Osmolarity Calculator

osmolarity = Σ (i × c)

Add up the osmolarity of a solution from each solute's concentration and its van 't Hoff factor.

Solutes

Concentration

van 't Hoff factor (i)

particles

Particles released per formula unit — enter the value your method uses.

Concentration

van 't Hoff factor (i)

particles

Particles released per formula unit — enter the value your method uses.

Result

305 mOsm/L

0.305 Osm/L, summed over 2 solutes.

Calculation

Contribution of NaCl
osmolarity = i × c
2 × 150 mmol/L
300 mOsm/L
Contribution of Glucose
osmolarity = i × c
1 × 5 mmol/L
5 mOsm/L
Total osmolarity
osmolarity(total) = Σ (i × c)
305 mOsm/L (0.305 Osm/L)

Inputs

NaCl: 150 mmol/L, i = 2
Glucose: 5 mmol/L, i = 1

This is an ideal calculation: it assumes the van 't Hoff factors you entered and complete, non-interacting behaviour. Measured osmolality is normally lower because of ion pairing and non-ideal activity (the osmotic coefficient of NaCl is ≈ 0.93, giving ≈ 1.86 rather than 2.00).

Osmolarity is osmoles per litre of solution; osmolality is osmoles per kilogram of solvent. They are not interconverted here — that requires the solution density and the solvent mass.

No van 't Hoff factor is chosen automatically; presets are idealised values that you can override.

Contribution to the total osmolarity

NaCl (i = 2)300 mOsm/L98.36%
Glucose (i = 1)5 mOsm/L1.64%

Total osmolarity305 mOsm/L

Idealised: calculated from the factors you entered, with no correction for the osmotic coefficient.

About this calculator

Osmolarity counts dissolved particles, not formula units. Each solute contributes its molar concentration multiplied by its van 't Hoff factor i — the number of osmotically active particles it releases. The contributions of all solutes are then added.

The van 't Hoff factor is never assumed for you. The presets shown are idealised values for complete dissociation: real electrolytes fall short of them because of ion pairing, so the effective factor of NaCl in physiological saline is closer to 1.86 than to 2.00. Enter the value your reference or method specifies.

This is osmolarity — osmoles per litre of solution. Osmolality, in osmoles per kilogram of solvent, is what a freezing-point osmometer measures. The two are numerically close for dilute aqueous solutions but are not the same quantity, and this calculator does not convert between them.

LabReadyTools is provided as a calculation aid. Users remain responsible for verifying calculations, units, chemical compatibility, experimental procedures and applicable safety requirements before laboratory use.