Osmolarity Calculator

Osmolarity counts the dissolved particles in a litre of solution rather than the molecules you started with, so a salt that splits into two ions counts twice. Enter the molar concentration of each solute in mmol/L and how many particles one formula unit breaks into, and the tool adds up the contributions and returns the total in mOsm/L. A second mode starts from a mass weighed into a volume and uses the molar mass to get there, with room for an osmotic coefficient. The figure is the ideal one, which real electrolytes fall a few percent short of, and it is the chemistry calculation rather than the clinical estimate made from a blood report.

Calculator Type

One row per solute. The concentration is the molar concentration of the compound you dissolved, before it splits up; the particle count does the splitting.

Solute 1 Concentration (mmol/L) *

Solute 1 Particles Per Formula Unit

Solute 2 Concentration (mmol/L)

Leave blank if you have fewer than 2 solutes.

Solute 2 Particles Per Formula Unit

Solute 3 Concentration (mmol/L)

Leave blank if you have fewer than 3 solutes.

Solute 3 Particles Per Formula Unit

Solute 4 Concentration (mmol/L)

Leave blank if you have fewer than 4 solutes.

Solute 4 Particles Per Formula Unit

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Mass In

Grams (g) Milligrams (mg)

Milligrams for the small weighings a balance actually reports; the working is shown in grams either way.

Volume In

Millilitres (mL) Litres (L)

Litres for a batch made up by the tens of litres, rather than typing six figures of millilitres.

One solute, starting from what went on the balance. The molar mass turns grams into moles, and the particle count turns moles into osmoles.

Mass of Solute (g) *

Final Volume of Solution (mL) *

Molar Mass (g/mol) *

NaCl is 58.44, glucose is 180.16, KCl is 74.55. Grams per mole is the unit everywhere; it has no second form.

Particles Per Formula Unit

Osmotic Coefficient

Leave at 1 for the ideal figure. Sodium chloride near physiological strength is about 0.93.

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