Osmolal Gap & Anion Gap Calculator
Enter same-draw chemistry values to compare the standard osmolal gap, an ethanol-adjusted gap, the Khajuria-Krahn equation, and albumin-corrected anion gap in one place.
Ethanol-adjusted osmolal gap0
- Raw osmolal gap
- 0
- Standard calculated osmolality
- 290
- Expected ethanol contribution
- 0
- Khajuria-Krahn residual gap
- -0.75
- Anion gap
- 12
- Albumin-corrected anion gap
- 12
Equation comparison
| Measure | Result |
|---|---|
| Standard calculated osmolality, mOsm/kg | 290 |
| Measured osmolality, mOsm/kg | 290 |
| Raw osmolal gap, mOsm/kg | 0 |
| Ethanol contribution, mOsm/kg | 0 |
| Ethanol-adjusted osmolal gap, mOsm/kg | 0 |
| Khajuria-Krahn calculated osmolality, mOsm/kg | 290.75 |
| Khajuria-Krahn residual gap, mOsm/kg | -0.75 |
| Anion gap, mEq/L | 12 |
| Albumin-corrected anion gap, mEq/L | 12 |
Use laboratory values from the same blood draw whenever possible.
The osmolal gap is reported in mOsm/kg; the anion gap is reported in mEq/L.
How to use this calculator
- Enter sodium, glucose, BUN, and the measured serum osmolality from the same specimen or draw.
- Enter serum ethanol if it was measured, or zero only when it was undetected or intentionally excluded.
- Enter chloride, bicarbonate, and albumin from the same clinical context.
- Review the raw and ethanol-adjusted osmolal gaps alongside both anion-gap results.
How the gaps are calculated
The osmolal gap compares a measured serum osmolality with an osmolality estimated from common measured solutes. In this calculator, the standard calculated osmolality excludes ethanol:
COs = 2 x Na + glucose / 18 + BUN / 2.8
The raw osmolal gap is measured osmolality - COs. If ethanol is present, its expected contribution is estimated as ethanol / 3.7, with ethanol in mg/dL. The ethanol-adjusted gap is therefore measured osmolality - COs - ethanol / 3.7. When ethanol is zero, the raw and adjusted gaps are identical.
The calculator also shows the Khajuria-Krahn calculated osmolality:
COkk = 2 x Na + 1.15 x (glucose / 18) + BUN / 2.8 + 1.20 x (ethanol / 4.6)
The Khajuria-Krahn residual gap is measured osmolality - COkk. Showing both methods side by side helps make the equation choice visible, especially when glucose or ethanol is high.
How the anion gap is handled
The uncorrected anion gap is Na - Cl - HCO3. Albumin is a major unmeasured anion, so hypoalbuminemia can make the observed anion gap look lower than the acid-base disturbance would otherwise suggest. This calculator applies the common correction corrected AG = AG + 2.5 x (4 - albumin), with albumin in g/dL. If albumin is above 4 g/dL, the correction moves the result downward rather than upward.
What moves the result most
Measured osmolality moves each osmolal gap directly: one higher mOsm/kg measured value raises the gap by one. Sodium has a large effect on calculated osmolality because it is doubled. Glucose, BUN, and ethanol matter most when markedly elevated. Chloride and bicarbonate move the anion gap one-for-one in the opposite direction, while low albumin raises the corrected anion gap.
What this calculator leaves out
The calculation does not identify a cause of an elevated gap and does not account for every endogenous or exogenous osmole. Ketones, lactate, mannitol, glycols, contrast agents, other alcohols, severe renal failure, assay methods, and timing all can matter. A normal osmolal gap cannot rule out methanol or ethylene glycol poisoning after parent alcohol has been metabolized. Values entered in mmol/L for glucose, BUN, or ethanol will produce the wrong result because the equations here expect mg/dL.
Worked example
Suppose sodium is 140 mEq/L, glucose is 90 mg/dL, BUN is 14 mg/dL, measured osmolality is 320 mOsm/kg, ethanol is 46 mg/dL, chloride is 104 mEq/L, bicarbonate is 18 mEq/L, and albumin is 2.8 g/dL.
The standard calculated osmolality is 2 x 140 + 90 / 18 + 14 / 2.8 = 290 mOsm/kg. The raw osmolal gap is 320 - 290 = 30 mOsm/kg. Ethanol contributes 46 / 3.7 = 12.43 mOsm/kg, so the ethanol-adjusted gap is 17.57 mOsm/kg. The Khajuria-Krahn calculated osmolality is 302.75 mOsm/kg, giving a residual gap of 17.25 mOsm/kg. The anion gap is 140 - 104 - 18 = 18 mEq/L, and the albumin-corrected anion gap is 18 + 2.5 x (4 - 2.8) = 21 mEq/L.
Common questions
What is the difference between osmolality and osmolarity?
Serum osmolality is measured per kilogram of water and is the laboratory value used for the osmolal gap. Osmolarity is calculated per liter of solution. The terms are sometimes used loosely in bedside discussion, but this calculator compares measured serum osmolality with calculated osmolality.
What is considered an elevated osmolal gap?
There is no single cutoff that fits every laboratory and equation. Many clinicians become concerned when the gap is clearly above the local reference interval or roughly above 10 to 15 mOsm/kg, but interpretation depends on timing, assay method, and clinical context. Small positive or negative gaps can occur from normal biological and laboratory variation.
Why should ethanol be subtracted from the osmolal gap?
Ethanol is an effective osmole and can raise the measured osmolality. Subtracting its expected contribution helps estimate how much gap remains unexplained by ethanol. This is useful when the question is whether another unmeasured osmole could be contributing.
Why do the standard and Khajuria-Krahn formulas give different results?
The standard equation uses simple clinical conversion factors for sodium, glucose, and BUN, then handles ethanol as a separate correction. The Khajuria-Krahn equation applies empiric coefficients to glucose and ethanol inside the calculated osmolality. The difference is usually modest at ordinary values and more visible when glucose or ethanol is high.
Can a normal osmolal gap rule out methanol or ethylene glycol poisoning?
No. The osmolal gap is highest when parent alcohol is still present and can fall as toxic metabolites accumulate. Suspected toxic alcohol exposure needs urgent poison-control and emergency evaluation even if a calculator result appears normal.
Why correct the anion gap for albumin?
Albumin contributes to the normal unmeasured anion pool. When albumin is low, the measured anion gap may understate the degree of unmeasured anions. The correction raises the anion gap by 2.5 mEq/L for each 1 g/dL that albumin is below 4 g/dL.