Osmolarity Calculator
Calculated serum osmolarity from sodium, glucose, and BUN.
What it does
Blood labs estimate particle load with one compact sum: double the sodium for its partner anions, then add glucose over 18 and BUN over 2.8 to convert milligrams into milliosmoles. At sodium 140, glucose 90, and BUN 15 the total is 290.4 mOsm/kg. The tool above runs that sum with the normal band beside it.
Sodium carries the result. Its doubled share is 280 of the 290.4, while glucose adds 5 and urea about 5.4. That split explains why sodium swings move osmolarity most, and why very high sugar or urea can push totals past 295 on their own.
Reference ranges anchor the reading. StatPearls and nursing lab tables both place normal serum osmolality from 275 to 295 mOsm/kg, with water flowing toward the higher side. Calculated values screen and trend; freezing point machines measure the definitive number.
The gap matters more than the sum in toxicology. Measured minus calculated is the osmolar gap, normally small, which widens with unmeasured alcohols and sugar alcohols. This page prices the calculated half only; gap reading and treatment belong to the care team.
How to use
- Type serum sodium in mEq/L.
- Type glucose in mg/dL.
- Type BUN in mg/dL.
- Read calculated osmolarity first.
- Read the band: low under 275, normal to 295.
Formula
- 2 x Na
- Sodium doubled for chloride and bicarbonate partners.
- /18
- Converts glucose mg/dL to mmol/L.
- /2.8
- Converts BUN mg/dL to urea mmol/L.
Simplest standard estimate, not the fitted clinical equations with extra constants. SI inputs in mmol/L skip the divisors: 2xNa + glucose + urea.
Worked example
Example: Na 140, glucose 90, BUN 15
Defaults: Na 140, glucose 90, BUN 15.
- Sodium: 2 x 140 = 280.
- Glucose: 90 / 18 = 5.
- BUN: 15 / 2.8 = 5.4.
- Total: 280 + 5 + 5.4 = 290.4 mOsm/kg.
- Tool shows 290.4 mOsm/kg, Normal, 280 + 5 + 5.4.
Reference table
| Na / Glu / BUN | Result | Band |
|---|---|---|
| 140 / 90 / 15 | 290.4 mOsm/kg | Normal |
| 135 / 180 / 28 | 290.0 mOsm/kg | Normal |
| 130 / 100 / 20 | 272.7 mOsm/kg | Low |
| 150 / 300 / 40 | 331.0 mOsm/kg | High |
Limits
- Estimate only; measured osmolality rules.
- Ignores ethanol, mannitol, and toxins.
- Unit locked: mg/dL inputs assumed.
- Study aid, never a treatment order.
Common mistakes
- Forgetting to double the sodium.
- Dividing glucose by 2.8 instead of 18.
- Using urea instead of BUN with 2.8.
- Mixing mmol/L inputs with mg/dL divisors.
- Treating calculated as measured.
Try it yourself
Quick checks. Answers sit tucked under each question.
Na 140, glucose 100, BUN 14?
280 + 5.6 + 5 = 290.6 mOsm/kg, Normal.
Na 130, glucose 100, BUN 20?
260 + 5.6 + 7.1 = 272.7 mOsm/kg, Low.
Na 145, glucose 200, BUN 30?
290 + 11.1 + 10.7 = 311.8 mOsm/kg, High.
Questions
What is normal osmolarity?
275 to 295 mOsm/kg.
Why double sodium?
It accounts for partner anions chloride and bicarbonate.
What raises it most?
Usually sodium; extreme glucose or BUN can too.
Calculated versus measured?
Calculated estimates; freezing point depression measures.
What is the osmolar gap?
Measured minus calculated; wide gaps flag hidden osmoles.
Can urea cross membranes?
Freely enough that tonicity math sometimes drops it.