Osmolarity Calculators

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Osmolarity (mOsm/L) is the total concentration of dissolved solute particles in one liter of solution, accounting for the dissociation of electrolytes into multiple ions. It differs from osmolality (mOsm/kg H₂O) in that osmolarity is volume-based while osmolality is mass-based. For biological fluids: osmolarity = Σ(concentration of each solute × number of particles it dissociates into). Normal plasma osmolarity is approximately 285–295 mOsm/L. Osmolarity drives osmosis — water moves from low-osmolarity to high-osmolarity solutions across semipermeable membranes, determining cell volume and fluid balance.

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Osmolarity Formula

Osmolarity (mOsm/L) = Σ cᵢ × φᵢ

cᵢ = molar concentration (mmol/L) of each solute; φᵢ = osmotic coefficient (number of particles it dissociates into: 2 for NaCl, 3 for MgCl₂, 1 for glucose).

Simplified plasma osmolarity: ≈ 2 × [Na⁺] + [glucose (mmol/L)] + [BUN (mmol/L)]

Example: [Na⁺] = 140 mEq/L, glucose = 5 mmol/L, BUN = 5 mmol/L: Osm ≈ 2×140 + 5 + 5 = 290 mOsm/L.

Osmolarity vs. Osmolality

  • Osmolarity (mOsm/L): volume-based; temperature-dependent; calculated
  • Osmolality (mOsm/kg H₂O): mass-based; temperature-independent; directly measurable by freezing-point depression
  • For dilute biological fluids: osmolarity ≈ osmolality
  • Clinical labs measure osmolality; osmolarity is calculated

Tonicity vs. Osmolarity

Tonicity refers specifically to the effect of a solution on cell volume — only membrane-impermeant solutes contribute to tonicity. Urea, for example, contributes to osmolarity (it's a solute) but not tonicity (freely crosses cell membranes → equilibrates across membrane → no net water movement).

Common Osmolarity Values

  • Normal plasma: ~285–295 mOsm/L
  • Isotonic saline (0.9% NaCl): 308 mOsm/L
  • 5% dextrose (D5W): 252 mOsm/L
  • 3% NaCl: ~1026 mOsm/L (hypertonic)
  • Seawater: ~1000 mOsm/L

Glossary

Osmolarity
Total dissolved particle concentration in mOsm/L: Σ(concentration × dissociation number); normal plasma ≈ 285–295 mOsm/L; drives osmosis from low to high osmolarity.
Tonicity
The effective osmolarity considering only membrane-impermeant solutes; determines cell volume changes; urea contributes to osmolarity but not tonicity (freely crosses membranes).
Osmolal Gap
Measured osmolality − calculated osmolarity; normal <10 mOsm/kg; elevated gap indicates unmeasured solutes (ethanol, methanol, ethylene glycol) in toxicology workup.

Frequently Asked Questions

Osmolarity (mOsm/L) = total molar concentration of all dissolved particles. For electrolytes, multiply molar concentration by the number of ions produced: 1 mM NaCl → 2 mOsm/L (Na⁺ + Cl⁻); 1 mM MgCl₂ → 3 mOsm/L (Mg²⁺ + 2Cl⁻); 1 mM glucose → 1 mOsm/L (no dissociation). Plasma osmolarity ≈ 2×[Na⁺] + [glucose mmol/L] + [BUN mmol/L]. Normal value: 285–295 mOsm/L. In the US, labs often report glucose in mg/dL (divide by 18 for mmol/L) and BUN in mg/dL (divide by 2.8 for mmol/L).

Osmolarity counts all dissolved particles (both permeant and impermeant solutes). Tonicity counts only membrane-impermeant solutes — those that cannot cross the cell membrane and therefore exert an osmotic pressure that affects cell volume. Urea distributes freely across cell membranes (it is an effective osmole but not a tonic solute) — it contributes to osmolarity but not tonicity. NaCl is impermeant (the ion transport is too slow to equilibrate rapidly) → contributes to both osmolarity and tonicity. Isotonic = same tonicity as plasma; does not cause net water movement across cell membranes.

Water moves by osmosis from a compartment of lower osmolarity (more dilute) to higher osmolarity (more concentrated) across a semipermeable membrane (permeable to water, not solutes). This continues until osmolarity equilibrates or until hydrostatic pressure opposes it. The osmotic pressure (π = iMRT) that must be applied to prevent flow equals the force driving water movement. In biology: cells placed in hypertonic solutions lose water (crenation/shrinkage); cells in hypotonic solutions gain water (swelling → lysis if severe). Red blood cells lyse (hemolysis) in water or very hypotonic solutions; crenate (shrink) in concentrated salt.

Calculated serum osmolarity (mOsm/L) = 2×[Na⁺] + glucose(mg/dL)/18 + BUN(mg/dL)/2.8. Used with measured osmolality (by freezing-point depression) to calculate the osmolal gap = measured − calculated osmolality (normal < 10 mOsm/kg). Elevated osmolal gap indicates unmeasured osmoles: ethanol (most common), methanol, ethylene glycol, isopropanol, mannitol. Used in toxicology: an osmolal gap of 25–30 mOsm/kg with an anion gap acidosis strongly suggests toxic alcohol poisoning. Each 46 mg/dL of ethanol increases the osmolal gap by ~10 mOsm/kg.