Osmotic Pressure Calculator

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Choose whether to enter molarity directly or derive it from solute mass and solution volume.
Molar concentration of the solute in moles per litre (mol/L = M).
mol/L
Select a common solute to auto-fill n, molar mass and osmotic coefficient, or choose Custom to set them manually.
Temperature of the solution. Default is 25 °C (298.15 K), a standard lab condition.
°C
Osmotic pressureModerate osmolarity
6.8258

Minimum pressure needed to prevent osmosis across a semipermeable membrane.

Pressure unitatm
Osmotic pressure (Pa)691,629.01Pa
Osmotic pressure (bar)6.9163bar
Osmotic pressure (atm)6.8258atm
Effective dissociation (n)2
Osmotic coefficient (Phi)0.93
Temperature298.15K
6.8258 atm
Very low<1Physiological1-10Moderate10-30High30+

Osmotic pressure: 6.8258 atm

  • This range is close to blood plasma osmotic pressure (~7.6 atm), important in clinical and pharmaceutical formulations.
  • The solute dissociates into 2 ions per formula unit, amplifying the effective particle count and thus the osmotic pressure by a factor of 2 compared with a non-electrolyte at the same concentration.
  • At 298.15 K the calculation uses the universal gas constant R = 8.314 J/(mol·K). Raising temperature increases osmotic pressure proportionally.

Next stepFor reverse osmosis design, apply a safety factor of 1.2-1.5 over the calculated osmotic pressure when sizing pumps and membranes.

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