Osmosis Rate Calculator

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Water Moves Toward Wherever It's More Crowded

Osmosis isn't water being "pulled" by solute — it's water diffusing down its own concentration gradient, moving from the side with more free water molecules to the side with fewer, across a membrane that blocks the solute itself. The bigger the difference in solute concentration between two sides of a membrane, the stronger that driving force, and this calculator quantifies exactly how strong.

The Formula

π = C × R × T (van't Hoff equation for osmotic pressure)
Jv = Lp × A × (π1 − π2) (Kedem-Katchalsky flow equation)

Each solution's osmotic pressure is calculated first using the van't Hoff equation, then the difference between the two drives net volumetric water flow across the membrane, scaled by the membrane's permeability (Lp) and surface area (A).

Where This Calculation Matters

  • Cell physiology — predicting whether a cell placed in a given solution will swell, shrink, or stay stable depends on this exact pressure difference.
  • Kidney dialysis — dialysis membranes are engineered around known permeability and surface area to control fluid removal rate.
  • Plant water transport — osmotic gradients across root cell membranes drive water uptake from soil.
  • Food preservation — osmotic dehydration in salting and sugaring processes relies on this same pressure-driven water movement out of cells.

How to Use This Calculator

  1. Enter Solution 1 Concentration (mol/L) and Solution 2 Concentration (mol/L) for the two sides of the membrane.
  2. Enter the Temperature (degC) of the system.
  3. Enter the Membrane Permeability (Lp) and Membrane Area (cm^2).
  4. Select Calculate to get the net osmotic pressure difference and the resulting flow rate and direction.
Note: The osmotic pressure figures (π1 and π2) are computed directly from the van't Hoff equation using your entered concentration and temperature; the resulting flow rate scales linearly with whatever permeability and area values you provide, so results are only as realistic as those membrane parameters.

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Working in the mOsm/L unit typically used in clinical and lab settings instead? See the chemistry section's Osmolarity Calculator. For the concentration values feeding into this calculation, use the chemistry section's Molarity Calculator.