Osmolarity Calculator — mOsm Conversion & Isotonicity Check
What is the osmolarity of 0.9% saline?
By the textbook calculation: 0.9 g/100 mL ÷ 58.44 g/mol = 154 mM, and NaCl dissociates into Na⁺ and Cl⁻ — two particles — giving 154 × 2 = 308 mOsm/L.
The measured value is 286 mOsm/kg H₂O.
Where the gap comes from
The difference is the osmotic coefficient φ. Strong electrolytes do not dissociate ideally — ion pairing and electrostatic interactions reduce the effective particle count below the stoichiometric number:
Theoretical osmolarity = Σ(concentration × number of dissociated particles n)
Actual osmolarity = Σ(concentration × n × φ)
NaCl has φ ≈ 0.93, and 286 ÷ 308 = 0.929 — a perfect match.
This is not a pedantic detail. Plasma osmolality is approximately 275–295 mOsm/kg: using 308, you would conclude that normal saline is slightly hypertonic; using 286 is what actually explains why it is called “isotonic.”
Dissociation particle counts for common solutes
| Solute | n | Notes |
|---|---|---|
| Glucose, mannitol, glycerol, urea | 1 | Non-electrolytes; do not dissociate |
| NaCl, KCl | 2 | One monovalent cation + one monovalent anion |
| CaCl₂, MgCl₂ | 3 | One cation + two anions |
| Na₂HPO₄ | 3 | Two Na⁺ + one hydrogen phosphate |
| Sodium citrate (trisodium) | 4 | Three Na⁺ + one citrate |
n is the stoichiometric particle count. In a real solution the effective count is always slightly lower — that is what φ captures. Non-electrolytes have φ ≈ 1; strong electrolytes are typically around 0.9.
Osmolarity and osmolality are not the same thing
- Osmolarity: osmotically active particles per litre of solution, unit mOsm/L.
- Osmolality: particles per kilogram of solvent (water), unit mOsm/kg.
At low concentrations the two values are nearly identical and the terms are often used interchangeably, but when solute concentrations are high (e.g. concentrated sugar solutions) the volume occupied by solute itself causes a measurable divergence. Freezing-point osmometers in the lab measure osmolality.
When this number matters
- Cell culture: buffer or wash solutions that deviate from isotonicity cause cells to shrink or lyse; worth calculating when preparing custom buffers.
- Injection and perfusion: in animal experiments, the osmolarity of the vehicle directly affects tolerability.
- Cryopreservation and dialysis: osmotic gradients determine which way water moves.
What this tool does not cover
- Tonicity ≠ osmolarity. Urea crosses membranes freely and contributes to osmolarity but produces virtually no transmembrane tension; predicting whether cells will lyse requires looking at effective osmolytes only.
- Colloid osmotic pressure (albumin and similar) operates on a completely different scale.
- The precise value of φ varies with concentration and temperature; the tool uses standard approximations.
Related tools
For mass concentration ↔ molar concentration conversion, see Percent Concentration Converter; for single-solute preparation, see Molarity Calculator; for multi-component buffer preparation, see Buffer Recipe Calculator.
Theoretical and measured values for normal saline are from Why 0.9% saline is isotonic (Pediatr Nephrol 2019, PMID 30215094).
FAQ
Is 0.9% saline 308 or 286 mOsm?
Both are correct — they answer different questions. By the "concentration × dissociation number" formula: 0.9 g/100 mL ÷ 58.44 = 154 mM, and NaCl dissociates into two particles, giving **308 mOsm/L** — the **theoretical** value. Freezing-point osmometry gives **286 mOsm/kg H₂O**. The difference comes from the osmotic coefficient: strong electrolytes do not dissociate ideally, and ion pairing reduces the effective particle count below the stoichiometric number. 286 ÷ 308 = 0.93, which is exactly φ for NaCl.
Does that 7% difference actually matter?
Yes. Plasma osmolality is approximately 275–295 mOsm/kg: using 308, saline exceeds the upper limit and you would conclude it is hypertonic; using 286 puts it within the range, which is why it is called isotonic. When assessing whether a custom buffer or injection solution is isotonic, using the theoretical value will systematically classify solutions as hypertonic.
What is the difference between osmolarity and osmolality?
Osmolarity is osmotically active particles per litre of **solution**, in mOsm/L; osmolality is particles per kilogram of **solvent water**, in mOsm/kg. At low concentrations the two are nearly identical and the terms are often used interchangeably, but at high solute concentrations (e.g. concentrated sugar solutions) the volume occupied by solute itself causes a measurable divergence. Freezing-point osmometers measure osmolality.
How do I determine the dissociation number n?
Count the ions from the chemical formula: non-electrolytes such as glucose, mannitol, glycerol, and urea have n = 1; NaCl and KCl are 2; CaCl₂, MgCl₂, and Na₂HPO₄ are 3; trisodium citrate is 4. Note that these are **stoichiometric** particle counts. In a real solution the effective count is always slightly lower — that is what φ captures. Non-electrolytes have φ ≈ 1; strong electrolytes are typically around 0.9.
If the calculated value is isotonic, will cells definitely not lyse?
Not necessarily. Osmolarity counts total particles, but whether cells lyse depends on **tonicity**, which is determined only by solutes that cannot freely cross the membrane. Urea enters and exits cells freely, so it contributes to osmolarity but almost no transmembrane tension — an "isotonic" urea solution will still cause hemolysis. This tool gives osmolarity; predicting cell fate also requires knowing whether each solute is membrane-permeant.
Why doesn't the tool apply φ by default?
Because φ varies with solute, concentration, and temperature — there is no single universal value to fill in for you. The tool shows the theoretical value and labels it clearly; enter φ yourself when needed. For NaCl, using 0.93 reproduces the published saline data exactly (308 → 286). Supplying an opaque default coefficient would be worse than making you aware that one exists.
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