Osmotic Pressure Calculator
The osmotic pressure of a solution — the pressure needed to stop water flowing across a membrane into it.
The van 't Hoff equation π = iMRT treats dissolved particles like a gas: pressure is proportional to concentration and absolute temperature.
How the osmotic pressure calculator works
The van 't Hoff equation π = iMRT treats dissolved particles like a gas: pressure is proportional to concentration and absolute temperature.
The magnitudes surprise people. Blood plasma at about 300 mosmol/L exerts 7.7 atmospheres — nearly eight times atmospheric pressure. Seawater is 27 atm, which is why desalination by reverse osmosis needs 55 to 80 bar of pump pressure.
Formula: π = i M R T
Worked examples
| Inputs | Osmotic pressure | Note |
|---|---|---|
| Physiological saline at 37 °C | 7.635 atm | 7.6 atm — isotonic with blood |
| Seawater | 29.3585 atm | about 29 atm |
| Glucose at the same molarity | 3.8175 atm | half — one particle, not two |
FAQFrequently asked questions
What is osmotic pressure?
The pressure that must be applied to a solution to stop water flowing into it across a semi-permeable membrane.
How strong is it in blood?
About 7.7 atmospheres at body temperature — which is why cells burst in pure water and shrivel in concentrated salt.
Why does desalination need such high pressure?
Seawater's osmotic pressure is about 27 atm, so reverse osmosis must exceed that — plants run at 55 to 80 bar.
Is it a colligative property?
Yes — it depends on the number of dissolved particles, not what they are.
Why does i matter so much?
Because sodium chloride produces two particles per formula unit. The same molarity of glucose exerts half the pressure.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- Royal Society of Chemistry — the UK chemistry professional body
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.