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 ≈ 7.8 atm (~5900 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 |