the laboratory's matched equation and reference interval.

Inputs used

  • Sodium in mmol/L
  • Glucose in mmol/L or mg/dL
  • Urea in mmol/L or BUN in mg/dL
  • Ethanol in mg/dL, zero when absent

Clinical interpretation

The displayed result is a calculated estimate, conventionally labeled mOsm/kg, not an osmometer measurement. Total osmolality includes urea and is not the same as effective osmolality or tonicity. A high calculated value does not identify an unmeasured toxic alcohol.

Worked example

Sodium 140, glucose 5 and urea 5 mmol/L with no ethanol give 280 + 5 + 5 = 290. Adding ethanol 46 mg/dL adds 46/4.6 = 10, giving 300. This explicitly follows the implemented coefficient; the empirically derived ethanol/3.7 convention would give a different estimate.

Limitations and safety notes

Ethanol coefficients and osmolality equations vary. Purssell's clinical study supports ethanol/3.7, whereas this code uses the traditional molar approximation /4.6; do not mix the two when interpreting a residual gap. Approximation error and unmeasured solutes remain. Laboratory-specific ranges take precedence over the tool's generic bands.

Frequently asked questions

Why does the tool include ethanol?

Its estimate includes the entered ethanol contribution. Do not subtract that same contribution a second time when doing a separate gap calculation.

Can I enter serum urea reported in mg/dL as BUN?

No. BUN measures nitrogen mass, while urea mass is a different quantity. Use the stated analyte and units.

References

  • Purssell RA et al. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med. 2001. PMID: 11719745. https://pubmed.ncbi.nlm.nih.gov/11719745/
  • Lynd LD et al. An evaluation of the osmole gap as a screening test for toxic alcohol poisoning. BMC Emerg Med. 2008. https://pmc.ncbi.nlm.nih.gov/articles/PMC2390580/

Editorial review and citation methodology

Maintained by the Quick Medical Calculator Editorial Team. Content record date: September 7, 2026. A content date is not evidence of independent clinical review. Individual clinical sign-off is not recorded on this page.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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