Osmolarity Calculator
Because cells don't care about fancy chemistry—they care about osmotic pressure. Calculate osmolarity, tonicity, and IV fluid concentrations that actually matter to living things.
Simple Osmolarity Calculation
For single solutes. Enter concentration and dissociation factor to get osmolarity.
Number of particles the solute dissociates into
Results
Osmolarity
-
osmol/L
Tonicity
-
vs plasma
Calculation Breakdown
Van't Hoff factor (i): -
Osmolarity = i × C: -
Osmotic Pressure: - mmHg
Freezing Point: - °C
Ready to Calculate Osmolarity?
Enter your solute concentration and dissociation factor above to calculate osmolarity and tonicity.
What Actually IS Osmolarity?
Cells don't care about your fancy molarity calculations. They care about osmolarity—the concentration of particles that can't cross their membranes.
When you dissolve table salt (NaCl) in water, it doesn't make 1 M NaCl. It makes 1 M Na⁺ and 1 M Cl⁻—two moles of particles per mole of salt. That's why saline solution is 0.9% NaCl but 308 mOsm/L.
Osmolarity vs Osmolality?
Osmolarity is per liter of solution (what you mix). Osmolality is per kilogram of solvent (what cells experience). For dilute solutions, they're basically the same. For medical fluids, we care about both.
Every IV bag, every dialysis solution, every cell culture medium—osmolarity matters. Get it wrong, and cells swell, shrink, or die. Get it right, and biology works.
The Van't Hoff Factor: Why Salt Makes Cells Swell
Jacobus van't Hoff discovered that solutes depress freezing points and elevate boiling points twice as much as expected. Why? Because electrolytes dissociate.
NaCl in Water
NaCl → Na⁺ + Cl⁻
i = 2
0.9% NaCl = 308 mOsm/L
Glucose in Water
Glucose stays as glucose
i = 1
5% glucose = 278 mOsm/L
Medical Reality:
Normal saline is isotonic not because it's 0.9% (that would be hypotonic), but because the ions make it 308 mOsm/L—perfectly matching blood plasma.
Important Medical Disclaimer
This calculator provides theoretical values for educational purposes. Actual osmolarity measurements require laboratory equipment. Clinical decisions should never be based solely on calculated values. Consult healthcare professionals for medical applications.