Osmolal Gap Calculator

Work out osmolal gap instantly with clear inputs, formula shown and shareable results.

About Osmolal Gap Calculator

Subtracts the calculated osmolality from a measured value; the residual gap points to unmeasured osmoles such as methanol or ethylene glycol.

Formula

Osmolal gap

Calculated osmolality = 2 x Na + glucose/18 + BUN/2.8; gap = measured - calculated

Worked example

With the default values below, Osmolal Gap Calculator returns Osmolal gap of 9.4. Change any input above and every figure here is recalculated in your browser.

Inputs used

Measured serum osmolalityaccepts 200–450 mOsm/kg: 300 mOsm/kg
Sodiumaccepts 100–180 mmol/L: 140 mmol/L
Glucoseaccepts 40–1,200 mg/dL: 100 mg/dL
Blood urea nitrogenaccepts 2–200 mg/dL: 14 mg/dL

Results

Osmolal gap: 9.4 mOsm/kg
Calculated osmolality: 290.6 mOsm/kg
Interpretation: Normal osmolal gap (below about 10 mOsm/kg)

Step by step

1Values used
2Osmolal gap
3Osmolal gap= 9.4 mOsm/kg
4Calculated osmolality= 290.6 mOsm/kg
5Interpretation= Normal osmolal gap (below about 10 mOsm/kg)

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.
This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.

Frequently Asked Questions

What gap is significant?

Above roughly 10 mOsm/kg is usually treated as raised, though normal values range from -10 to +10.

Should ethanol be included?

Yes. Add ethanol (mg/dL) / 3.7 to the calculated value when a known ethanol level is present.