Osmolarity (mOsmol/L)

Calculate the milliosmoles per litre contributed by a dissolved solute from its concentration, molecular weight, and number of dissociated particles.

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What is this for?

Osmolarity determines whether a parenteral solution can be given peripherally and drives fluid shifts across membranes. Each dissolved particle contributes one osmole, so a salt that dissociates into two ions contributes twice the osmoles of an equimolar non-electrolyte.

How to use it

Select the substance from the list, or choose "Other" and supply the molecular weight and particle count yourself.
Enter the concentration, choosing whether you are expressing it as % w/v or g/L.
Optionally enter the container volume to obtain the total milliosmoles delivered.

Worked example

Calculate the osmolarity of 0.9% sodium chloride injection, and the total milliosmoles in a 1 litre bag.

Answer: 0.9% = 9 g/L. (9 ÷ 58.5) × 2 × 1000 = 308 mOsmol/L; a 1 L bag delivers 308 mOsmol. This is why normal saline is described as isotonic with plasma.

Clinical pearls & pitfalls

Osmolarity is per litre of solution; osmolality is per kilogram of solvent. In dilute aqueous solutions they are nearly equal, but laboratories measure osmolality while labels state osmolarity.
This calculation assumes complete dissociation. Real solutions dissociate incompletely, so measured osmolality is consistently a little lower than calculated osmolarity — the "osmolar gap".
Multiple solutes are additive. For a parenteral nutrition bag, calculate each component separately and sum them.
Dextrose does not dissociate, so its particle count is 1 — but note that dextrose monohydrate has a molecular weight of 198, not 180, which matters when working from the powder.

Assumptions & limitations

Idealised complete dissociation. It overestimates true osmolality, particularly for divalent salts such as magnesium sulfate, which associate substantially in solution.
Does not sum multiple components. Calculate each separately and add.
The peripheral-administration band is a general guide only; individual institutional policies and the specific vesicant properties of the drug take precedence.

References

United States Pharmacopeia. General Chapter <785> Osmolality and Osmolarity.
Boullata JI, et al. ASPEN safe practices for parenteral nutrition. JPEN J Parenter Enteral Nutr. 2014;38(3):334-377.
Gorski LA, et al. Infusion Therapy Standards of Practice. J Infus Nurs. 2021;44(1S):S1-S224.