Solution Osmolarity Calculator
Solution Osmolarity Calculator
Calculate the theoretical osmolarity of a solution you are making up, from the millimolar concentration of each solute and the number of particles it dissociates into — and see why an osmometer will read lower.
Calculated osmolarity
3 solutes → mOsm/L0.9% sodium chloride: 154 mmol/L of NaCl, which dissociates into 2 particles
Formula
Osmolarity = osmoles per litre of SOLUTION · Osmolality = osmoles per kilogram of SOLVENT
- concentration
- millimoles of the compound per litre of finished solution — the same denominator as molarity, and not the same as the millimoles of each ion it releases
- particles per molecule
- the van ‘t Hoff factor at complete dissociation: 1 for glucose and urea, 2 for NaCl, 3 for CaCl₂, 4 for trisodium citrate
- mOsm/L
- milliosmoles per litre of solution. This is a CALCULATED quantity; no instrument measures it directly
- mOsm/kg
- milliosmoles per kilogram of water. This is what a freezing-point-depression osmometer measures, and it is the quantity to compare a measured result against
- what is missing
- the osmotic coefficient. Real electrolytes do not dissociate completely or behave ideally, so the measured value is lower than this sum — by about 7% for sodium chloride
Worked example
0.9% sodium chloride: 154 mmol/L of NaCl, which dissociates into 2 particles
9 g/L ÷ 58.44 g/mol = 154 mmol/L of NaCl
154 × 2 = 308.0 mOsm/L, which is the figure printed on the bag
The measured osmolality of the same fluid is 286 mOsm/kg
The 22-unit gap is the osmotic coefficient: 308 × 0.926 = 285, and sodium and chloride are about 92.6% osmotically active
This is why a fluid whose calculation says 308 is nonetheless isotonic at the bedside
Calculated against measured: the three fluids everyone uses
| Fluid | Composition | Calculated osmolarity (mOsm/L) | Measured osmolality (mOsm/kg) |
|---|---|---|---|
| Sodium chloride 0.9% | 154 mmol/L NaCl | 308 (US package insert) | 286 — isotonic in use |
| Lactated Ringer’s / Hartmann’s | Na 130, K 4, Ca 1.5 mmol, Cl 109, lactate 28 | 273 (US package insert) | 256 — genuinely hypotonic |
| Dextrose 5% | 50 g/L dextrose monohydrate = 252 mmol/L | 252 (US package insert) | 0 in vivo, once the glucose is metabolised |
| Plasma, for comparison | — | 291 calculated | 288 measured |
Why 5% dextrose is quoted as both 252 and 278 mOsm/L
| Assumption | Molecular weight | 50 g/L works out at | Calculated osmolarity |
|---|---|---|---|
| Dextrose monohydrate — what USP 5% Dextrose Injection contains | 198.17 g/mol | 252 mmol/L | 252 mOsm/L |
| Anhydrous glucose | 180.16 g/mol | 278 mmol/L | 278 mOsm/L |
Osmolarity, osmolality and tonicity are three different things
| Quantity | Denominator | Measured or calculated? | What it tells you |
|---|---|---|---|
| Osmolarity | One litre of solution | Calculated only | How many osmotically active particles you put in. Temperature-dependent, because the solution’s volume changes. |
| Osmolality | One kilogram of water | Measured, by freezing point depression | How many particles are actually there and active. Independent of temperature, because mass is. |
| Tonicity | Relative to a membrane | Neither — inferred | What the solution does to cells. Depends on whether the solutes can cross the membrane: urea contributes to osmolality and nothing to tonicity. |
Why the calculated number and the measured number never agree
The calculation is a particle count. Take the millimolar concentration of each solute, multiply by the number of particles it releases on dissolving, and add them up. Sodium chloride at 154 mmol/L releases 154 mmol of sodium and 154 mmol of chloride, so it counts twice and the answer is 308 mOsm/L. Glucose releases one particle and counts once. Calcium chloride releases three. That is the whole method, and the figure printed on an intravenous fluid bag is exactly this sum.
It is also, reliably, too high. Ions in solution do not behave as independent particles: they associate into transient pairs and their charges interact, so the osmotic effect of a mole of dissolved sodium chloride is less than the effect of two moles of an ideal solute. The ratio between the two is the osmotic coefficient, and for sodium and chloride it is about 0.926 — they are, as the 2025 European Journal of Medical Research review puts it, only 92.6% osmotically active. Apply that to 0.9% saline and 308 mOsm/L becomes about 285, which is what an osmometer finds: the measured figure for 0.9% saline is 286 mOsm/kg. The bag says 308, the instrument says 286, and neither is wrong. This is why a fluid whose calculated osmolarity is above the plasma figure is nonetheless the standard isotonic crystalloid.
