Osmolarity Calculator (mOsm/L of a Solution)
Calculate the osmolarity (mOsm/L) of a solution such as normal saline, D5W or potassium chloride from its % w/v, mg/mL, g/L or mmol/L, molar mass and number of dissolved particles.
How it works
g/L = % w/v × 10 = mg/mL · mmol/L = g/L ÷ molar mass (g/mol) × 1,000 · osmolarity (mOsm/L) = mmol/L × number of particles each formula unit dissolves into
Osmolarity counts the dissolved particles per litre of solution. A non-electrolyte such as glucose gives one particle per molecule, while salts dissociate: sodium chloride gives two ions and calcium chloride three. This ideal value is what IV fluid labels quote — 0.9% sodium chloride is 308 mOsm/L and D5W (dextrose monohydrate) 252 mOsm/L. Measured osmolality is a little lower because ions do not behave completely independently (the osmotic coefficient of saline is about 0.93). Plasma is about 285–295 mOsm/kg; solutions above roughly 900 mOsm/L are normally given through a central vein to avoid phlebitis (Infusion Nurses Society standards).
Worked example
0.9% sodium chloride contains 9 g/L. Dividing by the molar mass, 9 ÷ 58.44 × 1,000 = 154.00 mmol/L, and each NaCl gives two ions, so the osmolarity is 154.00 × 2 = 308.0 mOsm/L — roughly isotonic with plasma.
Sources: Wikipedia — Osmotic concentration · Infusion Nurses Society — Infusion Therapy Standards of Practice
Frequently asked questions
What is the osmolarity of normal saline?
0.9% sodium chloride is 308 mOsm/L: 154 mmol/L of Na⁺ plus 154 mmol/L of Cl⁻. Half-normal (0.45%) saline is 154 mOsm/L and 3% saline about 1,027 mOsm/L.
Why is D5W isotonic in the bag but hypotonic in the body?
5% dextrose (as monohydrate) is 252 mOsm/L, close to plasma, so it does not damage red cells when infused. Once the glucose is metabolised only free water remains, so its effect on the body is that of a hypotonic fluid.
What is the difference between osmolarity and osmolality?
Osmolarity is per litre of solution (mOsm/L) and is what you calculate from a recipe; osmolality is per kilogram of water (mOsm/kg) and is what a laboratory osmometer measures. In dilute solutions they are almost the same.
How do I calculate osmolarity for a mixture?
Calculate each solute separately and add the results. D5 0.9% NaCl is 252 + 308 = 560 mOsm/L; adding 20 mmol of KCl per litre adds 40 mOsm/L.
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Educational calculation for pharmacists, students and health professionals only — not medical advice or a prescription. Check every result against the product label, an approved compounding reference and local protocols, and have a second person verify any dose before it is given.