Pharmaceutics
Osmolality Calculators
Five related calculations — a general solute mixture, serum, urine, plasma and whole blood — each with its own formula and clinical interpretation.
Calculated osmolality
306mOsm/kg H₂O
Hypertonic solution
Additional details
Calculator
Solutes
Each solute contributes concentration × its dissociation factor i.
| Solute | mmol/kg | i | mOsm/kg | Actions |
|---|---|---|---|---|
| NaCl | 150 | 2 | 300.0 | |
| Glucose | 5.5 | 1 | 5.5 |
NaCl = 2, glucose = 1, CaCl₂ = 3.
Correction is 0.1% per °C from 25 °C.
Working
Σ(Concentration × i) × Temperature correction
General. Every solute contributes its molar concentration multiplied by its dissociation factor i (NaCl = 2, CaCl₂ = 3, glucose = 1), with an approximate temperature correction of 0.1% per °C away from 25 °C.
Osmolality = Σ(concentration × i) × [1 + (T − 25) × 0.001]
Serum uses the conventional US-unit formula, where 18 converts glucose and 2.8 converts BUN from mg/dL to mmol/L:
2 × Na⁺ + Glucose/18 + BUN/2.8 (+ Ethanol/4.6)
Urine is either 2(Na⁺ + K⁺) + urea + glucose in mmol/L, or the specific-gravity approximation (SG − 1) × 40 000.
Plasma works in SI units and adds a protein term of (albumin + globulin) × 0.08. Effective osmolality (tonicity) omits urea, which crosses membranes freely and so exerts no lasting osmotic force.
2 × (Na⁺ + K⁺) + Glucose + Urea + (albumin + globulin) × 0.08
Whole blood weights the plasma and red-cell compartments by haematocrit: plasma × (100 − Hct)/100 + cells × Hct/100.
Keep learning
References
- United States Pharmacopeia–National Formulary. General Chapter <785> Osmolality and Osmolarity. Rockville (MD): USP.
- Sinko PJ, editor. Martin's Physical Pharmacy and Pharmaceutical Sciences. 8th ed. Philadelphia: Wolters Kluwer; 2023.
Common questions
Osmolality is particles per kilogram of solvent and is what a laboratory osmometer measures; osmolarity is per litre of solution and is what you calculate from concentrations. In dilute aqueous solutions they differ by only a few per cent, and this page uses the terms loosely, as the original tool did.
It converts mg/dL to mmol/L. Glucose has a molar mass of 180 g/mol, so 1 mmol/L is 18 mg/dL. BUN uses 2.8 for the same reason, allowing for urea carrying two nitrogen atoms.
It is the difference between measured and calculated osmolality, and a gap above about 10 mOsm/kg suggests an unmeasured osmole such as methanol or ethylene glycol. Note the warning on this page: without a measured value entered, the gap shown here cannot do that job.
Because it crosses cell membranes freely and equilibrates on both sides, so it raises measured osmolality without driving any net water movement. Tonicity — what actually shifts water between compartments — counts only the effective osmoles, chiefly sodium and glucose.
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