Osmolarity Calculator
Concentration Isn't the Whole Story When a Solute Dissociates
A mole of glucose dissolved in water contributes one mole of dissolved particles. A mole of sodium chloride contributes two — one sodium ion and one chloride ion, once it dissociates. Osmolarity accounts for that difference by counting particles, not formula units, which makes it the number that actually predicts how a solution will behave across a semipermeable membrane.
The Formula
The van't Hoff factor, i, is the number of particles one formula unit produces upon dissociation — 1 for a non-electrolyte like glucose, 2 for NaCl, 3 for CaCl2, and so on for fully dissociating compounds.
Where This Calculation Matters
- IV fluid formulation — intravenous solutions are matched to the body's natural osmolarity (roughly 275–295 mOsm/L) to avoid damaging red blood cells.
- Cell culture media — osmolarity affects cell viability and must be controlled within a narrow range for healthy growth.
- Dialysis solution design — the osmolarity gradient between dialysate and blood drives fluid and solute movement across the membrane.
- Food preservation — high-osmolarity brines and syrups draw water out of microbial cells, which is part of how salting and sugaring preserve food.
Osmolarity of Common Solutions
| Solution | Concentration | van't Hoff Factor (i) | Osmolarity |
|---|---|---|---|
| NaCl (0.9%, "normal saline") | 154 mmol/L | 2 | 308 mOsm/L |
| Glucose solution | 278 mmol/L | 1 | 278 mOsm/L |
| CaCl2 solution | 100 mmol/L | 3 | 300 mOsm/L |
Normal saline's calculated osmolarity of 308 mOsm/L sits close to human plasma osmolality (roughly 275–295 mOsm/L), which is why it's used as a baseline isotonic IV fluid.
How to Use This Calculator
- Enter the Molar Concentration (mmol/L or mol/L) of the solute.
- Enter the Van't Hoff Factor (i) — the number of particles the solute dissociates into (defaults to 1 for non-dissociating solutes).
- Select Calculate to get the osmolarity, in matching units.
Related Calculations
Need the molarity of a solution from a measured mass first? Use the Molarity Calculator. For the osmotic pressure driving water movement across a membrane in a biological context, see the biology section's Osmosis Rate Calculator.