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Osmotic pressure calculator

Compute osmotic pressure with the van't Hoff equation Π = i·M·R·T — the pressure that drives water across a semipermeable membrane. Enter molarity, temperature and the van't Hoff factor to get the pressure in atmospheres, kilopascals, bar and mmHg. Key to IV fluids, cells and reverse osmosis.

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Enter Molarity (mol/L), Temperature, Temperature unit, van't Hoff factor (i) and the Osmotic pressure calculator works out Osmotic pressure (atm), Pressure (kPa), Pressure (mmHg) straight away. For instance, with Molarity (mol/L) = 0.15, Temperature = 37, Temperature unit = °C and van't Hoff factor (i) = 1 it returns Osmotic pressure (atm) = 3.818, Pressure (kPa) = 386.809 and Pressure (mmHg) = 2,901.

How to use it

  1. Enter your values: Molarity (mol/L), Temperature, Temperature unit, van't Hoff factor (i).
  2. Read the result instantly: Osmotic pressure (atm), Pressure (kPa), Pressure (mmHg).

Frequently asked questions

What does the Osmotic pressure calculator actually compute?

It takes Molarity (mol/L), Temperature, Temperature unit and van't Hoff factor (i) and derives Osmotic pressure (atm), Pressure (kPa) and Pressure (mmHg) from them. The calculation is live as you type, so the result updates on every change.

What information do I need to provide?

4 values: Molarity (mol/L), Temperature, Temperature unit and van't Hoff factor (i). Nothing else is required — no account, no file upload.

Can you show a worked example?

With Molarity (mol/L) = 0.15, Temperature = 37, Temperature unit = °C and van't Hoff factor (i) = 1, the calculator returns Osmotic pressure (atm) = 3.818, Pressure (kPa) = 386.809 and Pressure (mmHg) = 2,901. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

What happens if I enter larger values?

It moves a lot. Using Molarity (mol/L) = 0.3, Temperature = 74, Temperature unit = K and van't Hoff factor (i) = 2 instead, Osmotic pressure (atm) goes from 3.818 to 3.643 — which is why it is worth testing a few scenarios rather than trusting a single figure.

Which “Temperature unit” option should I choose?

You can pick between « °C », « K » and « °F ». Each one changes what the calculator works out, so switch and compare — the default is « °C ».

What does it give for smaller values?

Scaled down to Molarity (mol/L) = 0.075, Temperature = 18.5, Temperature unit = °C and van't Hoff factor (i) = 0.5, Osmotic pressure (atm) comes out at 0.897. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

Preparing a solution at a stated concentration, working out how much of a reagent a reaction needs, and converting between mass, moles and volume.

What is the most common mistake?

Confusing molarity with molality. One is per litre of solution and the other per kilogram of solvent; they diverge as concentration rises, and only one of them is temperature-independent.

What is the difference between the Osmotic pressure calculator and the Boiling point calculator (altitude & pressure)?

This one returns Osmotic pressure (atm) and Pressure (kPa); the Boiling point calculator (altitude & pressure) returns Boiling point and Boiling point (°F). That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

Normality calculator is the closest one after this: Compute the normality of a solution — gram-equivalents of solute per litre. Enter equivalents directly or a mass with its molar mass and n-factor, plus the volume, and it returns the normality (N) and the equivalent molarity (M = N ÷ n-factor).

Further reading

All guides →
ExplainerThe Ideal Gas Law Explained: PV = nRT, R in Every Unit, and Where It BreaksPV = nRT holds when the gas is dilute and far from condensing. The value of R depends entirely on the units you feed it, and the temperature is never in degrees Celsius.How-toHow to Calculate a Dilution with C1V1 = C2V2Solve for the missing term, then convert the answer into a bench instruction. The step almost everyone skips is that the solvent you add is V2 minus V1, not V2.How-toHow to Calculate pH — And When the Weak-Acid Shortcut Stops WorkingFour routes for four kinds of solution, each worked with real numbers. Including the part most pages leave out: the square-root formula for a weak acid is an approximation with a validity limit, and it fails quietly.ExplainerHow Buoyancy Works: Archimedes' Principle, and Why Ice Floats With 10.5 % Above WaterThe upward force equals the weight of the fluid pushed aside. That one sentence decides whether something floats, and if it floats, exactly how much of it stays under.ExplainerHow Half-Life Works: What Is Left After 1, 3 and 10 Half-LivesOne half-life leaves 50%, three leave 12.5%, ten leave less than a tenth of a percent. The formula is a single line — the part worth understanding is why the answer never reaches zero.How-toHow to Solve Stoichiometry Problems: Grams to Moles to Ratio to GramsEvery problem in the chapter is the same four moves. The mole ratio is the hinge, and it is the step students lose — worked here end to end, limiting reactant and percent yield included.