Pharmaceutics
Osmolarity Calculator Suite
Six osmolarity calculators in one place — any solution from its solutes, serum, plasma, IV fluids, TPN and laboratory buffers — in mOsm/L.
Solution osmolarity
306mOsm/L
Isotonic — Solution matches physiological osmolarity
Solutes in solution
Each solute contributes concentration × dissociation factor (i).
| Solute | mmol/L | i | mOsm/L | Remove |
|---|---|---|---|---|
| NaCl | 150 | 2 | 300 | |
| Glucose | 5.5 | 1 | 5.5 |
Add a common solute
Each tap adds a new row; remove rows you do not need.
Add a solute
Name, concentration and dissociation factor of any other solute.
Any label you like.
Molar concentration of the solute.
Particles per formula unit: glucose 1, NaCl 2, CaCl₂ 3.
Working
The numbers you entered, plugged into the formula.
Osmolarity = Σ(Concentration × Dissociation factor)
Osmolarity reference values
Common fluids for comparison.
| Solution / fluid | mOsm/L | Tonicity | pH | Clinical use |
|---|---|---|---|---|
| Normal Saline (0.9% NaCl) | 308 | Isotonic | 5.5 | Fluid resuscitation |
| Lactated Ringer's | 273 | Isotonic | 6.5 | Surgery, burns |
| D5W (5% Dextrose) | 252 | Isotonic (initially) | 4.0 | Free water replacement |
| TPN Standard | 1200-1800 | Hypertonic | 5.5-6.5 | Nutrition support |
| Human Plasma | 275-295 | - | 7.35-7.45 | Physiological reference |
General
Osmolarity = Σ(Concentration × Dissociation factor i)
Concentration in mmol/L; i is the number of particles each formula unit gives in solution. Below 250 mOsm/L is read as hypotonic, above 375 as hypertonic.
Serum
2 × Na + Glucose/18 + BUN/2.8
Sodium in mmol/L is doubled for its accompanying anions; glucose (mg/dL) ÷ 18 and BUN (mg/dL) ÷ 2.8 convert to mmol/L. Normal 275–295 mOsm/L.
Plasma
2 × (Na⁺ + K⁺) + Glucose + Urea
All values already in mmol/L. Normal band used here 280–300 mOsm/L.
IV fluid (custom)
(NaCl % ÷ 0.9) × 308 + (Dextrose % ÷ 5) × 278
0.9% NaCl = 154 mmol/L = 308 mOsm/L; 5% dextrose = 278 mmol/L = 278 mOsm/L. Standard fluids use pre-calculated values.
TPN
AA × 100 + Dextrose × 50 + Lipids × 20 + (Na + K) × 2 + Ca × 3 + Mg × 2 + PO₄ × 4
Rule-of-thumb factors of about 100, 50 and 20 mOsm/L per 1% amino acids, dextrose and lipid. Under 900 mOsm/L is read as suitable for a peripheral line, 900–1199 as borderline, 1200 and above as central.
Buffer
Base osmolarity × Concentration + |pH − 7.4| × 10
Base values are approximate 1× figures (custom buffers use 300 mOsm/L). 250–350 mOsm/L is read as physiological.
Keep learning
References
- United States Pharmacopeia–National Formulary. General Chapter <785> Osmolality and Osmolarity. Rockville (MD): USP.
- Stockton SJ. Stoklosa and Ansel's Pharmaceutical Calculations. 16th ed. Philadelphia: Wolters Kluwer; 2021.
Common questions
Osmolarity counts particles per litre of solution (mOsm/L); osmolality counts them per kilogram of water (mOsm/kg). Osmometers measure osmolality, and formulas estimate osmolarity. For dilute body fluids the two numbers are within a few percent of each other.
They convert mg/dL to mmol/L. Glucose has a molar mass of 180 g/mol, and BUN is reported as the nitrogen in urea (28 g/mol), so dividing the mg/dL value by 18 and 2.8 gives mmol/L. If your lab already reports mmol/L, use the Plasma mode instead.
Non-electrolytes such as glucose and urea have i = 1. Ideal values for salts are the number of ions: NaCl and KCl 2, CaCl₂ 3. Real solutions dissociate a little less (NaCl about 1.85), so ideal values slightly overestimate osmolarity.
Strongly hypertonic fluids damage the lining of small peripheral veins (phlebitis). This calculator reads up to 375 mOsm/L as safe peripherally and 900 mOsm/L and above as needing a central line; local policies differ.
No. Tonicity depends only on solutes that cannot cross cell membranes. Urea crosses freely, so a urea solution can be iso-osmolar yet behave as hypotonic to red cells. D5W starts isotonic but becomes hypotonic once the glucose is metabolised.
Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy