Osmotic Pressure Calculator
Enter a van 't Hoff factor, a molarity, and a temperature to get the osmotic pressure in atmospheres — plus kilopascals — straight from Π = iMRT.
Use kelvin
Temperature must be absolute (kelvin) and concentration in mol/L — add 273.15 to a Celsius value before you start.
What is osmotic pressure?
The pressure that drives osmosis
The osmotic pressure calculator finds the pressure needed to stop a solvent from flowing across a semipermeable membrane into a solution. Osmotic pressure is a colligative property: it depends on how many dissolved particles are present, not on what they are. This tool turns three values — the van 't Hoff factor, the molar concentration, and the absolute temperature — into the pressure in atmospheres and kilopascals.
Enter the van 't Hoff factor, molarity, and temperature to get the osmotic pressure in atm and kPa instantly.
The van 't Hoff equation multiplies the van 't Hoff factor (i), the molar concentration (M), the gas constant (R = 0.082057 L·atm/(mol·K)), and the absolute temperature (T) to give the osmotic pressure.
Π = i × M × R × TThe van 't Hoff factor counts how many particles each formula unit releases: 1 for glucose, 2 for NaCl, 3 for CaCl₂. Because the equation mirrors the ideal gas law (PV = nRT), use kelvin for temperature and mol/L for concentration and the pressure comes back in atmospheres; multiplying by 101.325 converts it to kilopascals.
Suppose you have a 0.1 mol/L glucose solution at body temperature, 310 K.
Set the van 't Hoff factor
Glucose does not dissociate, so i = 1 — one particle per formula unit.
Multiply i × M × R
1 × 0.1 × 0.082057 = 0.0082057 — the concentration scaled by the gas constant.
Multiply by the temperature
0.0082057 × 310 = 2.543767 atm. In kilopascals that is 2.543767 × 101.325 ≈ 257.7472 kPa.
The van 't Hoff equation is an idealisation, so keep a few accuracy limits in mind.
Dilute, ideal solutions only
The equation assumes a dilute, ideal solution and a fully known van 't Hoff factor. Real electrolytes fall short of the whole-number i at higher concentrations because of ion pairing, so the calculated pressure is an upper estimate. The result also depends on using absolute temperature — kelvin, not Celsius — and concentration in mol/L, or the pressure will be wrong.