Work out serum osmolality instantly with clear inputs, formula shown and shareable results.
Applies the standard 2Na + glucose/18 + urea/2.8 formula, and also reports effective osmolality, which excludes freely permeable urea.
Calculated osmolality
Osm = 2 x Na (mmol/L) + glucose (mg/dL)/18 + BUN (mg/dL)/2.8; effective Osm omits the urea term
Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.
Urea crosses cell membranes freely, so it raises osmolality without creating an osmotic gradient.
With glucose and urea already in mmol/L the formula becomes 2 x Na + glucose + urea.