Serum Osmolality Calculator

Written by Great Medico

Updated September 24, 2026 •2 min read

Share Print Save

When sodium, bicarbonate, osmolality, or an infusion rate is changing, clean arithmetic matters. Serum Osmolality Calculator focuses on the exact variables shown in the tool and returns the estimate its engine supports.

What this calculator does

Serum Osmolality Calculator uses Sodium, BUN, Glucose, Serum alcohol concentration, and Measured serum osmolality to calculate calculated serum osmolality and osmolal gap. Where supported, it also shows Osmolal gap so the primary result is easier to check. It follows the exact field flow and engine behavior in this plugin rather than describing a different calculator with the same name. Fluid and electrolyte formulas are sensitive to units, timing, ongoing losses, kidney function, and rapidly changing physiology. They are most useful as structured estimates alongside repeat measurements and bedside assessment.

How to use it

Enter or select Sodium, BUN, Glucose, Serum alcohol concentration, and Measured serum osmolality. Use the units offered by the calculator and keep paired laboratory or treatment values from the same clinical episode whenever the formula assumes that. If an entry is unknown, do not substitute a guess simply to obtain a number.

How the calculation works

Calculated serum osmolality uses 2 × sodium + glucose/18 + BUN/2.8 + ethanol/3.7. If measured osmolality is supplied, the tool subtracts the calculated value from it to display an osmolal gap.

Example

Using the sample values prefilled in this calculator, the primary result is 290.4 mOsm/kg (Calculated serum osmolality) and Osmolal gap is -0.4 mOsm/kg. This example demonstrates the plugin’s own calculation path; replace the sample inputs with verified values before interpreting a real result.

How to interpret the result

Read the output as calculated serum osmolality from the implemented formula. Compare it with measured values, volume status, ongoing inputs/losses, and serial trends; treatment decisions should not rely on the calculator in isolation.

Limitations and notes

Calculated osmolality and osmolal gap depend on the equation, assay, units, ethanol, and unmeasured osmoles. A gap cannot identify a toxic alcohol by itself and should not delay urgent toxicology management.