Osmolarity Calculator

How to calculate the osmolarity of a solution

1. List each solute with its concentration in mM and the number of particles it splits into: 2 for NaCl or KCl, 3 for Na2HPO4 or CaCl2, 1 for glucose or sucrose.
2. The total osmolarity appears in mOsm/L, with each solute's share in the table.
3. Compare the estimated measured osmolality with your target, such as about 280 to 300 for cells.

Frequently asked questions

What is the formula?

Osmolarity = the sum of concentration × particles for every solute. 154 mM NaCl (normal saline) gives 154 × 2 = 308 mOsm/L.

Why is measured osmolality lower?

Ions in solution do not act as fully independent particles. For saline the osmotic coefficient is about 0.93, so normal saline measures about 286 mOsm/kg rather than 308. The estimate uses that factor for every solute.

Osmolarity or osmolality?

Osmolarity is per liter of solution and osmolality per kilogram of water. For dilute water solutions they are almost the same.