Osmolarity is the total concentration of osmotically active particles per litre of solution, measured in osmoles or milliosmoles per litre. It governs the osmotic pressure a solution exerts and therefore how it moves water across cell membranes.

How is the van't Hoff factor used?

Each solute contributes its molar concentration multiplied by its van't Hoff factor, which is the number of particles it splits into. Glucose stays as one particle, so its factor is 1, while NaCl splits into a sodium and a chloride ion, giving a factor of 2.

What is the difference between osmolarity and osmolality?

Osmolarity is per litre of total solution while osmolality is per kilogram of solvent water. They are nearly identical in dilute aqueous solutions but diverge as concentration rises, because solutes take up volume that the per-litre figure includes and the per-kilogram figure does not.

What osmolarity is isotonic with human plasma?

Human plasma sits at roughly 285 to 295 mOsm per litre. Solutions in that band are isotonic, lower values are hypotonic and tend to swell cells, and higher values are hypertonic and tend to shrink them. The tool flags the result against this range.

Why might my measured osmolarity be lower than calculated?

The calculator assumes each electrolyte dissociates completely. In real solutions ion pairing at higher concentrations reduces the effective number of free particles, an effect captured by the osmotic coefficient, so measured osmolarity is usually slightly below the ideal calculated value.

Osmolarity of Solution Calculator

Osmolarity tells you how many osmotically active particles a solution carries per litre, which determines the osmotic pressure it exerts and how it will move water across a membrane. It is the number that decides whether a solution is isotonic, and getting it right is essential when preparing infusions, culture media, or buffers.

How it works

Every solute contributes particles in proportion to both its concentration and how far it dissociates. The osmolar contribution of a solute is its molar concentration multiplied by its van’t Hoff factor i:

osmolarity = sum over solutes of ( C_i x i_i )

The factor i is the number of independent particles one formula unit produces. A non-electrolyte like glucose or urea has i = 1. A salt that splits into two ions, such as NaCl or KCl, has i = 2. One that splits into three, such as CaCl2 or Na2SO4, has i = 3. Summing the weighted concentrations and expressing the result in milliosmoles gives the total in mOsm/L.

Tonicity and notes

Human plasma osmolarity is about 285 to 295 mOsm/L. A solution below that band is hypotonic and tends to drive water into cells, while one above it is hypertonic and tends to draw water out. The tool compares your total against this reference.

Two caveats matter for accuracy. First, osmolarity is per litre of solution, whereas osmolality is per kilogram of water; the two agree closely only in dilute solutions. Second, the van’t Hoff factors used here assume complete dissociation, so for concentrated electrolytes the real osmolarity is a little lower because of ion pairing, an effect described by the osmotic coefficient.