Osmolarity Calculator
Calculate osmolarity from solute concentrations and Van t Hoff factors.
Supports up to 4 solutes.
Compare to plasma osmolarity and understand osmotic pressure.
Osmolarity is the total concentration of solute particles in a solution, expressed in osmoles per liter (Osm/L or mOsm/L).
Formula for one solute:
Osmolarity = i × M
For multiple solutes:
Osmolarity = Σ(iₙ × Mₙ) = i₁M₁ + i₂M₂ + i₃M₃ + ...
Where:
- i = Van’t Hoff factor (number of particles per formula unit)
- M = molar concentration (mol/L)
Osmolarity vs Osmolality:
- Osmolarity = osmoles per liter of solution (Osm/L), more common in lab
- Osmolality = osmoles per kg of solvent (Osm/kg), used clinically, independent of temperature
For dilute aqueous solutions, osmolarity ≈ osmolality.
Clinical reference values:
| Fluid | Osmolarity |
|---|---|
| Normal plasma | 285–295 mOsm/L |
| Isotonic saline (0.9% NaCl) | ~308 mOsm/L |
| D5W (5% dextrose) | ~252 mOsm/L |
| Urine (normal) | 50–1200 mOsm/L |
| Seawater | ~1000 mOsm/L |
Tonicity (clinical):
- Isotonic: same osmolarity as plasma (~285–295 mOsm/L), no net water movement
- Hypotonic: lower osmolarity, cells swell (water enters)
- Hypertonic: higher osmolarity, cells shrink (water leaves)
Osmotic pressure:
π = i × M × R × T
Where R = 0.08206 L·atm/(mol·K) and T is temperature in Kelvin. For plasma at 37°C: π ≈ 7.4 atm (~5,600 mmHg).
Serum osmolarity estimation (clinical formula):
Serum Osm ≈ 2[Na⁺] + [glucose]/18 + [BUN]/2.8
(concentrations in mEq/L and mg/dL)
Common i values:
| Solute | i |
|---|---|
| Glucose (non-electrolyte) | 1 |
| Urea | 1 |
| NaCl (dilute) | 2 |
| KCl | 2 |
| MgCl₂ | 3 |
| Na₂SO₄ | 3 |
| CaCl₂ | 3 |
Osmolarity or osmolality?
The two words differ by one letter and by their denominator. Osmolarity is osmoles per litre of solution; osmolality is osmoles per kilogram of solvent. Clinical laboratories measure osmolality, because it is what a freezing-point osmometer actually determines and because it does not shift with temperature. Textbooks and IV bag labels usually quote osmolarity.
For dilute body fluids the two are close enough to swap in conversation, since a litre of plasma is nearly a kilogram of water. They diverge when a solution carries a lot of protein or lipid, which is exactly the situation where the distinction starts to matter clinically.
Reading the tonicity verdict
Tonicity is not the same as osmolarity, and this is worth holding onto. Osmolarity counts every dissolved particle; tonicity counts only the ones that cannot cross the cell membrane. A urea solution can be high in osmolarity yet effectively hypotonic, because urea diffuses into the cell and stops pulling water outward. That is why 5% dextrose is treated as isotonic in the bag and hypotonic in the patient: once the glucose is metabolised, what remains is free water.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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