What it does
Water moves across a cell membrane towards the side with more dissolved particles. How many particles a solution holds per litre is its osmolarity, in milliosmoles per litre (mOsm/L). This calculator adds up the particles from every solute you list:
osmolarity = Σ concentration × i × φ
where i is the number of particles one formula unit gives (NaCl gives Na⁺ and Cl⁻, so 2) and φ is the osmotic coefficient, a correction for ions that do not act fully independently.
It then gives the osmotic pressure by van ‘t Hoff’s law (π = cRT) and compares the solution with a reference you choose, such as the inside of a cell.
For study and teaching, not for clinical use.
Osmotic or tonic?
- Hyper-, iso- or hypo-osmotic compares all particles.
- Hyper-, iso- or hypotonic compares only the particles that cannot cross the membrane, because those are the ones that hold water back. Urea, glycerol and ethanol enter most cells, so a urea solution can be hyperosmotic and still hypotonic: cells in it swell.
How to use it
- Add solutes. Pick from the list (the molar mass comes from the formula) or choose “Other…” and type a name, molar mass and i.
- Amount and unit: mmol/L, mol/L, g/L, % w/v (grams per 100 mL) or mg/dL.
- i fills in for listed solutes; change it if a salt does not dissociate fully.
- φ is 1 for an ideal solution. Use a measured value if you have one.
- Crosses membranes removes that solute from the effective (tonic) osmolarity.
- Set the temperature, the reference osmolarity and how close counts as equal.
Worked example
The made-up example is 0.9 % w/v NaCl (φ 0.93) with 100 mmol/L urea, at 37 °C, compared with 290 ± 10 mOsm/L.
- 0.9 % NaCl is 154 mmol/L, giving 286.4 mOsm/L. Urea adds 100.
- Osmolarity 386.4 mOsm/L: hyperosmotic.
- Effective osmolarity 286.4 mOsm/L: isotonic, because urea crosses membranes.
- Osmotic pressure 996.5 kPa (9.835 atm, 7475 mmHg); from the NaCl alone 738.7 kPa.
Tips
- Osmolarity is per litre of solution; osmolality is per kilogram of water. In dilute solutions they are close.
- Hydrated salts (such as CaCl₂·2H₂O) have a larger molar mass: pick the form on your bottle.
- Proteins add very little osmolarity because there are few of them, even at many grams per litre.
FAQ
Why is my answer different from a freezing-point osmometer? Real solutions are not ideal. Use a measured φ, or measure osmolality directly.
Does anything leave my device? No. Everything is worked out in your browser.
Where the numbers come from
- Osmotic pressure: J. H. van 't Hoff (1887), π = cRT. Gas constant R = 8.314462618 J/(mol·K) (CODATA).
- Molar masses from the formula and the IUPAC/CIAAW standard atomic weights; the van 't Hoff factors are ideal dissociation counts (general chemistry). Which solutes cross cell membranes (urea, glycerol, ethanol) is general physiology.
- Checked in the tests against a Python rewrite with separately typed IUPAC atomic weights. The example solution is made up.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.