Osmolarity & Osmolality Calculator
Calculate osmolarity, osmolality, approximate conversions, estimated serum osmolality, osmolar gap, and solution tonicity with formulas, presets, and step-by-step explanations.
Background
Osmolarity measures osmoles per liter of solution, while osmolality measures osmoles per kilogram of solvent. This calculator helps students keep the difference clear by showing formulas, units, particle factors, tonicity interpretation, and common fluid examples side by side.
How to use this calculator
- Choose whether you want osmolarity, osmolality, a conversion, a serum estimate, or an osmolar gap.
- Pick an input method, such as molarity, mass per volume, molality, or osmoles directly.
- Use a solute preset like NaCl, glucose, or CaCl₂ when helpful.
- Turn on tonicity interpretation to classify the result as hypotonic, isotonic, or hypertonic.
- Click Calculate to see the result, formula, substitution, interpretation, and steps.
How this calculator works
- Osmolarity uses volume of solution: osmoles per liter.
- Osmolality uses mass of solvent: osmoles per kilogram.
- The dissociation factor i estimates how many particles one formula unit forms in solution.
- Ideal values like i ≈ 2 for NaCl and i ≈ 3 for CaCl₂ are useful classroom approximations, but real solutions can deviate from ideal behavior.
- For dilute water-like solutions, osmolarity and osmolality are often numerically similar because density is close to 1 kg/L.
Formula & Equations Used
Osmolarity: Osm/L = M × i
mOsm/L: mOsm/L = Osm/L × 1000
Osmolality: Osm/kg = m × i
mOsm/kg: mOsm/kg = Osm/kg × 1000
Mass to moles: mol = mass / molar mass
Approximate conversion: mOsm/kg ≈ mOsm/L ÷ density
Estimated serum osmolality, US units: 2Na + glucose/18 + BUN/2.8
Estimated serum osmolality, SI units: 2Na + glucose + urea
Osmolar gap: Osmolar gap = measured osmolality − calculated osmolality
Example Problems & Step-by-Step Solutions
Example 1 — Osmolarity of 0.10 M NaCl
- NaCl is often approximated as forming 2 particles, so i ≈ 2.
- Use Osm/L = M × i.
- 0.10 × 2 = 0.20 Osm/L = 200 mOsm/L.
Example 2 — Osmolality of 0.20 mol/kg glucose
- Glucose does not dissociate, so i = 1.
- Use Osm/kg = m × i.
- 0.20 × 1 = 0.20 Osm/kg = 200 mOsm/kg.
Example 3 — Osmolar gap
- Suppose measured osmolality is 305 mOsm/kg.
- Suppose calculated osmolality is 288 mOsm/kg.
- Use osmolar gap = measured − calculated.
- 305 − 288 = 17 mOsm/kg.
Frequently Asked Questions
Q: What is the difference between osmolarity and osmolality?
Osmolarity is osmoles per liter of solution. Osmolality is osmoles per kilogram of solvent.
Q: When are osmolarity and osmolality almost the same?
They are often numerically similar for dilute, water-like solutions because the density is close to 1 kg/L.
Q: What does tonicity mean?
Tonicity describes how a solution may affect cell water movement compared with a reference fluid. In classroom use, lower effective osmoles are called hypotonic, similar values isotonic, and higher values hypertonic.
Q: Are ideal dissociation factors exact?
No. Values like i ≈ 2 for NaCl and i ≈ 3 for CaCl₂ are useful approximations. Real solutions can deviate from ideal behavior.
Q: What is osmolar gap?
Osmolar gap is the difference between measured osmolality and calculated osmolality. This calculator includes it for educational practice.
Q: Is the serum osmolality estimate diagnostic?
No. It is included for educational practice only and should not be used for medical decisions.
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