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Serum Osmolality Calculator

Work out serum osmolality instantly with clear inputs, formula shown and shareable results.

About Serum Osmolality Calculator

Applies the standard 2Na + glucose/18 + urea/2.8 formula, and also reports effective osmolality, which excludes freely permeable urea.

Formula

Calculated osmolality

Osm = 2 x Na (mmol/L) + glucose (mg/dL)/18 + BUN (mg/dL)/2.8; effective Osm omits the urea term

Worked example

With the default values below, Serum Osmolality Calculator returns Calculated serum osmolality of 290.6. Change any input above and every figure here is recalculated in your browser.

Inputs used

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

Calculated serum osmolality
290.6 mOsm/kg
Effective osmolality (tonicity)
285.6 mOsm/kg
Band
Within the usual 275-295 mOsm/kg range

Step by step

  1. 1Values used
  2. 2Calculated osmolality
  3. 3Calculated serum osmolality= 290.6 mOsm/kg
  4. 4Effective osmolality (tonicity)= 285.6 mOsm/kg
  5. 5Band= Within the usual 275-295 mOsm/kg range

Assumptions and limits

  • The result is derived only from the 3 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

Why exclude urea for tonicity?

Urea crosses cell membranes freely, so it raises osmolality without creating an osmotic gradient.

What if results are in SI units?

With glucose and urea already in mmol/L the formula becomes 2 x Na + glucose + urea.

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Serum Osmolality Calculator | Calcrivo