Calculate serum osmolality, effective osmolality (tonicity), osmol gap, corrected sodium, and a rough free-water estimate in one interpretation worksheet.
Serum osmolality measures the total concentration of dissolved solutes per kilogram of body water, normally maintained within a narrow range by thirst and water-balance regulation. Calculated osmolality, derived from sodium, glucose, and blood urea nitrogen, provides a quick estimate that can be compared with a measured value to show the osmol gap.
This calculator computes the standard calculated osmolality (2×Na + Glu/18 + BUN/2.8), effective osmolality or tonicity (which excludes the freely permeable BUN and reflects the osmotic force driving water across cell membranes), the osmol gap (measured minus calculated), and the corrected sodium for hyperglycemia. It also estimates free water deficit in hypernatremic patients.
The goal is to keep the related osmolality numbers together in one worksheet so you can compare measured and calculated values without doing the arithmetic by hand. The output is an interpretation aid, not a poisoning diagnosis or a fluid-replacement protocol.
This calculator brings the main osmolality-related calculations into one place so you can compare calculated osmolality, effective osmolality, osmol gap, corrected sodium, and free water deficit without doing the math by hand. That makes it easier to separate hypotonic hyponatremia from hyperosmolar states and to recognize when a widened osmol gap should trigger a toxic alcohol workup.
Calculated Osmolality = 2 × Na + Glucose/18 + BUN/2.8 (mOsm/kg). Effective Osmolality (Tonicity) = 2 × Na + Glucose/18. Osmol Gap = Measured Osm − Calculated Osm | Normal ≤ 10. Corrected Na = Na + 1.6 × ((Glucose − 100) / 100). Free Water Deficit = TBW × (Na/140 − 1).
Result: Calculated Osmolality 281 mOsm/kg, Osmol Gap 39 → elevated, investigate toxic alcohol ingestion
The calculated osmolality of 281 is normal, but the measured osmolality of 320 creates an osmol gap of 39. That is a meaningful gap and should be interpreted alongside the acid-base picture, renal function, and any direct alcohol or toxicology testing.
Calculated osmolality and effective osmolality answer different questions. Calculated osmolality estimates the total solute concentration, while effective osmolality focuses on the osmoles that actually move water across cell membranes. That is why BUN is included in the calculation but excluded from tonicity.
An elevated osmol gap is most useful when it is interpreted alongside the acid-base picture. A wide gap with an anion gap metabolic acidosis can raise concern for toxic alcohols, while an isolated gap may reflect ethanol, isopropanol, ketoacidosis, renal failure, or another unmeasured osmole. The same number can mean different things depending on the clinical setting.
Corrected sodium and free water deficit help translate the chemistry into a fluid-balance discussion. In hyperglycemia, the corrected sodium estimates what the sodium would look like once the glucose effect is removed, while the free water deficit gives a rough sense of replacement needs in hypernatremia rather than a stand-alone treatment order.
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This page calculates total osmolality from sodium, glucose, and BUN, then shows effective osmolality, corrected sodium for hyperglycemia, the osmol gap when a measured value is entered, and a rough free-water estimate for hypernatremia. Ethanol can be added separately so the worksheet can distinguish baseline calculated osmolality from an ethanol-adjusted osmolality value.
The result is an interpretation aid, not a poisoning diagnosis or a fluid-order generator. Osmol-gap interpretation still depends on timing, acid-base status, kidney function, measured laboratory values, and the broader clinical picture, and the free-water estimate is only a rough planning number.
Osmolality is measured per kilogram of solvent (water), while osmolarity is per liter of solution. Osmolality is more accurate because it is unaffected by dissolved solutes or temperature. Clinically, the terms are often used interchangeably since values are very similar in dilute solutions like plasma.
BUN (urea) crosses cell membranes freely and distributes equally between intracellular and extracellular compartments. It does not create an osmotic gradient and therefore does not cause water movement. Effective osmolality (tonicity) reflects only the osmoles that drive water shifts, primarily sodium and glucose.
An elevated osmol gap is most useful when it is read together with the acid-base picture, kidney function, and the exposure history. The same gap can mean different things in different settings, including ethanol, toxic alcohols, ketoacidosis, renal failure, or lab variation.
No. A normal osmol gap does not exclude toxic alcohol exposure. Timing, partial metabolism, small ingestions, and baseline variation can all narrow the gap even when the clinical concern remains real.
The Katz correction adds 1.6 mEq/L to the measured sodium for each 100 mg/dL increase in glucose above 100. This reveals the "true" sodium that would be measured if glucose were normal, since hyperglycemia draws water into the vascular space doubling measured sodium. Some guidelines use 2.4 mEq/L per 100 mg/dL for glucose >400.
Treat it as a rough planning estimate rather than a stand-alone fluid order. Real replacement depends on symptoms, acuity, ongoing losses, volume status, and repeat sodium checks.