The denominators differ too, and the distinction is worth getting right because almost nothing written about these fluids observes it. Osmolarity is osmoles per litre of solution; osmolality is osmoles per kilogram of water. Osmolarity can only ever be calculated, because it counts particles per unit volume of a mixture and there is no instrument that does that. Osmolality can be measured, and in practice is measured one way: by freezing point depression, which the same review describes as currently the only method in use. So a laboratory osmometer reports mOsm/kg, a fluid bag reports mOsm/L, and comparing them involves assuming that a litre of solution contains about a kilogram of water. For a dilute crystalloid that is close to true. For plasma with a high protein or lipid content it is not, and the discrepancy is the mechanism behind pseudohyponatraemia.
A third quantity, tonicity, is what actually happens to cells, and it is not the same as either. Tonicity counts only the solutes that cannot cross the membrane in question. Urea contributes fully to measured osmolality and almost nothing to tonicity, because it equilibrates across cell membranes; glucose in a 5% dextrose infusion contributes fully in the bottle and nothing at all once it has been metabolised, which is why that fluid is described as hypotonic in vivo despite a perfectly respectable calculated osmolarity of 252 mOsm/L. A solution can be iso-osmolar and hypotonic at the same time, and the calculation on this page cannot tell you which it is.
None of this is the serum osmolar gap, which is a different calculation on a different sample for a different purpose: it subtracts a calculated plasma osmolality, estimated from sodium, glucose and urea, from a measured one, in order to detect an unmeasured solute such as methanol or ethylene glycol. If that is what you need, the site has calculated serum osmolality and the osmolal gap. This page is for a solution you are making up, where you know the composition exactly and want to know what you have built — and where the gap between the calculation and the osmometer is not a clinical finding but a property of electrolytes in water.
Frequently asked questions
What is the difference between osmolarity and osmolality?
Osmolarity is osmoles per litre of solution; osmolality is osmoles per kilogram of water. Osmolarity is always calculated, because nothing measures particles per unit volume of a mixture. Osmolality is measured, by freezing point depression, which is the only routine method.
Which one does an osmometer measure?
Osmolality, in mOsm/kg. Freezing point depression responds to the number of particles per kilogram of water, so a laboratory result is never osmolarity, however the request form is worded.
Why is 0.9% saline 308 on the bag but 286 on the osmometer?
Because sodium and chloride are only about 92.6% osmotically active — ion interactions mean a mole of dissolved NaCl exerts less osmotic effect than two moles of an ideal solute. 308 × 0.926 is 285, close to the measured 286. The bag prints the theoretical sum; the instrument measures what is really there.
Why do I see 5% dextrose quoted as both 252 and 278 mOsm/L?
The US product is dextrose monohydrate, molecular weight 198.17, so 50 g/L is 252 mmol/L and the label says 252 mOsm/L. Calculations based on anhydrous glucose, molecular weight 180.16, give 278 mmol/L and 278 mOsm/L. Both arithmetics are right; they count the water of crystallisation differently.
Is this the same as the osmolar gap?
No. The osmolar gap subtracts a calculated plasma osmolality from a measured one, to detect an unmeasured solute such as a toxic alcohol. This page calculates what is in a solution whose composition you already know. The site covers the gap separately.
Can a solution be iso-osmolar and still hypotonic?
Yes. Tonicity counts only solutes that cannot cross the cell membrane. 5% dextrose is close to iso-osmolar in the bottle and behaves as free water once the glucose is metabolised; urea raises measured osmolality without raising tonicity at all.
How do I enter a fluid with more than three solutes?
Combine solutes that release the same number of particles into one row — three monovalent salts at 130, 4 and 28 mmol/L can be entered as a single 162 mmol/L row with two particles each. The sum is what the calculation uses.
Related calculators
References
- Osmolality (mosmol/kg H₂O) versus osmolarity (mosmol/L): applied physiology to improve patient safety. Eur J Med Res 2025;30. doi:10.1186/s40001-025-03652-7 — definitions, the 92.6% osmotic activity of sodium and chloride, freezing point depression as the only measurement method, and measured osmolalities of 0.9% saline, Hartmann’s and 5% glucose.
- DailyMed / US Food and Drug Administration. 0.9% Sodium Chloride Injection USP, Lactated Ringer’s Injection USP and 5% Dextrose Injection USP package inserts — calculated osmolarities of 308, 273 and 252 mOsmol/L.
- Neonatal and Pediatric Peripheral Parenteral Nutrition: What Is a Safe Limit? Nutrition in Clinical Practice, 2013 — the A.S.P.E.N. 900 mOsm/L, Infusion Nurses Society 600 mOsm/L and ESPEN 850 mOsm/L peripheral limits.
- Deranged Physiology. Osmolarity, osmolality, tonicity and the reflection coefficient — the colligative basis of freezing point osmometry and the distinction between osmolality and tonicity.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.