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Osmotic Pressure Calculator

Find a solution's osmotic pressure.

How it is calculated

The van't Hoff equation gives osmotic pressure for a dilute solution: Π = i × M × R × T. i is the van't Hoff factor (roughly the number of particles one formula unit gives, such as 2 for NaCl, 1 for glucose; blank or invalid values are treated as 1), M is molarity in mol/L, R = 0.0820573 L·atm/(mol·K) and T is the absolute temperature, calculated as °C + 273.15. The result is in atmospheres and also converted to kilopascals (× 101.325). Molarity cannot be negative. The equation is ideal-solution behaviour, so it is least accurate at high concentrations or for strongly interacting ions, where the effective i is below the ideal value.

Worked example

Saline of 0.154 mol/L NaCl (i = 2) at body temperature 37 °C. Absolute temperature: 37 + 273.15 = 310.15 K. Π = 2 × 0.154 × 0.0820573 × 310.15 = 7.84 atm. In kilopascals: 7.84 × 101.325 = 794.25 kPa. This is why physiological saline is considered isotonic with blood.

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Common mistakes

  • Typing the temperature in kelvin when the field expects °C; the tool adds 273.15 itself.
  • Using molality or percent concentration instead of molarity (mol/L).
  • Forgetting the van't Hoff factor for salts, which makes the pressure half or a third of the real value.
  • Assuming the ideal i for concentrated solutions where ions pair up.
  • Mixing units by reading the answer in atmospheres when you need kPa, or vice versa.

About this calculator

This calculator finds a solution's osmotic pressure using the van't Hoff equation: Π = i·M·R·T, where i is the van't Hoff factor (number of particles a solute dissociates into), M is molarity, R is the gas constant, and T is absolute temperature. This is the pressure that must be applied to a solution to stop osmotic flow across a semipermeable membrane.

Chemistry and biology students, pharmacists, and lab scientists use it to predict how a solution will behave across a cell membrane or dialysis membrane, to formulate isotonic IV fluids and eye drops, and to solve colligative-properties problems.

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Comments (1)

  • Editorial team

    For a quick sanity check, a 1 mol/L non-dissociating solute at 25 degrees C gives about 24.47 atm, and the result scales directly with the van't Hoff factor and the concentration. The formula assumes an ideal dilute solution, so strong electrolytes at higher molarity will deviate from the real i. If you leave the factor blank the tool uses 1. Which solute and temperature are you working with?

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