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 |
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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