Osmolarity Calculator

Osmolarity of a lab solution from its solutes (mM, g/L or %) using dissociation factors, and serum osmolality from sodium, glucose and BUN with the osmolal gap.

The Osmolarity Calculator sums molar concentration × number of dissociated particles for every solute to give mOsm/L, classifies the solution as hypo-, iso- or hypertonic relative to plasma, and computes calculated serum osmolality = 2·Na + glucose + urea (mmol/L) with the osmolal gap against a measured value.

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How to use it

Choose solution or serum mode.
Add solutes with concentrations, or enter Na, glucose and BUN.
Read osmolarity and tonicity.

What you enter

Solutes and concentrations, or Na/glucose/BUN

What you get

mOsm/L
Tonicity classification
Calculated serum osmolality
Osmolal gap

Formula

Osmolarity = Σ (Cᵢ × iᵢ)   |   Serum osmolality ≈ 2[Na⁺] + [glucose] + [urea] (mmol/L)   |   Gap = measured − calculated

Each solute contributes its molar concentration times the number of particles it forms (NaCl → 2, CaCl₂ → 3, glucose → 1). Ideal dissociation is assumed; real osmotic coefficients are slightly below 1.

Worked example

Input: 154 mM NaCl

Result: 308 mOsm/L — approximately isotonic

0.9 % saline: 154 mM × 2 particles.

Questions

Osmolarity vs osmolality?

Osmolarity is per litre of solution (mOsm/L); osmolality is per kilogram of solvent (mOsm/kg). For dilute aqueous solutions they are nearly equal.

What is a normal osmolal gap?

Usually below 10 mOsm/kg. A larger gap suggests unmeasured osmoles such as ethanol, methanol, ethylene glycol or mannitol.

Limitations

Ideal dissociation; measured osmolality of 0.9 % NaCl is ~286 mOsm/kg because of ion pairing.
Clinical formula is educational; it does not replace laboratory measurement.

References

Fazekas AS et al. (2013). Evaluation of 36 formulas for calculating plasma osmolality. Intensive Care Med 39:302-308.

More Chemistry

All free tools · By Dr. Zubair Khalid, DVM, MS, PhD.