Osmolality Osmolarity Calculator – Formula, Gap & Examples

Understanding osmolality and osmolarity is essential for clinicians, lab technicians, pharmacists and anyone interpreting fluid and electrolyte results. This short, practical article explains the difference, shows when to use each measure, how to calculate them, and links to two easy online tools so you can get results instantly.

What they are (simple definitions)

Osmolality — concentration of osmotically active particles per kilogram of solvent (mOsm/kg). Measured in the lab using methods like freezing-point depression; preferred when accuracy matters (e.g., in hyponatremia workups, critical care).
Osmolarity — concentration of osmotically active particles per liter of solution (mOsm/L). Calculated from solute concentrations; useful for quick estimates and non-clinical settings.

Why the difference matters

The two values are closely related and often numerically similar, but they can diverge when the solution’s density or solute concentration changes (dehydration, hyperglycemia, high lipid/protein levels). Clinically, osmolality (mOsm/kg) is the gold standard for precise assessment; osmolarity (mOsm/L) is more convenient for calculations from lab values.

Common formulas (useful for understanding)

Estimated serum osmolarity (commonly used clinical formula): Osmolarity (mOsm/L) ≈ 2 × [Na+] (mmol/L) + [Glucose] (mmol/L) + [Urea] (BUN or urea; mmol/L) (Different labs may use BUN in mg/dL versions — convert units consistently.)
Measured osmolality comes from lab instruments and is reported in mOsm/kg.

Tip: When converting glucose or BUN between mg/dL and mmol/L always use the right conversion factors before plugging numbers into formulas.

When to use each calculator

Use the Osmolality Calculator when you have measured osmolality or when you want values that reflect mass of solvent (best for clinical accuracy). Try it here: https://calcifylabs.com/osmolality-calculator/
Use the Osmolarity Calculator when you want a quick estimate from common lab values (Na⁺, glucose, BUN). Try it here: https://calcifylabs.com/osmolarity-calculator/

Both calculators are useful together: estimate osmolarity from labs, then compare to measured osmolality to calculate an osmolar gap (measured − calculated). A significant osmolar gap suggests the presence of unmeasured osmoles (e.g., toxic alcohols, mannitol, ethylene glycol).

Clinical interpretation — quick rules

Normal serum osmolality: ~275–295 mOsm/kg (lab-dependent).
Osmolar gap: normally <10 mOsm/kg. A gap >10 (or >15 depending on local cutoff) warrants investigation for unmeasured osmoles or lab/units error.
Common causes of high osmolality/osmolarity: hypernatremia, hyperglycemia, dehydration, ingestion of osmotically active toxins.
Common causes of low osmolality: hyponatremia (water excess), SIADH, excessive hypotonic fluid administration.

Example (walkthrough)

Lab values: Na⁺ = 140 mmol/L, glucose = 90 mg/dL (≈5 mmol/L), BUN = 14 mg/dL (≈5 mmol/L).
Estimated osmolarity ≈ 2×140 + 5 + 5 = 290 mOsm/L.
If measured osmolality from the lab is 305 mOsm/kg → osmolar gap = 305 − 290 = 15 mOsm → investigate for unmeasured osmoles.

How to incorporate these calculators into your workflow

Use the Osmolarity Calculator first when you have basic lab values to get a quick estimate.
Use the Osmolality Calculator to check measured values or to convert units when precision is required.
Compute the osmolar gap automatically by entering both values — our calculators are designed so you can easily move from one to the other using the links above.

Osmolality vs Osmolarity — quick guide + free calculators to try