Osmolarity Calculator

Work out osmolarity instantly with clear inputs, formula shown and shareable results.

About Osmolarity Calculator

Multiplies molar concentration by the number of particles released on dissolution and by the osmotic coefficient that accounts for non-ideal behaviour.

Formula

Osmolarity

Osmolarity = molarity x particles per molecule x osmotic coefficient; plasma is about 290 mOsm/L

Worked example

With the default values below, Osmolarity Calculator returns Osmolarity of 279.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Solute concentrationaccepts 0–10 mol/L: 0.15 mol/L
Particles per moleculeaccepts 1–6: 2
Osmotic coefficientaccepts 0.5–1.2: 0.93

Results

Osmolarity: 279.00 mOsm/L
Ideal osmolarity before the coefficient: 300.00 mOsm/L
Osmolarity: 0.27900 Osm/L
Ratio to plasma at 290 mOsm/L: 0.96

Step by step

1Values used
2Osmolarity
3Osmolarity= 279.00 mOsm/L
4Ideal osmolarity before the coefficient= 300.00 mOsm/L
5Osmolarity= 0.27900 Osm/L
6Ratio to plasma at 290 mOsm/L= 0.96

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is the osmotic coefficient below one?

Ion pairing means dissolved salts behave as slightly fewer independent particles than full dissociation implies.

Osmolarity or osmolality?

Osmolarity is per litre of solution; osmolality is per kilogram of solvent and is what osmometers measure.